Harvard Psychological Studies Volume II

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BY HUGO MÜNSTERBERG

I. EXPERIMENTAL PSYCHOLOGY IN HARVARD

On the 27th of December, 1905, Harvard University opened its new house of philosophy, Emerson Hall. The presence of the American Philosophical and Psychological Associations gave national significance to the completion of this building.

The psychologist will find quarters in all parts of Emerson Hall. The general courses in psychology will be held on the first floor in the large lecture-room, which has nearly four hundred seats; and close by are the psychological seminary-room and smaller lecture-rooms for the advanced psychological courses. On the second floor the psychologist finds his special library as a wing of the large library hall. But the exclusive domain of the psychologist is the third floor,—a psychological laboratory with twenty-five rooms. A large attic hall for laboratory purposes on the fourth floor completes the psychologist's allotment.

The work to be reported in future in the Harvard Psychological Studies will be work done in this new building, and while the researches reported in the following pages were completed in the smaller quarters of the old laboratory, it seems natural that this volume, which appears at this new epoch of our work, should give an account both of our psychological past and of the development and purpose of Emerson Hall.

The Harvard Psychological Laboratory was founded in 1891 by Professor William James, who had introduced some experimental features into his psychological lecture courses for some time before the formal opening of a regular workshop. Professor James started with two large rooms on the second floor of Dane Hall, and secured an excellent equipment, especially for the study of the psychology of the senses. He was assisted by Dr. Herbert Nichols, and at once gathered a number of graduate students for research.

In the following year Professor James withdrew from the experimental work, and the conduct of the laboratory was given over to me. In the years which followed, Dr. Arthur Pierce, Dr. J. E. Lough, and Dr. Robert MacDougall were the assistants until three years ago, when the development of the laboratory demanded a division of the assistant functions; since that time Dr. E. B. Holt has been the assistant for the work in human psychology, while Dr. R. M. Yerkes has had charge of the work in comparative psychology. Since from the first I laid special emphasis on research work, a greater number of small rooms was soon needed. In the year 1893, we divided a part of the adjacent lecture-room into four rooms for special investigations, and two years later the larger of the two original rooms was divided into five. As the lecture-room also was finally made part of the research laboratory, we had at last eleven rooms in Dane Hall. The activities of the laboratory, however, went far beyond the research work. We had regular training-courses in experimental practice, and the lecture courses in human and in comparative psychology drew largely on the resources of our instrument cases. Yet the original investigations absorbed the main energy of the laboratory, and demanded a steady expansion of its apparatus. An illustrated catalogue of the instruments has been published as part of the Harvard Exhibition at the Chicago World's Fair.

The participation of the students has been controlled by a principle which has characterized our Harvard work through all these years, and distinguished it from the methods of most other institutions. I insist that no student shall engage in one investigation only, but that every one who has charge of a special problem shall give to it only half of his working time, while in the other half he is to be subject in four, five, or more investigations by other members of the laboratory. In this way each research is provided with the desirable number of subjects, and all one-sidedness is avoided. Every experimenter thus comes in contact with a large range of problems and gets a fair training in manifold observations, besides the opportunity for concentration on a special research. It is true that this demands a complicated schedule and careful consideration of the special needs of every research, but it gives to the work a certain freshness and vividness, and banishes entirely the depression which is unavoidable whenever a student is for any length of time a passive subject in one psychological enquiry only. In both capacities, as experimenter and as observing subject, only graduate students have been acceptable. In this way about one hundred investigations on human psychology have been carried on, for most of which I have proposed the problems and the special lines of work, taking care that the research of succeeding years and of succeeding generations of graduate students should show a certain internal continuity. Whenever the results seemed fit for publication, the papers have been published under the names of the students who had the responsibility for the conduct of the experiments. Until three years ago the publication was scattered; most of the papers, however, appeared in the Psychological Review. The Harvard Psychological Studies, beginning in 1903, are to gather the bulk of our material, although not a few of the researches of recent years have been published in other places.

The laboratory has always sought to avoid one-sidedness, and this the more as it was my special aim to adjust the selection of topics to the personal equations of the students, many of whom came with the special interests of the physician, the zoölogist, the artist, the pedagogue, and so on. My own special interests may have emphasized those problems which refer to the motor functions and their relations to attention, apperception, space-sense, time-sense, feeling, etc. At the same time I have tried to develop the psychological-æsthetic work, which has become more and more a special branch of our laboratory, and there has been no year in which I have not insisted on some investigations in the fields of association, memory, and educational psychology. On the other hand, in a happy supplementation of interests, Dr. Holt has emphasized the physiological psychology of the senses, and Dr. Yerkes has quickly developed a most efficient experimental department of animal psychology.

As the work thus became more manifold, the old quarters in Dane Hall appeared less and less sufficient. And yet this laboratory development has been merely parallel to the growth of general philosophical studies in the whole University. The demand for a new hall, exclusively devoted to philosophy, was thus suggested from many sides. The idea of linking it with the name of Ralph Waldo Emerson has been for years a cherished plan of Professor Palmer.

An especially appropriate time for the realization of such a plan came in the approach of the hundredth anniversary of Emerson's birthday. Almost two years before this date the Department took the first steps in seeking to interest the members of the Visiting Committee for the collection of the necessary funds. This Committee, consisting of Mr. G. B. Dorr, chairman, Mr. R. H. Dana, Dr. R. Cabot, Mr. J. Lee, Mr. D. Ward, and Mr. R. C. Robbins, showed not only warm interest, but lent itself to the furtherance of the plans with such an energy and devotion that the Philosophical Department owes to these friends of philosophy in Harvard the most lasting gratitude. Various means were taken by the Committee and by the Department to stir the interest of the public, and soon the gifts began to come in, gifts of which some were clearly given from sympathy with the work of the Philosophical Department, some evidently in memory of Emerson. The original plans of the architect called for $150,000 for the building. When, on the 25th of May, 1903, the hundredth anniversary of Emerson's birthday was celebrated, the University had contributions amounting to more than this sum, and given by one hundred and seventy persons.

It was soon found, however, that this sum was inadequate; yet we never asked in vain. Additional gifts came in for the building fund, just as later the generosity of several friends furnished the building with a handsome equipment and the laboratory with new instruments. Mr. R. C. Robbins gave the books for a philosophical library to be placed in the new Hall.

The architect chosen was Mr. Guy Lowell, who has had to labor under the difficulties involved in the fact that the best and quietest available place was on Quincy Street opposite Robinson Hall. This spot demanded that the new building be harmonized with Robinson and Sever Halls, two structures most unlike in their architectural style. There was not even the possibility of making it a companion to Robinson Hall, since the latter has but two stories, while it was evident that Emerson Hall needed three stories. The plan finally accepted, a Greek, brick building with brick columns and rich limestone trimmings, provided for the work of the whole Philosophical Division with the exception of education. The Education Department, with its large library, will soon need a whole building of its own, and has thus had no interest in being housed under the roof of Emerson Hall. On the other hand, the building was to give full space to that part of our Philosophical Division which now forms, like education, an administrative unity,—the Department of Social Ethics. A special library, museum rooms, etc., for social ethics were planned for the second floor by the munificence of an anonymous benefactor. Altogether we have six large lecture-rooms, two library halls, two collection-rooms, a department-room, a seminary-room, two studies and conference-rooms, twenty-five laboratory-rooms, all connected by very spacious, well-lighted halls and broad, imposing stairways. Surely never before in the history of scholarship has such a stately house been built for philosophy. And while the nature of the work is certainly not determined by the luxury of stone and carved wood, teachers and students alike must feel these superb surroundings as a daily stimulus to their best efforts.

At Christmas, 1905, the building stood ready for use, and Duveneck's bronze statue of Emerson was unveiled in the entrance hall. At the opening meeting, after short dedicatory orations by President Eliot and Dr. Edward Emerson, a real exchange of ideas in a joint debate of the Philosophical and Psychological Associations was substituted for the usual formal exercises. The question debated was suggested by the fact that Emerson Hall was to house the psychological laboratory. Does psychology really belong to philosophy or rather to the natural sciences? As the representative of Harvard, it was my part to open the debate and to characterize the attitude of the Harvard laboratory.

My remarks on that occasion may thus serve as the most direct introduction to our work. They are printed here, together with a short sketch of the equipment of the laboratory. I venture to add also two other papers, one of which points to the administrative, the other and longer one to the philosophical background of Emerson Hall. Inasmuch as I was Chairman of the Philosophical Department throughout the five years in which the plan for Emerson Hall was growing and became finally realized, it has been my official duty repeatedly to express our hopes and ideals. Thus I had to formulate the wishes of the Department at the outset in a letter to the Visiting Committee, a letter which was used as a circular in asking the public for funds. Two years later when Harvard celebrated the Emerson anniversary, I delivered an address on Emerson as philosopher. This epistemological paper may seem far removed from the interests of the Harvard Psychological Studies, and yet I am glad to print it in this laboratory volume, and thus emphatically to indicate that I for one consider philosophy the true basis for the psychologist.

There follow thus, first, the letter to the Visiting Committee, with which the Emerson Hall movement took its official inception in 1901; secondly, the address delivered at Harvard on the celebration of the Emerson anniversary in May, 1903; thirdly, the paper contributed to the debate of the philosophers at the opening meeting in December, 1905; and, finally, a description of the present status of the laboratory in January, 1906.

[The letter addressed to the Visiting Committee of the Overseers of Harvard University, in 1901, reads as follows:]

Gentlemen,—The philosophical work in Harvard has in the last twenty years gone through an inner development which has met with a hearty response alike on the part of the University and of the students. The students have attended the courses in constantly growing numbers, the Governing Boards have provided the Division amply with new teachers, steadily increasing the number of professors, instructors, and assistants. The outer growth of the Division has thus corresponded most fortunately to the internal development, by an harmonious coöperation of the administration, the teachers, and the students of the University. And yet there remains one other factor as an essential condition for the healthy life of the Department, a factor which cannot be provided by the University itself and for which the help must come from without. Our work needs a dignified home where under one roof all the varied philosophical work now carried on at Harvard may be united. The need has been urgently felt for many years, but only with the recent growth has the situation become intolerable. It is therefore the unanimous opinion of the Department that we must ask the public for the funds to build at Harvard a "School of Philosophy," in the interest of the students and of the teachers, in the interest of the Department and of the University, in the interest of culture and of scholarship.

The present work of the Division of Philosophy can be indicated by a few figures. We entered the current year with a teaching-staff of six full professors, two assistant professors, four instructors, two teaching-fellows, and six assistants. The instruction of these twenty men covers the ground of history of philosophy, metaphysics, theory of knowledge, psychology, logic, ethics, æsthetics, philosophy of religion, philosophy of science and sociology. Thirty-two courses have been offered. These courses are grouped in three classes: the introductory courses, intended primarily for Sophomores and Juniors; the systematic and historic courses, planned for Juniors, Seniors, and Graduates; and the research courses for Graduates only. But the students whom we try to reach differ not only with regard to their classes, their corresponding maturity, and their degree of philosophical preparation, but also with regard to the aims and interests for which they elect philosophical studies in the University. The one group seeks in our field liberal education. The fundamental problems of life and reality, and the historic solutions of them which the great thinkers developed, the values of truth and beauty and morality, the laws of the mental mechanism and of the social consciousness, all these promise and prove to be incomparable agencies for widening the soul and giving to our young men depth, strength, and ideals. Not a few of the students who belong to this group remain loyal to philosophy through three or four years. A second group of students need our courses as preparation for divers scholarly or practical aims. The future lawyer, teacher, physician, minister, scientist, or philanthropist knows that certain courses in ethics or psychology, in education or logic afford the most solid foundations for his later work; there is hardly a course in our Division which is not adjusted to some kind of professional study. The third group finally, naturally the smallest, but to the teachers the most important, consists of those to whom philosophy itself becomes a life's work. The Harvard Department believes that there is nowhere else in this country or abroad such an opportunity for systematic and all-round training for an advanced student of philosophy as is offered here, covering easily a man's full work for six years, advancing from the introductory courses of the Sophomore year to the six seminaries of the graduate years and finally reaching the doctor's thesis in the third year after graduation.

The extent to which the Harvard students make use of these opportunities is to be inferred from the figures which the last Annual Report of the President offers. These refer to the year 1899-1900; the current year will show somewhat the same proportions, perhaps even an increase of graduate work. The figures are necessarily too low, inasmuch as they refer merely to those students who take examinations in the courses and omit those who merely attend the lectures. The attendance in the philosophical courses was last year over one thousand students. They belonged to all parts of the University, 188 Graduates, 210 Seniors, 218 Juniors, 175 Sophomores, 59 Specials, 57 Scientifics, 55 Divinity students, and the rest from the Freshman class, the Law School, and the Medical School. The introductory courses were attended by almost four hundred students, that is, by a number corresponding to the size of the Junior class. As, in spite of natural fluctuations, this figure is pretty constant,—in 1897 reaching its maximum with 427,—it can be said that in Harvard under the system of absolutely free election practically every student who passes through Harvard required of himself at least a year of solid philosophical study.

An even higher interest, however, belongs to the figures which refer to the most advanced courses offered, especially to the courses of research. It has always been the most characteristic feature of the Harvard Philosophical Department to consider the advancement of knowledge as its noblest function. The productive scholarship of the Department is shown by the fact that the last two years alone brought before the public eight compendious scholarly works from members of our Department, besides a large number of smaller contributions to science. To train also in the students this highest scholarly attitude, that of the critical investigator as contrasted with that of the merely receptive hearer of lectures, is thus the natural aim of our most advanced work; it is this spirit which has given to the Department its position in the University and in the whole country. This prevalence of the spirit of research is the reason why, as the Report of the Dean of the Graduate School points out, the Philosophical Department has a larger number of graduate students who have carried on graduate studies elsewhere than any other Department of the University. The table of the Dean which records these migrating graduate students who come to us for advanced work after graduate studies at other universities, is as follows: Mathematics 6, Natural History 7, Political Science 7, Modern Languages 11, Classics 14, History 15, English Literature 16, Philosophy 20. If we consider the whole advanced work of the University, that is the totality of those courses which are announced as "primarily for Graduates," we find that the following number of graduate students, including the graduate members of the professional schools, have taken part: Classics 103, Philosophy 96, English 75, German 61, History and Government 52, Romance Languages 45, Mathematics 39, Economics 23, Chemistry 21, in the other departments less than twenty. But this situation turns still more strongly in favor of philosophy as soon as we consider the technical research courses, those which in the language of the catalogue are known as the 20-courses, and omit those graduate courses which are essentially lecture courses. In these research courses the number of Graduate Students is: Philosophy 71, History and Government 34, Chemistry 13, Zoölogy 12, Geology 10, and in the other departments less than ten.

These few figures may be sufficient to indicate not only the extent of the Department and its influence, but above all the harmonious character of this development. The most elementary courses, the solid routine courses, and the most advanced courses, show equal signs of growth and progress, and the whole work with its many side branches remains a well-connected unity with a clear systematic plan. All this must be understood before one can appreciate the striking contrast between the work and the workshop. It is of course easy to say at once that the truth of a metaphysical thought does not depend upon the room in which it is taught, and that the philosopher is not, like a physicist or chemist, dependent upon outer equipments. Yet, this is but half true, and the half of the statement which is false is of great importance.

The dependence upon outer conditions is perhaps clearest in the case of psychology, which has been for the last twenty-five years an objective science with all the paraphernalia of an experimental study: the psychologist of to-day needs a well-equipped laboratory no less than the physicist. Harvard has given the fullest acknowledgment to this modern demand and has spent large sums to provide the University with the instruments of an excellent psychological laboratory; the one thing which we miss is room, simply elbow-room. Our apparatus is crowded in the upper story of Dane Hall, and even that small story must give its largest room for the lectures of other departments and another room to a philosophical reading-room. The space which remains for the psychological work is so absolutely out of proportion to the amount of work going on that the problem how to bring all the men into those few rooms has become the most difficult of all our laboratory problems. During the current year, besides the training-courses, twenty-three men are engaged there in original research, each one with a special investigation and each one anxious to devote as much time as possible to his research; only the most complicated adjustment makes it possible at all, and yet the mutual disturbance, the necessity of passing through rooms in which other men are working, and of stopping the work when other men need the place interfere every day with the success of the instruction. A mechanical workshop is an urgent need of our laboratory, and yet we cannot afford the room; and while the only desirable arrangement would be to have the psychological lectures in the same building where the apparatus is stored,—as the instruments are necessary for the experimental demonstrations,—there is no room for the lectures under the roof of Dane Hall, which houses the Bursar's Office and Coöperative Stores. The result is that the instruments must be carried through the yard in rain or shine, an effective way to damage our valuable equipment. But the evils connected with the present locality of the psychological laboratory are not only such as result from its narrowness. Its position on Harvard Square, with the continuous noise and the vibration of the ground, is perfectly prohibitory for large groups of psychological studies and disturbing for every kind of work for which concentration of attention is a fundamental condition. Finally a psychological laboratory, perhaps still more than a physical one, needs in its whole construction a perfect adaptation to its special purpose; the walls, the shape, and the connection of the rooms, everything must be built, as has been done in other universities, for the special end. We have merely the rooms of the old Law School with thin partitions dividing them. In short everything is in a state which was tolerable during the last few years only because it was felt as provisional, but the time when the psychological laboratory must have really adequate quarters cannot be postponed much longer.

The needs of the psychological work can thus be easily demonstrated to every beholder; but while perhaps less offensive on the surface, the outer conditions of the other branches of the Philosophical Department are not therefore less unsatisfactory. The advanced student of logic or ethics does not need a laboratory, but he needs seminary-rooms with a working library where his work may have a local centre, where he can meet his instructors and his fellow students engaged in related researches, where he may leave his books and papers. To-day all this theoretical work has no home at all; the seminaries seek refuge in an empty room of the laboratory at a late evening hour, in a chance lecture-room, or in private homes; there is nowhere continuity, no place to collect or to deposit, no opportunity to meet beyond official hours, no feeling of coherence suggested by surroundings. The most advanced research work of the country is thus done under external conditions which suggest the spirit of a schoolroom, conditions which deprive students and instructors equally of the chance to make our seminaries the fitting forms for their rich content. But if all this is most deeply felt by the advanced students, it is not less true and not less deplorable for the undergraduate courses. There is nowhere fixity of association between the work and the room. The philosophy courses are scattered over the whole yard, wandering each year from one quarter to the other, creeping in wherever a vacant room can be found, not even the instructors knowing where their nearest colleagues are meeting students. The dignity and the unity of the work are equally threatened by such a state of affairs. There remains not even a possibility for the instructor to meet his students before or after the lecture; his room is filled up to the time when he begins and a new class rushes in before he has answered questions. A business-like restlessness intrudes into the instruction, and yet philosophy above all needs a certain repose and dignity.

Thus what we need is clear. We need a worthy monumental building at a quiet central spot of the Harvard yard, a building which shall contain large and small lecture-rooms, seminary-rooms, a reading-room, and one whose upper story shall be built for a psychological laboratory, so that under one roof all the philosophical work, metaphysical and ethical, psychological and logical, may be combined. Here the elementary and the advanced work, the lecture courses and the researches, the seminaries and the experiments, the private studies in the reading-room and the conferences and meetings of the assistants would go on side by side. Here would be a real school of philosophy where all Harvard men interested in philosophy might find each other and where the students might meet the instructors.

Such a home would give us first, of course, the room and the external opportunities for work on every plane; it would give us also the dignity and the repose, the unity and the comradeship of a philosophical academy. It would give us the inspiration resulting from the mutual assistance of the different parts of philosophy, which in spite of their apparent separation are still to-day parts of one philosophy only. All this would benefit the students of philosophy themselves, but not less good would come to the University as a whole. The specialization of our age has brought it about that in the organization of a university, even philosophy, or rather each of the philosophical branches, has become an isolated study coördinated with others. The average student looks to psychology as to physics or botany; he thinks of ethics as he thinks of economics or history; he hears about logic as coördinated with mathematics, and so on. The University has somewhat lost sight of the unity of all philosophical subjects and has above all forgotten that this united philosophy is more than one science among other sciences, that it is indeed the central science which alone has the power to give inner unity to the whole university work. Every year our universities reward our most advanced young scholars of philology and history, of literature and economics, of physics and chemistry, of mathematics and biology with the degree of Ph.D., that is of Doctor Philosophiae, thus symbolically expressing that all the special sciences are ultimately only branches of philosophy; but the truth of this symbol has faded away from the consciousness of the academic community. All knowledge appears there as a multitude of scattered sciences and the fact that they all have once been parts of philosophy, till one after the other has been dismissed from the mother arms, has been forgotten. A school of philosophy as a visible unity in the midst of the yard will renew this truth and thus give once more to the overwhelming manifoldness of intellectual efforts of our University a real unity and interconnection; the external connection of administration will be reënforced by the inner unity of logical interdependence.

The time is ripe for a school of philosophy to play this rôle and to fulfil again its old historical mission, to be the unifying principle of human knowledge and life. The second half of the nineteenth century was essentially controlled by realistic energies, by the spirit of analysis, by the triumph of natural science and technique. But a long time before the century came to an end a reaction started throughout the whole intellectual globe; the synthetic energies again came to the foreground, the idealistic interests were emphasized in the most different quarters; the historical and social sciences make to-day the same rapid progress which fifty years ago characterized the natural sciences, and everywhere in the midst of the empirical sciences there is awakening again the interest in philosophy. In the days of anti-philosophical naturalism scientists believed that philosophy had come to an end and that an unphilosophical positivism might be substituted for real philosophy; to-day the mathematicians and physicists, the chemists and biologists, the historians and economists eagerly turn again and again to philosophy, and on the borderland between philosophy and the empirical sciences they seek their most important problems and discussions. The world begins to feel once more that all knowledge is empty if it has no inner unity, and that the inner unity can be brought about only by that science which enquires into the fundamental conceptions and methods of thought with which the special sciences work, into the presuppositions and ultimate axioms with which they begin, into the laws of mental life which lie at the basis of every experience, into the ways of teaching the truth, and above all into the value of human knowledge, its absolute meaning and its relation to all the other human values—those of morality, beauty, and religion. The most advanced thinkers of our time are working to-day in all fields of knowledge to restore such a unity of human life through philosophy. To foster this spirit of the twentieth century in the life of our University there is no more direct way possible than to give a dignified home to the philosophical work. Such a building ought to be a Harvard Union for scholarly life.

The beautiful building which we see in our minds should not be devoted to a single system of philosophy. In its hall we hope to see as greeting for every student the busts of Plato the Idealist and Aristotle the Realist, of Descartes and Spinoza, of Bacon and Hobbes, of Locke and Hume and Berkeley, of Kant and Fichte and Hegel, of Comte and Spencer, of Helmholtz and Darwin. The School of Philosophy will be wide open to all serious thought, as indeed the members of the Department to-day represent the most various opinions and convictions. This ought never to be changed; it is the life-condition of true philosophy. Yet there is one keynote in all our work: a serious, critical, lofty idealism which forms the background of the whole Department and colors our teaching from the elementary introductions to the researches of our candidates for the doctor's degree. All the public utterances which have come from the Department in recent years are filled with this idealism, in spite of the greatest possible variety of special subjects and special modes of treatment. Here belong The Will to Believe and the Talks to Teachers, by William James, the Noble Lectures and the Glory of the Imperfect, by George Herbert Palmer, Poetry and Religion, by George Santayana, The Principles of Psychology, and Psychology and Life, by Hugo Münsterberg, Jesus Christ and the Social Question, by Francis Peabody, Educational Aims and Educational Values, by Paul Hanus, Shaftesbury, by Benjamin Rand, the Conception of God, and The World and the Individual, by Josiah Royce.

We have sought a name which might give symbolic expression to this underlying sentiment of idealism and might thus properly be connected with the whole building. It cannot be that of a technical philosopher. Such a name would indicate a prejudice for a special system of philosophy, while we want above all freedom of thought. It ought to be an American, to remind the young generation that they do not live up to the hopes of the School of Philosophy if they simply learn thoughts imported from other parts of the world, but that they themselves as young Americans ought to help the growth of philosophical thought. It ought to be a Harvard man—a man whose memory deserves that his name be daily on the lips of our students, and whose character and whose writing will remain a fountain of inspiration. Only one man fulfils all these demands perfectly: Ralph Waldo Emerson. It is our wish and hope that the new, dignified, beautiful home of philosophy may soon rise as the moral and intellectual centre of Harvard University and that over its doors we shall see the name: Emerson Hall—School of Philosophy.

[The following address was delivered at Harvard University, May, 1903, as part of the Emerson Celebration:]

At the hundredth anniversary of Emerson's birthday, Harvard University is to take a noble share in the celebration. For years it has been one of the deepest desires of the Harvard community to erect in the college yard a building devoted to philosophy only. To-day this building is secured. To be sure, the good-will of the community must still do much before the funds allow the erection of a building spacious enough to fulfil our hopes; but whether the hall shall be small or large, we know to-day that it will soon stand under the Harvard elms and that over its door will be inscribed the name: Ralph Waldo Emerson. No worthier memorial could have been selected. Orations may be helpful, but the living word flows away; a statue may be lasting, but it does not awaken new thought. We shall have orations and we shall have a statue, but we shall have now, above all, a memorial which will last longer than a monument and speak louder than an oration: Emerson Hall will be a fountain of inspiration forever. The philosophical work of Harvard has been too long scattered in scores of places; there was no unity, philosophy had no real home. But Emerson Hall will be not only the workshop of the professional students of philosophy, will be not only the background for all that manifold activity in ethics and psychology, in logic and metaphysics, in æsthetics and sociology, it will become a new centre for the whole University, embodying in outer form the mission of philosophy to connect the scattered specialistic knowledge of the sciences. Harvard could not have offered a more glorious gift to Emerson's memorial.

But the spirit of such a memorial hour demands, more than all, sincerity. Can we sincerely say that the choice was wise, when we look at it from the point of view of the philosophical interests? It was beautiful to devote the building to Emerson. Was it wise, yes, was it morally right to devote Emerson's name to the Philosophy Building? Again and again has such a doubt found expression. Your building, we have heard from some of the best, belongs to scientific philosophy; the men who are to teach under its roof are known in the world as serious scholars, who have no sympathy with the vague pseudo-philosophy of popular sentimentalists; between the walls of your hall you will have the apparatus of experimental psychology, and you will be expected to do there the most critical and most consistent work in methodology and epistemology. Is it not irony to put over the door, through which daily hundreds of students are to enter, the name of a man who may be a poet and a prophet, a leader in literature and a leader in life, but who certainly was a mystic and not a thinker, an enthusiast but not a philosopher? Not only those who belittle him to-day and who short-sightedly deny even his immense religious influence, but even many of Emerson's warmest admirers hold such an opinion. They love him, they are inspired by the superb beauty of his intuitions, but they cannot respect the content of his ideas, if they do not wish to deny all their modern knowledge and scientific insight. Yes, for the most part they deny that his ideas form at all a connected whole; they are aphorisms, beautiful sparks. Did he not himself say: "With consistency a great soul has simply nothing to do. He may as well concern himself with his shadow on the wall." And yet how can there be philosophy without consistency; how can we interpret reality if we contradict ourselves? If Emerson's views of the world did really not aim at consistency and did really ignore our modern knowledge, then it would be better to go on with our philosophical work in Harvard without shelter and roof than to have a hall whose name symbolizes both the greatest foe of philosophy, the spirit of inconsistency, and the greatest danger for philosophy, the mystic vagueness which ignores real science.

But Emerson stands smiling behind this group of admirers and says, "To be great is to be misunderstood." Yes, he did say, "A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines;" but he soon adds, "Of one will the actions will be harmonious however unlike they seem." Emerson despises the consistency of the surface because he holds to the consistency of the depths, and every sentence he speaks is an action of the one will, and however unlike they seem they are harmonious, and, we can add, they are philosophical; and, what may seem to these anxious friends more daring, they are not only in harmony with each other, they are in deepest harmony with the spirit of modern philosophy, with a creed which ought to be taught by the most critical scholars of Harvard's Philosophy Hall.

What is the essence of Emerson's doctrine in the realm of philosophy? It seems like sacrilege to formulate anything he said in the dry terms of technical philosophy. We must tear from it all the richness and splendor of his style, we must throw off the glory of his metaphor, and we must leave out his practical wisdom and his religious emotion. It seems as if we must lose all we love. It is as if we were to take a painting of Raphael and abstract not only from the richly colored gowns of the persons in it, but from their flesh and blood, till only the skeletons of the figures remained. All beauty would be gone, and yet we know that Raphael himself drew at first the skeletons of his figures, knowing too well that no pose and no gesture is convincing, and no drapery beautiful if the bones and joints fit not correctly together. And such a skeleton of theoretical ideas appears not only without charm, it appears necessarily also uninteresting, without originality, commonplace. All the philosophies, from Plato to Hegel, brought down to their technical formulas, sound merely like new combinations of trivial elements, and yet they have made the world, have made revolutions and wars, have led to freedom and peace, have been mightier than traditions and customs; and it is true for every one of them that, as Emerson said, "A philosopher must be more than a philosopher."

There are, it seems, three principles of a philosophical character without which Emerson's life-work cannot be conceived. To bring them to the shortest expression we might say, Nature speaks to us; Freedom speaks in us; the Oversoul speaks through us. There is no word in Emerson's twelve volumes which is inconsistent with this threefold conviction, and everything else in his system either follows immediately from this belief or is a non-essential supplement. But that threefold faith is a courageous creed indeed. The first, we said, refers to Nature; he knew Nature in its intimacy, he knew Nature in its glory; "Give me health and a day and I will make the pomp of emperors ridiculous." And this Nature, that is the assertion, is not what natural sciences teach it to be. The Nature of the physicist, the dead world of atoms controlled by the laws of a dead causality, is not really the Nature we live in; the reality of Nature cannot be expressed by the record of its phenomena, but merely by the understanding of its meaning. Natural science leads us away from Nature as it really is. We must try to understand the thoughts of Nature. "Nature stretches out her arms to embrace man; only let his thoughts be of equal greatness;" and again Emerson says, "All the facts of natural history taken by themselves have no value, but are barren like a single sex; but marry it to human history and it is full of life;" and finally, "The philosopher postpones the apparent order of things to the empire of Thought."

And in the midst of Nature, of the living Nature, we breathe in freedom; man is free. Take that away and Emerson is not. Man is free. He does not mean the freedom of the Declaration of Independence, a document so anti-Emersonian in its conception of man; and he does not mean the liberty after which, as he says, the slaves are crowing while most men are slaves. No, we are free as responsible agents of our morality. We are free with that freedom which annuls fate; and if there is fate, then freedom is its most necessary part. "Forever wells up the impulse of choosing and acting in the soul." "So far as man thinks he is free." "Before the revelations of the soul, time, space, and nature shrink away." "Events are grown on the same stem with the personality; they are sub-personalities." "We are not built like a ship to be tossed, but like a house to stand." This freedom alone gives meaning to our life with its duties, and puts the accent of the world's history on the individual, on the personality: "All history resolves itself very easily into the biography of a few stout and earnest persons," and "An institution is the lengthened shadow of a man."

Nature speaks to us, Freedom speaks in us, but through us speaks a Soul that is more than individual, an over-individual soul, an "Oversoul, within which every man is contained and made one with all others." Now even "Nature is a great shadow, pointing always to the sun behind her." Every one of us belongs to an absolute consciousness which in us and through us wills its will; "Men descend to meet" and "Jove nods to Jove from behind each of us." Yes, "Man is conscious of a universal soul within or behind his individual life, wherein as in a firmament justice, truth, love, freedom arise and shine." The ideals, the duties, the obligations, are not man's will but the will of an Absolute.

Does not all this sound like a wilful denial of all that has been fixed by the sciences of our time? Does not every Sophomore who has had his courses in Physics, Psychology, and Sociology know better? He knows, we all know, that the processes of Nature stand under physical laws, that the will of man is the necessary outcome of psychological laws, that the ideals of man are the products of human civilization and sociological laws. And if every atom in the universe moves according to the laws which physics and chemistry, astronomy and geology, have discovered, is it not anti-scientific sentimentality to seek a meaning and thoughts in the mechanical motions of the dead world of substance? So the poet may speak, but we ought not to say that his fanciful dreams have value for scholarly philosophers. The philosophy of the scientist ought to be the acknowledgment that matter and energy, and space and time are eternal, and that the smallest grain of sand and the largest solar system move meaningless by blind causality.

And emptier still is the naïve belief that man is free. Do we not profit from decades of psychological labor, whereby the finest structure of the brain has been discovered, wherein the psychological laws have been studied with the exactitude of a natural science, wherein we have studied the mental life of animals and children, and have observed the illusions of freedom in the hypnotized man and in the insane? Yes, we know to-day that every mental act, that every psychological process is the absolutely necessary outcome of the given circumstances; that the functions of the cells in the cortex of the brain determine every decision and volition, and that man's deed is as necessary as the falling of the stone when its support is taken away. Yes, modern psychology does not even allow the will as an experience of its own kind; it has shown with all the means of its subtle analysis that all which we feel as our will is only a special combination of sensations which accompany certain movement-impulses in our body. Can we still take it seriously, when the philosopher steps in and pushes sovereignly aside all the exact knowledge of mankind, and declares simply "Man's will is free!"

Finally, the claim for the over-personal, absolute consciousness in man. It is a triumph of modern science to understand how the duties and ideals have grown up in the history of civilization. What one nation calls moral is perhaps indifferent or immoral for another people or for another time; what the one calls beautiful is ugly for the other; what one period admires as truth is absurdity for another; there is no absolute truth, no absolute beauty, no absolute religion, no absolute morality; and sociology shows how it was necessary that just these ideals and just these obligations should have grown up under a given climate and soil, a given temperament of the race, a given set of economical conditions, a given accumulation of technical achievements. Man has made his Absolute, not the Absolute made man, and whatever hopes and fears make men believe, the scholarly mind cannot doubt that these beliefs and idealizations are merely the products of the feelings and emotions of individuals bound together by equal conditions of life. Leave it to the raptures of the mystic to ignore all scientific truth, to get over-soul connection beyond all experience. In short, to accept Emerson's philosophy, the scientist would say, means to be a poet where Nature is concerned, means to be ignorant where man is concerned, and means to be a mystic where moral and religious, aesthetic and logical ideals are concerned. Can such be the herald of modern philosophy?

But those who are so proud and so quick are not aware that the times have changed and that their speech is the wisdom of yesterday. In the history of human knowledge the periods alternate. Great waves follow each other, and while one tendency of scientific thought is ebbing, another is rising; and there is no greater alternation than that between positivism and idealism. The positivistic period of natural science has ebbed for ten or fifteen years; an idealistic one is rising. Emerson once said here in Harvard that the Church has periods when it has wooden chalices and golden priests, and others when it has golden chalices and wooden priests. That is true for the churches of human knowledge too, and for knowledge of all denominations. Forty, fifty years ago, in the great period when Helmholtz discovered the conservation of energy and Darwin the origin of species, one naturalistic triumph followed the other, golden high priests of natural science were working with wooden chalices in narrow, awkward laboratories; to-day natural science has golden chalices provided in luxurious institutions, but there are too many wooden priests. The fullest energies of our time are pressing on to an idealistic revival, are bringing about a new idealistic view of the world, and turning in sympathy to that last foregoing period of idealism of which Ralph Waldo Emerson was perhaps the last original exponent. But also with his period idealism was not new. When he came to speak on the Transcendentalist, he began, "The first thing we have to say respecting the new views here in New England is that they are not new." Yes, indeed; since the beginnings of Greek philosophy, more than two thousand years ago, the two great tendencies have constantly followed each other. Each one must have its time of development, must reach its climax, must go over into undue exaggeration, and thus destroy itself to make room for the other, which then begins in its turn to grow, to win, to overdo, and to be defeated.

Glorious had been the triumph of Positivism in the middle of the eighteenth century when the French encyclopædists were at work, those men who wrote the decrees for the French Revolution. But before the last consequences of the Positivism of the eighteenth century were drawn, the idealistic counter-movement had started. Immanuel Kant gave the signal, he fired the shot heard round the world; and Fichte followed, whose ethical Idealism changed the map of Europe, and his spirit went over the Channel to Carlyle, and finally over the ocean to these shores of New England and spoke with the lips of Emerson. It is unimportant whether Emerson studied the great transcendental systems in the original; he knew Kant and Schelling probably at first through Coleridge, and Fichte through Carlyle. But in the mean time Idealism too had exaggerated its claims, it had gone forward to Hegel, and while Hegelian thought, about 1830, held in an iron grasp the deepest knowledge of his time, his neglect of positive experience demanded reaction, a counter-movement became necessary, and in the midst of the nineteenth century the great idealistic movement with all its philosophical and historical energies went down, and a new Positivism, full of enthusiasm for natural science and technique and full of contempt for philosophy, gained the day. With logical consistency, the spirit of empiricism went from realm to realm. It began with the inorganic world, passed into physics, then forward to chemistry, became more ambitious and conquered the world of organisms, and when biology had said its positivistic say, turned from the outer nature of being to the inner nature. The mind of man was scrutinized with positivistic methods; we came to experimental psychology, and finally, as the highest possible aim of naturalism, to the positivistic treatment of society as a whole, to sociology. But naturalism again has overdone its mission, the world has begun to feel that all the technique and all the naturalistic knowledge makes life not more worth living, that comfort and bigness do not really mean progress, that naturalism cannot give us an ultimate view of the world. And above all, the reaction has come from the midst of the sciences themselves. Twenty years ago scientific work received its fullest applause for the neglect of philosophical demands. Ten years ago the feeling came up that there are after all problems which need philosophy, and to-day philosophers, with good or bad philosophy, are at work everywhere. The physicists, the chemists and the biologists, the astronomers and the mathematicians, the psychologists and the sociologists, the historians and the economists, the linguists and the jurists, all are to-day busily engaged in philosophical enquiries, in enquiries into the conditions of their knowledge, into the presuppositions and methods of their sciences, into their ultimate principles and conceptions; in short, without a word of sudden command, the front has changed its direction. We are moving again towards philosophy, towards Idealism, towards Emerson.

Does all this mean that we are to forget the achievements of natural science, and ignore the results of empirical labor, of labor which has given us an invincible mastery of stubborn nature and an undreamed-of power to calculate all processes of the physical and of the psychical world? No sane man can entertain such a notion. Yes, such ideas would contradict the laws which have controlled the alternation of Idealism and Positivism through the ages of the past. Whenever Positivism returned, it always showed a new face, and the teaching of the intervening period of Idealism was never lost. The naturalism of the middle of the nineteenth century was not at all identical with the naturalism of the middle of the eighteenth; and so Idealism too, as often as it returned to mankind after periods of neglect and contempt, had every time gained in meaning, had every time found increased responsibilities, had every time to do justice to the new problems which the preceding period of Positivism had raised. If Idealism to-day wants to gain new strength, nothing must be lost of all that the last fifty years have brought us, no step must be taken backward, the careful scientific work of the specialists must be encouraged and strengthened, and yet the totality of this work must be brought under new aspects which allow a higher synthesis; yes, a higher synthesis is the problem of the philosopher of to-day. He does not want to be ignorant of natural science and simply to substitute idealistic demands in the place of solid, substantial facts; and he should feel ashamed of the foul compromise with which half-thinkers are easily satisfied, a compromise which allows science its own way till it comes over the boundaries of human emotions, a compromise which accepts rigid causality but pierces little holes in the causal world, making little exceptions here and there that human freedom may be saved in the midst of a world-machinery; a compromise which accepts the social origin of ideals, but claims a mystic knowledge that just our own private pattern will remain in fashion for eternity. No philosophy can live by compromises. If natural science is to be accepted and Idealism is to hold its own, they must be combined, they must form a synthesis in which the one no longer contradicts the other. Just such synthetic harmonization, and not at all a stubborn ignorance of the other side or a compromise with cheap concessions, was the aim of the period from Kant to Emerson. It is merely the naturalistic period which ignores its idealistic counterpart, which delights in its one-sidedness, which is afraid of harmony because it is suspicious of demands for concessions. It is naturalism only which thinks that mankind can walk on one leg.

If we ask where such harmonization can be found, where the great Idealists of the beginning of the last century have sought it, and where our modern philosophy is seeking it again, well aware that by the progress of science in the mean time the difficulties have been multiplied, the logical responsibilities have become gigantic, we cannot do more here than to point out the direction; we cannot go the way. And it is clear, of course, too, that such an answer has its individual shape, and that no one can promise to give a bird's-eye view of the marching movement while he is himself marching among his comrades. But the individual differences are non-essential. The one great tendency, the Emersonian spirit, if it is rightly understood, is common to them all. What has modern philosophy all over the world to say about that threefold claim concerning Nature, Freedom, and Oversoul? What has it to say when natural science has fully said its say and had its fair hearing, and has been approved as sound and welcome?

A philosopher might answer, perhaps, as follows: You Positivists have done wonderfully with your microscopes and your telescopes, with your chronoscopes and spectroscopes; you have measured and weighed and analyzed and described, and finally explained the whole world which you perceive, and there is nothing in space and time and causality which can escape your search. But did not all that work of yours involve certain presuppositions which you had accepted and which it was not your business to look on critically, but which, nevertheless, may be open to enquiry? Your first claims granted, all may follow; but how is it with the first claims? You examine all that is in space and time, but what are space and time? You examine the material substances and the contents of consciousness, but what is consciousness, and what is matter? You seek the special applications of causality, but what is causality? Well, you reply, you give the facts just as you find them; but do you do that really? And what do you mean by saying that you find the facts? Let us look, at least for a moment, at the very simplest facts with which your work begins. You say there are physical objects made up of atoms, and you describe them as a physicist; and there are mental ideas in consciousness made up of sensations, and you describe them as a psychologist; and both, you say, you are finding. But what does it mean, that you find the physical object outside there and the mental idea of the object inside in you; is that really a statement of your immediate experience? The physicist speaks of this table here before me, outside of me; and the psychologist speaks of my idea of this table, enclosed in my consciousness. Both may do well to speak so; but will you make me believe that I find that doubleness in my experience? If I see this table and want to use it, I am not aware of one table of wooden stuff and another in me of mental stuff. I am not aware of a two-ness at all, and if the physicist says that this wooden table is made up of molecules and has in itself no color and no continuity, and that the mental idea in me furnishes all those qualities of color and smoothness, but has no solidity, then they speak of two interesting worlds about which I am anxious to know, but certainly neither of them is the world I live in. If I lean on this table I am not aware of a table in my mind at all. I know the one table only, and this one table has its color and its smoothness.

I know what you will answer. You will say, in your immediate experience there are indeed not two worlds of objects, a physical and a psychical; the real thing to which our interests in life refer is not differentiated into a molecular object outside of us and a sensational object in us, but it is clear that every real thing allows a kind of double aspect; we can consider this table in so far as it is common to all of us, in so far as it is a possible object for every one of us, and in so far as it becomes an object for the individual, and we can then call the objects, in so far as they are common property, physical; and in so far as we take the aspect of individual relations, psychical; and as it must be of the highest importance for our practical purposes to discriminate between those two aspects, we have clearly the right to consider the world from the point of view of both the physicist and the psychologist. It is, of course, an abstraction if we leave out in the one case the one side, in the other case the other side of our objective experience; but we gain by that the possibility of constructing two closed causal systems of which each one must have its special conditions of existence, inasmuch as the one is conceived as related to individuals and the other as independent of individuals.

Very true, we should answer. Something like that saves you completely, justifies fully your claim to separate the physical and the psychical worlds of objects, the world of matter and the world of ideas; but can you deny that you have lost your case, are you not now yourself in the midst of philosophical, methodological discussions, which your physics and psychology themselves cannot settle, yet which must be settled before they can enter into their rights; and above all, do you not yourself see now that your whole physics, for instance, is not at all an account of reality, but merely a certain logical transformation of reality; that you do not find the world of physics at all, just as little as you find the psychical ideas, but that you can merely work over and reshape the reality which you find till you construct out of it your world of matter and your world of consciousness? What you believed you would find you have never found, while your construction of physical things may have been most necessary for your purposes; but do not deny that you have left reality far behind you.

And so it is with all your doings. You tell us proudly, for instance, that you show us the deepest nature of the world by showing us the elements which the object contains, and that you thus bring us at least nearer to the essence of things; and yet if we begin to look into your real achievements, we are disappointed again to find that you are far away from even attempting anything of the kind. You tell us that water is hydrogen and oxygen, and if we say "Prove it," you show us simply that you can transform the water into hydrogen and oxygen, and that you can transform these two elements again into water. Is that really what you promise? We want to know what the thing is, and you show us simply how the one thing can be transformed into another thing; and whenever we turn to your wisdom, it is always the same story. You show us always, and most nicely, how the one goes over into the other, but you never show us what the one or the other really is in itself. For your practical purposes the first may be the most important aspect, but do not make us believe, therefore, that it is the only possible aspect. In short, whether science describes or explains, it never gives us what we find in reality, but makes out of reality a new ideal construction in the service of certain purposes, and never gives us the things as they are, but merely the effects and changes which they produce. Are we still, then, to be deeply impressed with the claim of the naturalist that he alone has the monopoly of knowing reality, while we see now that every step of his leads us away from reality? And have we still to be afraid to raise the voice as philosophers with the claim that reality itself must find its expression, that there must be a science which shall give account of reality as we really find it, of nature before it is made up and repolished for the purposes of the physicist? Only if we have such other account of nature, then only do we speak of that nature in which we live and in which we act, and compared with such an account of the fuller reality, the constructed schematism of the physicist must appear, indeed, as Emerson said, "barren like a single sex." Not the slightest result of natural science is depreciated, not the slightest discovery ignored, if we insist that all these so-called facts have a meaning only under certain artificial conditions which set them apart from the reality of our life; and in this reality lies the interest of the philosopher. We have thus no reason to reproach the scientist so long as the scientist does not fancy that his science gives an account of nature as it really is. Both kinds of work are necessary, and the scientist may well speak, as the squirrel in Emerson's poem:

Natural science has to crack our nuts, but philosophy has to carry on its back the flourishing forests of life, in which we wander and breathe. And if Emerson is right, to-day and forever, in claiming that the facts of natural science are not expressions of reality, it is only a small step to see that he was not less right in saying that man is free. Consider man as a particle in the physical universe, consider his actions from the point of view of a causal science, and there is no possibility of escaping materialism and fatalism. We must understand every activity as a necessary outcome of foregoing conditions. Psychology must do so, and physics must do the same. The empirical sciences would be disloyal to their own principles if they allowed the slightest exception. The noblest gesture, the greatest word, the bravest action, must be considered by them under the category of causality. They are necessary effects of all the preceding causes. It may be interesting, it may be fascinating to follow such lines with the enthusiastic energy of scholarly research. But are we really obliged to accept the outcome as an ultimate word concerning the meaning of our freedom? "Forever wells up the impulse of choosing and acting in the soul." Is it really merely an illusion? Has responsibility still its moral value, are we the actors of our actions, are we still good, are we still guilty, when every deed follows as necessary effect? Is not, then, the whole constitution of the world, which has made us, responsible whenever we move our hand for good or for bad?

But we know now where we are standing; we know now that the world of objects, of psychical as well as of physical, is a constructed world, constructed for the purpose of satisfying our demand for causal connection; for that world holds causality because it is the world seen from the point of view of causality; and just as there cannot be anything in that world of physical and psychical objects which is not causally connected, just so it cannot have any meaning at all to ask for causal connection before the world is conceived in the service of this artificial construction. Reality in itself is not causal, and to ask for the causes of the real experience of our inner life has not more meaning than to ask how many pounds is the weight of a virtue, and how many inches is the length of our hopes. But we must go farther. To apply the question of cause and effect to our real will means not only that we apply to the real object a standard which belongs to the artificial or constructed object, but it means above all that we consider as an object something which in reality is not an object at all. The will which the psychologist describes and must describe, the will which has causes and which is thus not free, is a will conceived as an object found in our mind like an idea, something of which we are aware, something whose happening we perceive, and yet if anything is sure it is the immediate experience that we are aware of our will in a way which is absolutely different from the way in which we perceive objects. We do not perceive our will at all, we will it, we strive it, we fight it; yes, we feel ourselves, only in so far as we are the subjects of will. Our will is our personality, which we do not find but which we are, and which stands opposed and separated by the deepest gulf from the world of objects. Those objects are means and purposes of our will, are ends and aims and instruments; but they come in question for us only as we will them, as we like and dislike them, as we approve and reject them. And if we take this world of objects and reconstruct it into the artificial world of physical and psychical things connected by causality, in this very act of reconstruction we feel ourselves as willing, deciding, approving, aiming personalities, whose wills decide, who think the world as causally connected, whose freedom guarantees the value of our conception of a world not free. There is no knowledge but in our judgments; there is no judgment but in our affirming and denying; there is no affirming and denying but in our will. Our will chooses for its purposes to conceive reality as if it were unfree. What a climax of confusion to think that this conception of an unfree world, the conception of science, can itself now condemn the freedom of the will which has chosen. "Freedom is necessary," said Emerson. We can add, necessity itself is merely a purpose determined by freedom. "Intellect annuls fate," Emerson says. We may add, fate is merely an idea of intellect. Let us be psychologists if we want to analyze, to calculate, to explain the unfree man; but let us be philosophers to understand what it means to be a psychologist. Now the synthesis is reached; the real world is free, but we choose for our purposes to conceive the world as unfree, and thus to construct causal sciences.

And if we understand that in reality man is free and that the psychological aspect of man as unfree is a special way of looking on man for special purposes, then suddenly there opens itself before us the vast field of history, and the historical life, which seemed deprived of all interest by the psychological, iconoclastic mood, suddenly wins again a new importance. We feel instinctively that this free man of reality, this man who is a responsible actor of his actions, he only is the agent of history; and history is falsified and cheapened when it is brought down to a causal explanation of psychological man instead of real man. History had become an appendix of sociology, and what great historians aimed at in the interpretation of the few "stout and earnest personalities" seemed lost in favor of a construction in which the great man and the genius rank with the fool as mere extreme variations of psychological averages. Now suddenly do we understand that history has to deal with the world of freedom, that it has not to explain, but to interpret, that it has not to connect the facts by linking causes and effects, but by understanding the meaning of purposes, their agreement and disagreement, their growth and liberty. Now we understand why Fichte, why Carlyle, why Emerson believes in heroes and hero-worship, why Idealism has been at all times the fertile ground for writing history and for making history, while Naturalism has made technique, and thought in an anti-historical spirit. Our time begins again to think historically. It can do so because it again begins to emancipate itself from its positivistic disbelief in man's freedom and from its unphilosophic superstition that causal science alone is science, that we know only when we explain.

And when we at last stand man to man in full freedom, no longer as psycho-physical constructions but as free personalities, and when we debate and try to convince each other, will you deny that Jove stands behind each of us and Jove nods to Jove when we meet? Would it even have a meaning for us to go on with our talk, should we try at all to convince each other if you thought and I thought, each one for himself, that our will is only our personal will, that there is no over-individual will, no Oversoul behind us? Can we discuss at all if we do not presuppose that there is really a truth which we are seeking in common, that there are certain judgments which we are bound to will, which we are obliged to affirm, which we will, but not as individuals, and of which we take for granted that every one whom we acknowledge at all as a personality must will them too; and if you come with the flippant air of the sceptic and tell me, "No, there is no truth, all is only as it appears to me, there is no objective truth," do you not contradict yourself, are you not saying that at least this, your own statement, expresses objective truth; that you will this with a faith and belief that this will of yours is an over-individual will which is, as such, a duty, an obligation for every one who thinks? Every escape is futile. And all the over-individuality that lives in our will towards truth comes to us again in our will towards morality. Do not say sceptically that there is no absolute obligation, that you do not feel bound by an over-individual will in your action, that you will do in every moment what pleases you individually. You cannot even speak this sceptical word without contradicting yourself again, as you demand through the fact of your saying it that we believe that you speak the truth and that you thus feel yourself bound not to lie. If you leave us doubtful whether your word was not a lie, the word itself cannot have any meaning. Do not try to dodge the Oversoul. Men live and fight in its purposes, and men descend to meet. It is as Emerson said, "At first delighted with the triumph of the intellect, we are like hunters on the scent and soldiers who rush to battle; but when the game is run down, when the enemy lies cold in his blood at our feet, we are alarmed at our solitude." Let the sociologists triumphantly reduce the ideals to necessary social products of evolution in the same spirit in which the psychologist eliminates the freedom of the individual; but let us never forget that such a social mechanism is as much an artificial construction necessary for its purposes as is the psycho-physical mechanism of individuality. In that reality with which history deals, in which our freedom lies, there our over-individual will comes from deeper ground than from the soil and the food and the climate. Our logical obligations, our ethical duties, our æsthetic appreciations, our religious revelations, in reality they do not come from without, they come from within; but from within as far as we are souls in the Oversoul. There is no duty in the world but the duty which we will ourselves; no outer force, no training, no custom, no punishment can make us have duties. Duty is our will, it may be the duty to think for the ideal of truth, the duty to feel for the ideal of æsthetics, the duty to act for the idea of morality, the duty to have faith in the ideal of religion; but it is always our own will, and yet not our fanciful, personal, individual will. It is a system of purposes upon whose reality all knowledge of the world, and thus the world as we know it, is dependent forever. The wave of Idealism is rising. The short-sighted superstition of Positivism will not lurk under the roof of a new hall of philosophy. To be a true student of the most scientific, of the most scholarly, of the most insistent philosophy means to respect and to study the sciences, the physical and the psychical sciences, but at the same time to understand that natural science is not the science of reality, that psychology does not touch the freedom of man, that no life has a meaning without the relation to the Oversoul. We cannot write a whole system and a whole text-book on the front of the new building. It must be enough to write there a symbolic word; happy, forever happy, the university which can write over the door of its temple of philosophy the name: Ralph Waldo Emerson.

[At the opening of Emerson Hall, December 27, 1905, the American Psychological Association discussed the relation of psychology to philosophy; I opened the discussion with the following remarks:]

From the whole set of problems which cluster about psychology and its relation to neighboring sciences, this hour, in which Emerson Hall is completed, and this room, in which I hope to teach psychology to the end of my life, suggest to me most forcibly to-day the one question: Have I been right in housing psychology under this roof? I might have gone to the avenue yonder and might have begged for a psychological laboratory in the spacious quarters of the Agassiz Museum, to live there in peaceful company with the biologists; or I might have persuaded our benefactors to build for me a new wing of the physical laboratory. But I insisted that the experimental psychologists feel at home only where logic and ethics, metaphysics and epistemology keep house on the next floor.

I certainly do not mean that the psychologist ought to mix the records of his instruments with the demands of his speculations, and that he may seek help from the Absolute when the figures of the chronoscope or the curves of the kymograph are doubtful. Experimental psychology is certainly to-day and will be for all future an independent exact discipline with its own problems and methods. No one can insist more earnestly than I do on the demarcation line between the empirical study of mental phenomena and the logical enquiry into the values of life.

Yet I deny that it is a personal idiosyncrasy of mine to try to combine vivid interest in both. There is no antagonism between them; a man may love both his mother and his bride. I am devoted to philosophy, just as I love my native country; and I am devoted to psychology, just as I love the country in which I do my daily work; I feel sure there is no reason for any friction between them.

Of course, on the surface a psychological laboratory has much more likeness to the workshop of the physicist. But that has to do with externalities only. The psychologist and the physicist alike use subtle instruments, need dark rooms and sound-proof rooms, and are spun into a web of electric wires. And yet the physicist has never done anything else than to measure his objects, while I feel sure that no psychologist has ever measured a psychical state. Psychical states are not quantities, and every so-called measurement thereof refers merely to their physical accompaniments and conditions. The world of mental phenomena is a world of qualities, in which one is never a multiple of the other, and the deepest tendencies of physics and psychology are thus utterly divergent.

The complicated apparatus is therefore not an essential for the psychologist. Of course, we shall use every corner of our twenty-four laboratory-rooms upstairs, and every instrument in the new cases—and yet much of our most interesting work is done without any paraphernalia. Three of the doctor-dissertations which our psychological laboratory completed last year consisted of original research in which no instruments were involved; they dealt with memory-images, with associations, with æsthetic feeling, and so on. Yes, when, a short time ago, a Western university asked me how much it would cost to introduce a good practical training-course in experimental psychology, I replied that it would cost them the salary of a really good psychologist, and besides, perhaps, one dollar for cardboard, strings, rulers, colored paper, wire, and similar fancy articles at five cents apiece.

On the other hand, I do not know a psychological experiment which does not need a philosophical background to bring its results into sharp relief. Of course, you will say, the psychologist deals with facts, not with theories, and has to analyze and to describe and to explain those facts. Certainly he has to do all that; only he must not forget that the so-called "fact" in psychology is the product of complex transformations of reality. A will, an emotion, a memory-image, a feeling, an act of attention, of judgment, of decision—these are not found in the way in which stones and stars are noticed. Even if I choose perceptions or sensations as material for my psychological study, and still more when I call them my perceptions and my sensations, I mean something which I have found at the end of a long logical road, not at its starting-point, and that road certainly leads through philosophy. Emerson said wisely, "A philosopher must be more than a philosopher;" we can add: A psychologist must be more than a psychologist. First of all, he must be a philosopher.

What would be the result if our laboratory had moved to the naturalistic headquarters? It would be the beginning of a complete separation from philosophy. Our graduate students would flock to psychological research work without even being aware that without philosophical training they are mere dilettantes. And soon enough a merely psychological doctorate would be demanded. I do not deny at all that such pure psychologists would find enough to do; I should doubt only whether they know what they are doing. There are too many psychologists already who go their way so undisturbed only because they walk like somnambulists on the edge of the roof; they do not even see the real problem; they do not see the depths to which they may fall.

But does the laboratory itself gain from such divorce? Just the contrary. It is evident that everywhere in the world where the psychological laboratory turns to natural science, the experiments deal mostly with sensation, perception, and reaction; while those laboratories which keep their friendship with epistemology emphasize the higher mental functions, experimenting on attention, memory, association, feeling, emotion, thought, and so on. But is it not clear that only the latter work gives to the psychological laboratory a real right to existence, as the former is almost completely overlapped by the well-established interests of the physiologists? If psychology cannot do anything else than that which physiologists like Helmholtz, Hering, Kries, Mach, Bowditch, and the rest have always done so successfully, then experimental psychology had better give up the trade and leave the study of the mind to the students of the organism.

I have said that we ought not to depend on authorities here. Yet one name, I think, ought to be mentioned gratefully in this hour in which the new psychological laboratory is opened for work. I think of Professor Wundt of Leipzig. The directors of the psychological laboratories in Columbia, and Yale, in Clark and Chicago, in Pennsylvania and Cornell, in Johns Hopkins and Washington, in Leland Stanford and Harvard, and many more are his pupils. Some weeks ago, when I did not foresee our present discussion, I told him of Emerson Hall; and a few days ago I got an answer from which, as my closing word, I may quote in translation. Professor Wundt writes to me: "I am especially glad that you affiliated your new psychological laboratory to philosophy, and that you did not migrate to the naturalists. There seems to be here and there a tendency to such migration, yet I believe that psychology not only now, but for all time, belongs to philosophy: only then can psychology keep its necessary independence." Mr. Chairman, these are the words of the father of experimental psychology. I hope they indicate the policy to which Harvard University will adhere forever.

A monumental staircase leads from the first—the lecture-room—floor of Emerson Hall to the second, the library floor; at the two ends of its broad corridor smaller staircases lead to the third floor, the laboratory. Its general division of space is seen at a glance from the sketch of the ground plan (opposite page 1). Eighteen rooms of various sizes with outside windows form a circle around the central hall which is well lighted by large skylights; but at each end of the hall itself two large windowless spaces are cut off and each of these is divided into three dark rooms. We have thus twenty-four rooms, besides coat-room, toilet-rooms, etc. A further stair leads to the wide attic which is mainly a store-room for the institution.

In order that the laboratory should be adaptable to the most diverse purposes, the permanent differentiation of the rooms has been kept in narrow limits. It seemed unwise to give from the first every room to a special line of research, as the preponderance of special interests may frequently shift; there are years when perhaps studies in physiological and comparative psychology make the largest demand and others in which studies in æsthetical and educational psychology stand in the foreground. A thorough-going specialization, by which special rooms are reserved for tactual studies and others for chronoscope work or for kymograph researches, allows of course certain conveniences in the fixed arrangement of instruments and a certain elaboration of equipment that is built in, but it very much impairs the flexibility of the whole laboratory, and has thus not seemed advisable for an institution whose catholic attitude welcomes investigations as different as those contained in this volume.

To be sure certain constant requirements have demanded a special fitting up of one room as a workshop, one room for the more delicate instruments, one for the beginning course in experimental work, a lecture-room for the courses in comparative psychology, a photography-room, a battery-room, a sound-proof room, the chief animal rooms, and the dark rooms. We have seven light-proof rooms, finished in black, of which two have outside windows for heliostats; of the others, four can be used for optical research; the longest one contains the photometer. Six other rooms, including the lecture-room, may be darkened by opaque blinds. One contains a partition with door and a grooved window-frame fitted with screens in which openings of any desired size and shape may be cut. This window is opposite the main door of the room, and opposite this, across the central hall, some sixty feet away, is the door of another dark room; optical stimuli can thus be given from this window to a subject over seventy feet away.

Several rooms are fitted up with special reference to the investigation of the various forms of organic movement, animal behavior and intelligence. As one result of several investigations in animal psychology already pursued here, the laboratory has a considerable number of devices for testing and making statistical studies of the senses and intelligence, methods of learning and emotional reactions of animals.

Adequate provision is made for the keeping of animals in a large, well-lighted, and well-ventilated corner room. Instead of having aquaria built into the room, an aquarium-table eighteen feet long has been constructed to support moveable aquaria of various sizes. Whenever it is desirable for the purposes of an investigation, any of these aquaria may be moved to the research-room of the investigator or to such quarters as the special conditions of the experiment demand.

The vivarium-room contains, in addition to provisions for water-inhabiting animals, cages of a variety of forms and sizes. The largest of these cages, six and a half feet high, six feet wide, and four feet deep, may be used for birds, monkeys, or any of the medium-sized mammals. Cages for rabbits, guinea-pigs, and other small animals are arranged in frames which support four double compartments. Similarly, small cages suitable for mice, rats, and other small rodents are in supporting frames which carry four of the double cages, each of which is removeable and may be carried to the experimenting-room at the convenience of the experimenter.

In a large unheated room above the main laboratory are tanks for amphibians and reptiles. These tanks, since they can be kept at a low temperature during the winter, are very convenient and useful for frogs, tortoises, and similar hibernating animals.

In view of the prime importance of electricity to a modern psychological laboratory, a rather elaborate system of wiring has been designed and built in. The unit of this system is a small delivery-board six inches wide by eight inches high, which carries the following five circuits: a, a time-circuit for running magnetic signals; bb, two low-tension circuits for chronoscope, bells, signals, etc.; c, a high-tension alternating current (110 v. and 60 phases) for alt. current motors, to be used where great constancy of speed is desired; d, a high-tension direct current (110 v.) for dir. current motors, where it is desired to vary the speed continuously (by the introduction of resistance). Two such delivery-boards have been set on opposite walls of all except the smallest rooms, which have but one board. Circuits a and b are represented on the board by binding-posts, while the high-tension currents, c and d, appear as flush, protected sockets that take a double-pole plug.

Circuit a is a single circuit led from a time-pendulum permanently set in the battery-room, and carried once around the laboratory. It is connected with the a binding-posts of the individual delivery-boards in parallel. It follows that the time-circuit is alike for all the rooms at any one time; but in different hours the pendulum can be adjusted to give various impulse-rates. If an investigation requires some special rate of impulse, the special time-apparatus is set up in the investigator's room and current for it taken from one of the b pairs of posts.

Each b pair goes directly from the delivery-board to the battery-room and ends at a double-pole (telephone type) socket on a large switch-board. Thus every room has two or four direct and independent connections with the battery-room.

The c and d circuits do not come from the battery-room, but from their respective generators that are stationed outside of the building. They are of course connected at the delivery-boards in parallel.

The large switch-board in the battery-room consists of an upper and a lower part. The upper part bears the double-pole sockets from the b posts in all the rooms; the lower part carries some fifty pairs of single-pole sockets that are connected with the batteries stationed near by. These pairs are labelled, and some give a current from cells of the Leclanché type, others of a gravity type. The student has merely to choose the kind and number of cells that he needs, from the lower part, and connect them with one of the double-pole sockets of the upper part which runs to a b pair in his own room. By connecting two double-pole sockets with each other, the student can establish a circuit between any two rooms of the laboratory,—this for purposes of telephonic or other communication. Since every room has two, and most of the rooms have four of the b circuits, the greatest variety and elasticity of service is attained.

The large switch-board further carries a voltmetre and an ammetre, both of the Weston make, which are reached (electrically) from double-pole jacks (sockets) on the upper part of the board. Thus before connecting the current with his room, the student can in a moment measure its amount and intensity. These instruments are of the flushface type, and dead-beat.

All of the rooms are lighted by electricity, and the lighting system is independent of the delivery-boards. Nine of the rooms are provided with soapstone sinks, and six (not including the lavatories and service-room) with enamelled iron or porcelain sinks. All the sinks have two taps and each of these ends with a screw-thread so as to take a tip and rubber hose. The soapstone sinks were specially designed with soapstone drip-boards. This is probably the best material for a research-room, and the porcelain and enamel sinks were put only where a neater appearance was desired, or where chemicals were to be frequently used—as for instance in the battery and photographic rooms. Gas is not used for illumination, but six rooms are provided with jets for the smoking of drums, soldering, brazing, etc.

The instrument-room is equipped with large dust-proof cases for holding the more delicate and valuable instruments. The larger unused pieces are stored, out of sight but readily accessible, in an attic which has a clear floor-space of something more than half the total area of the laboratory. Dust-proof cases for demonstration and class-work material are provided in the lecture- and class-rooms.

The shop contains a wood-working bench with two vices, tool-racks, shelves, drawers, cupboards, and stock-racks, for the use of students; and a 9-in. lathe, circular saw, grinding- and buffing-machine, separate bench, vice, racks, and drawers for the use of the mechanic. The machinery is run by a 5 h.p. electric motor suspended from one of the outside brick walls, on brackets. One who selects the equipment of such a shop has to weigh carefully the respective merits of circular and band saws; the latter undoubtedly lends itself to a greater variety of uses, but it is also a far more dangerous machine to have running in a room to which students are to be given access. This latter consideration determined in the present case the choice of a circular saw. It is quite dangerous enough, and may be used only by, or under the supervision of, the mechanic.

It has been stated on competent authority that a truly sound-proof room cannot be built except under ground. This has not been attempted, but the laboratory contains one room (no. 17) which is virtually sound-proof. A double door separates it from the adjoining experimenter's room, and double doors also separate this from the main hall. The wall between these two rooms consists of two layers of plaster with special deadening material inserted between. Two small tubes, ordinarily stuffed with felt, connect these rooms. When the acoustical stimulus is a tuning-fork, it is placed in a distant room, connected with one of the b circuits of the sound-proof room, and then with a telephone receiver near the subject's ear.

The photographic-room contains the ordinary sink, red lights, shelves, etc. The indirect entrance is light-tight when the door is not closed, so that the experimenter may pass in and out even when developing is going on. This room, like all the others which have no window (except the sound-proof room), has forced ventilation.

The class-room is designed for the experimental training-courses. It has eight of the regular delivery-boards, ten tables, instrument-case, blackboard, and sink.

The lecture-room for specialized courses in comparative and experimental psychology seats eighty students. It is provided with two Bausch and Lomb electric projection-lanterns, horizontal and vertical microscope attachments, and attachment for the projection of opaque objects. On the lecturer's platform, besides the blackboard, projection-screen, and chart-racks (capable of holding twenty charts), is a large demonstration-table provided with a delivery-board, water, gas, sixteen chart-drawers, two other drawers, and three cupboards.

As has been said before, the general psychology course of the University is not given on the laboratory floor, but downstairs in the large lecture-hall with about 400 seats. A number of large demonstration instruments of the laboratory serve the special purpose of this course; this hall too has its own stereopticons.

Our instrumentarium is, of course, in first line, the collection of apparatus bought and constructed through the fourteen years of work. Yet with the new expansion of the institute a considerable number of psychological, physical, and physiological well-tested instruments has been added. Especially in the departments of kymographic, chronoscopic, and optical apparatus the equipment presents a satisfactory completeness; its total value may be estimated to represent about twelve thousand dollars. Yet the place of the laboratory which we appreciate most highly is not the instrument-room but the workshop, in which every new experimental idea can find at once its technical shape and form. Whether those experimental ideas will be original and productive, whether their elaboration will be helpful for the progress of our young science, in short, whether the work in the new laboratory will fulfil the hopes with which we entered it, may be better decided as soon as a few further volumes of the Harvard Psychological Studies shall have followed the present one, which is still from cover to cover a product of Harvard's pre-Emerson-Hall period.

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BY G. V. HAMILTON

The question which the Laboratory proposed to me for experimental enquiry was one which demanded a definite reply of yes or no. The positive answer seemed a necessary consequence of the traditional psycho-physiological theories, while a certain practical consideration seemed to suggest the negative solution. The question which seems to have been overlooked so far was this: According to the theory of stereoscopic vision two points of light which are seen by each of the two eyes under the same angle appear to lie in the same plane; as soon as the angle for the right eye is larger than that for the left, that is, as soon as the two stimulated retinal points in the right eye are more distant than the two retinal points stimulated in the left eye, the right light-point seems to be farther away than the left one. If we relate them to planes vertical on the ideal binocular fixation-line, the right point lies in a more distant plane. This principle, which, of course, controls all arrangements for stereoscopic effect, is deduced from experiences in which the fixation-line is vertical to the line that connects the nodal points of the two eyes; the plane in which the equally distant points lie is then parallel to the forehead. If, on the other hand, the eyes are turned to the side, that is, if the ideal fixation-line forms an acute angle with the line connecting the eyeballs, the two fixated light-points, which lie in a plane perpendicular to the fixation-line, cannot be seen by the two eyes under the same angle. Any object on my right side is somewhat nearer to my right eye than to my left, and therefore must throw a larger image on my right retina. The two light-points of a plane vertical to the fixation-line give thus with the eyes turned to the right two unequal pairs of retinal stimuli; and the difference of the retinal stimulations is evidently just the same as if the eyes were looking straight forward but the two lights were at different distances. If difference of retinal images really produces the conscious experience of seeing the lights in differently distant planes, vertical to the fixation-line, it follows that with the eyes turned to the right, lights which objectively lie in the same plane must appear subjectively to lie in different distances. The question arises whether the facts correspond to this conclusion. If we look with eyes turned sidewise towards a plane vertical to the direction of seeing, do the points of that plane remain in it for consciousness or do we see them in different planes? We see that practical considerations suggest a "No" to this question, because it would mean that everything which does not lie exactly in front of us must change its plastic form, and this the more strongly the more we see it on our right or our left, and this of course again the more strongly the nearer it is to the eyes, inasmuch as the relative difference of the retinal images must increase with the nearness of the object. If a short-sighted person fixates an object a few centimetres from the eyes strongly turned to the side, the distances in the retinal image of the one eye may be almost the double of those in the other. Under normal conditions the differences would be smaller, but yet everything would be necessarily distorted in its three-dimension shape as soon as it is seen in indirect vision or with sidewise fixation. On the other hand, if the objects keep their three-dimensional relations in spite of sidewise movements, it is evident that the accepted psycho-physiological theory of stereoscopic vision is incomplete and must be revised in a very essential way. The experiment had to decide. Of course the question might be approached experimentally in different ways. It would have been possible, for instance, to study the stereoscopic synthesis of two separate flat pictures seen with the eyeballs in different positions. But we preferred the simplest possible way, seeking the threshold of distance for two parallel vertical edges with eyes turned forward and to the side. We chose edges instead of hanging threads for the purpose of avoiding the possible influence of the apparent thickness of the threads on the judgment of distance. Of course, distance is here never distance from the one or the other eye, but from the centre of the line which connects the two nodal points of the eyes; the two vertical planes whose edges were to be compared stood always vertical on the ideal line of fixation which starts from that central point between the two eyeballs.

The apparatus used in these experiments consists of three parts, viz.:

(1) A plank 2.5 metres x 9.5 centimetres x 4 centimetres, set on edge and screwed to a table at either end.

(2) A head-rest 45 centimetres high, 35 centimetres broad and 15 centimetres deep. Attached to the centre of the lower strip of the frame is a concave trough for the chin. Another trough, shaped to fit over the vertex and with a strip of wood fastened to the front of it for the forehead, slides up or down within the frame. The attachment for the forehead can be moved and fixed at various positions antero-posteriorly. By means of these devices the head can be securely fixed in any position desired without discomfort to the subject.

In order to have the eyes always in the same plane and at a known distance from the apparatus at the other end of the plank, a hole was made in either side of the frame with its centre at a level of the eyes. Extending through the vertical diameter of each hole is a fine wire. Fitted into the inner portion of each hole is a cardboard tube 10 centimetres long: the inner end of each tube contains a vertical wire so arranged that the four wires all fall into a plane at right angles to the long direction of the plank. A mirror at the outer exit of either hole enables the experimenter to align the tips of the subject's corneæ with the wires.

Two parallel strips of wood are so attached to the "head-rest" end of the plank—one below and the other above it—that they can be rotated laterally upon the plank, with the bolt which secures them to it for a centre of rotation. Opposite this centre, and attached to the anterior surface of the upper parallel strip is a wire needle 25 centimetres long. By means of a quadricircular piece of cardboard attached to the plank at the end of the needle, the extent of rotation to the right or left can be read off in degrees. (The point midway between the two corneal tips when they are aligned with the wires is in the same axis of rotation as the head-rest.)

A vertical iron rod 50 centimetres long extends upwards from either end of the parallel strips, and upon these rods the frame of the head-rest can be moved up or down by means of thumb-screws upon which it rests.

(3) At the opposite end of the plank there is attached a flat board, 35 centimetres long and 30 centimetres wide. Attached to the edge of the board which faces the head-rest is a piece of black cardboard 40 centimetres long by 35 centimetres broad. In the centre of the cardboard is a rectangular aperture, 7 centimetres by 14 centimetres. On the upper surface of the board are two slots, one at either side. Sliding within each of these slots is a block of wood to which is attached an upright sheet of black-painted tin, 15 centimetres wide and 20 centimetres high. The surfaces of these tins lie in planes parallel to the plane of the four wires in the head-rest, when the latter is at right angles to the plank. When their surfaces are equidistant from the wires, the inner vertical edges of the tins are separated from each other by 3 centimetres. The sides of the slots, in which the blocks with their tins slide, are fitted with millimetre scales, thus enabling the experimenter to determine the distance of the edges from the corneæ. The point on the scale at which an edge was exactly 2 metres from the vertical plane of the wires was chosen as the "zero" point, and if this distance was decreased by moving an edge forward, the latter was said to stand at "minus" one, two, or more millimetres, as the case might be. Likewise, an edge was said to stand at "plus" the number of millimetres' distance beyond the zero point if it had been moved at a greater distance than 2 metres from the wires. A piece of ground glass attached to the distal end of apparatus enabled the experimenter to secure a uniform illumination, the room being darkened and the light coming from a 32-candle-power electric lamp set about a metre and a half behind and on a slightly lower level than the glass.

It was found that by shading the lamp itself and admitting a dim light to the room by means of drawing down only the ordinary thin window-shades, the edges could be made to seem almost isolated in space and to stand out in clear relief.

The subjects of the experiment were Messrs. Bell, Flexner, and Tait. Each subject determined the equality-point and the threshold for the normal primary position of the eyes, for the eyes in a lateral position of 15° and in a lateral position of 30°, both to the left and to the right.

Eyes at 0° means the following: that the most anterior part of the two corneæ lies in a plane parallel to and two metres' distance from the plane in which the two parallel edges lie at 0. Eyes at 30° to the left means that a line drawn in front of the two corneæ intersects such a line at an angle of 30°, the left eye being at the distal end of the line. In calculating the visual angles 7.4 mm. are added in order to compensate for the distance from the extreme anterior portion of the cornea to the nodal point of the eye.

The results for Mr. Tait are as follows:

The position of eyes 0°. The right edge was moved, at first from an evident + position to equality, then from equality to the - threshold, then from an evident - position to equality, then from equality to the + threshold. These four points were determined each fifteen times and the average taken. Then exactly the same fifteen sets of four determinations with the left edge moved. The averages of these 120 experiments are these: When the left edge is moved from + to =:-2.77, from = to -:-6.97, from - to =:+0.77, from = to +5.93. When the right edge is moved from + to =: +2.83, from = to -:-1.6, from - to =:+5.9, from = to +:+10.53. The first equality-point appears thus when the left edge is moved at -0.76, when the right edge is moved at +4.41, with a threshold of about 5 in either case. With the normal eye-position the edges must thus not be exactly in the same plane to appear equally distant; at a distance of 2000 mm. the left must be about 2 mm. nearer than the right to appear in the same plane, vertical to the line of regard.

If the position of the eyes is 15° to the left, we have the following results: When the left edge is moved from + to =:-4.17, from = to -:-8.5, from - to =:-1.33, from = to +:+1; when the right edge is moved from + to =:+4.17, from = to -:+1.17, from - to =:+4.5, from = to +:+8.67.

If the position of the eyes is 30° to the left, we find when the left edge is moved from + to =:-2.67, from = to -:-6.67, from - to =:+0.5, from = to +:+3.33. When the right edge is moved from + to =:+2.33, from = to -:-0.02, from - to =:+9., from = to +:+12.33.

If we take again the general averages, we have for the eye-position of 15° to the left an equality-point of -3.25 if the left edge is moved and judged and +4.63 if the right edge is moved and judged. That is, if the right edge stands at 2000 mm. the left edge must be moved to 1996.75, and if the left stands at 2000, the right must be moved to 2004.63. For the eye-position of 30° to the left, the equality-point lies at -1.49 if the left edge is moved and judged, and at +5.91 if the right edge is the variable. The threshold lies in all three cases, for eyes at 0°, at 15°, and at 30°, at about ±5 mm.; the position of the eyes has thus no influence on the threshold for the perception of distance in the direction of regard.

But the point essential for our investigation is of course not the threshold but the equality-point. To take the extremes of the eye-positions 0° and 30° we find the equality when the left edge is judged, at -0.76 for 0° and -1.49 for 30°, and when the right edge is moved, at +4.41 at 0° and +5.91 at 30°; the middle is thus +1.82 for 0° and +2.21 for 30°, that is a difference of less than 0.4 mm.

To understand this figure we must enter into the calculation of the angles. We have an eye-distance of 60 mm., a distance of the edges from the cornea 2000 mm., from the nodal points 2007.4 mm., the distance of each edge from the median line 15 mm., the distance of the two edges from each other thus 30 mm. as long as they are in the same plane. We have to determine the angle under which each eye sees the distance of the two edges. A simple trigonometric calculation gives the following figures: If both eyes are in normal position, at 0°, and both edges are in the same plane, 2000 mm. from the corneæ, the angle for each eye is 51' 22". If the left edge is now moved to +5, the left eye sees the distance of the edges at an angle of 51' 25", the right eye under 51' 10", the difference is thus 15"; if the left edge is at +10 mm., the left eye's angle is 51' 29", the right eye's angle 50' 59", the difference 30". If the left edge is moved to -5 mm., the left eye's angle is 51' 18", the right eye's angle 51' 33", the difference 15"; if the left edge is moved to -10 mm., the left eye's angle is 51' 14", the right eye's angle 51' 45", the difference 31". Now we saw that with normal eye-position when the left edge was moved the threshold was +5.93 and -6.97; a difference of 15" to 20" between the visual angles of the two eyes was thus amply sufficient to give a distinct experience of different distance. When the left eye's angle was about 15" smaller than the angle of the right eye, the difference of the retinal images gave a sure impression of the greater nearness of the left edge.

If we now bring the eyes into the position of 30°, the angles are of course different when both edges are in the same plane vertical to the direction of regard. If the two edges are in the same plane, the left eye's angle is 50' 59" and the right eye's angle 51' 45", the difference thus 46". If we move the left edge to +5, the left angle becomes 51' 1", the right angle 51' 34", the difference 33". If we move the left to +10, the left angle becomes 51' 4", the right 51' 24"; the difference is thus still 20", and we must move the left edge to +17 mm. to get an equal angle for the left and the right eye. If we move the left to -5, the difference becomes of course larger, the left eye sees under 50' 56", the right eye 51' 55", the difference 59"; and at -10, the left eye has the angle 50' 53", the right eye 52' 6", difference 1' 13". It is hardly necessary to state here the angles for the changes of the right edge or for an eye-position of 15°, inasmuch as the maximum differences bring out our case most clearly. With an eye-position of 15°, the edges at the same plane give angles of 51' 10" and 51'34", that is, a difference of 24"; if the left edge is moved to -5 mm. the difference becomes 38"; if it is moved to -10 mm. the difference is 54"; if the left edge is moved to +5 the difference decreases to 10" and at +10 mm. to 6".

We have thus the following fundamental result: If the eyes are in normal primary position, a movement of the left edge to ±6 mm. is constantly apperceived at threshold of distance and this corresponds to retinal images whose visual angles differ by about 17". A difference of 17" in the visual angles of the two eyes produces thus under the conditions of this experiment for this subject a strong stereoscopic effect when the eyes are in primary position. If the eyes are in the position of the head 30° to the left, the left eye thus much further from the edges than the right eye, the visual angle of the left image thus much smaller than that of the right image, we find the same equality-point with the same threshold. We saw that in this position the two visual angles would be equal if the left edge were moved to +17 mm.; instead of at +17, the equality-point—when the left edge is judged—lies at -1.49, that is, at a point at which the visual angle of the left eye is more than 46" smaller than the angle of the right eye. While in normal position a difference of the two retinal images of 17" constitutes a distinct threshold value; at a lateral position of the eyes of 30° the great difference of 46" becomes necessary to give the impression of equal plane, while a decrease of that difference to 30" gives a distinct feeling of greater distance. Equal retinal images produce for the lateral eyes thus the same effect which for the normal position very different images produce; and to get for the lateral eyes the effect which equal images produce for the normal position, the angles of the images must differ by 46".

The results for the second subject, Mr. Flexner, are practically the same. With the position of the eyes at 0°, when the left edge is judged and moved, we find the following averages: from + to =: +0.03, from = to -:-3.8, from - to =:-0.7, from = to +:+3.93; when the right edge is moved from + to =:-0.08, from = to -:-4.29, from - to =:+1.21, from = to +:+4.08. It is evident that the difference between right and left which existed for Mr. Tait does not enter into Mr. Flexner's results. The equality-point as average of 120 experiments lies for normal eye-position practically at zero, and the threshold is ±4 mm.; his sensibility for differences of retinal images is thus still finer than for Mr. Tait, as we saw that the threshold of ±4 mm. means a difference of visual angles of less than 15". If Mr. Flexner's head is turned 15° to the left, his left eye thus considerably farther away from the edges than the right eye, the results are these: If the left edge is moved and judged, we find from + to =:-0.02, from = to -:-3.17, from - to =:0, from = to +:+4.67; if the right edge is moved from + to =:-0.01, from = to -:-2.5, from - to =:-0.8, from = to +:+3.33. Experiments with lateral movement of 30° were not carried through, as the subject, accustomed to eye-glasses, became less accurate in the judgments; but the experiments with the position of 0° and of 15° are unequivocal. They show that the equality-point and the thresholds are exactly the same for 15´ as for 0°. For the lateral position of 15° again the average equality-point is exactly at 0° and the threshold at less than ±4 mm. We saw that for a lateral movement of 15° the difference of the angles at the equality-point is 24". We find thus for Mr. Flexner that with primary eye-position a difference of angles of less than 15" gives a distinct stereoscopic effect, while with a lateral position of the eyes a plane effect demands a difference of 24" for the two visual angles.

Experiments with Dr. Bell finally showed a rather strong fluctuation of judgments and the determination of the equality-point for normal eye-position has not only too large a middle variation to be a reliable basis, but is influenced by a constant tendency to underestimate the distance of the edge moved. Yet the general result is the same as with the other two subjects, that is, the equality-point is with him, too, practically the same for the eyes in normal and in lateral position.

The general conclusion from the results of all three subjects is thus evidently that the traditional physiological theory is untenable, the stereoscopic effect cannot be simply a function of the difference of the two retinal images. The same pair of unequal retinal images which gives a most striking stereoscopic effect for eyes in primary position, has no stereoscopic effect for eyes in lateral position and vice versa. The stereoscopic interpretation is thus the function of both the difference of the retinal images and the position of the eyeballs. Of course the two retinal images are in any case never felt as two pictures if they are not different enough to produce a double image. With the primary position of the eyes as long as the two different retinal views are sufficiently similar to allow a synthesis in a three-dimensional impression of our object, we perceive every point of the object not as double image but as one point of a given distance. The distance feeling of the normal stereoscopic vision demands thus itself more than the reference to the different retinal images, and the only factor which can explain the phenomena is the response of the eye-muscles which react on the double images by increase or decrease of convergence. The distance of a point in a stereoscopic image is determined by the impulse necessary for that particular act of convergence of the eyeballs by which the two retinal images on non-cor-responding points would be changed into images on corresponding points. The different retinal images are thus ever for the normal eye-position merely the stimuli for the production of that process which really determines the experience of distance, that is, the motor impulse to a change in convergence.

If thus the stereoscopic vision under normal conditions is ultimately dependent upon the central motor impulses, it is not surprising that a change in the psycho-physical conditions of movement produces a change in the resulting impulses. Such a change in the conditions is given indeed whenever the eyes are in a lateral position. Just as the same stimulus produces a different response when the arm or leg is in a flexed or an extended position, so the retinal double images stimulate different responses according to the particular position of the eyeballs. That pair of unequal retinal images that in primary eye-position produce in going from one end of the object to the other a strong increase of convergence and thus a feeling of greater nearness, may produce with the lateral eye-position no increase of convergence and thus a feeling of equal distance or even a decrease of convergence and thus a feeling of removal. The psycho-physical system upon which our three-dimensional visual perception depends is then much more complex than the usual theory teaches; it is not the retinal image of the double eye, but this image together with the whole distribution of contractions in the eye-muscles, which determines the stereoscopic vision: the same retinal images may give very different plastic perceptions for different positions of the eyeballs.

The experiments point thus to the same complex connection which Professor Münsterberg emphasized in his studies of the "Perception of distance."[1] I may quote the closing part of his article to bring out the intimate connection of the two problems. He reports his observations on the so-called verant and insists that the monocular verant almost as little as the ordinary binocular stereoscope can give the impression of normal distance of nature. Professor Münsterberg writes: "Whoever is able to separate seeing in three dimensions from seeing in natural distance cannot doubt that in both cases alike we reach the first end, the plastic interpretation, but are just as far removed from the other, the feeling of natural distance, as in the ordinary vision of pictures. The new instrument is thus in no way a real 'verant.'

"The question arises, Why is that so? If I bring my landscape picture on a transparent glass plate into such a distance from my one eye that every point of this transparent photograph covers for my resting eye exactly the corresponding point of the real landscape and yet accommodation is excluded, as, for instance, in the case of the short-sighted eye, or in the case of the normal eye with the verant lenses, then we have exactly the retinal images of the real view of nature and the same repose of the lens. Why are we, nevertheless, absolutely unable to substitute the near object for the far one? This problem exists in spite of all the theoretical assurances that the one ought to appear exactly like the other, and I think that it is not impossible to furnish an answer to it.

"If I am not mistaken, there is one point of difference between seeing the mere picture and seeing the far landscape, which has been neglected in the usual discussions. Every one knows, of course, that we see the picture and the landscape normally with the help of eye-movements. The eye moves from point to point; but psychologists have neglected the consideration that the relation between eye-movement and retinal image must be quite a different one for the landscape and for its photograph. Let us consider the simplest possible case, the case of the myopic eye without any lenses whatever, and without any need of accommodation for a picture as near to the eye as 10 cm. If I take a small landscape picture made with a camera whose distance from lens to plate is 10 cm., I have a splendid plastic view if I see it at a distance of about 10 cm. from my eye. I have before me just such a picture in which two mountain peaks are, in the photograph, 1 cm. distant from each other. If I now have my little picture at the distance of 10 cm. from the eye, these two mountain tops correspond in their distance of 1 cm. exactly to the retinal image which the two real mountains, which are ten miles away and one mile distant from each other, produce in my retina. The retinal image of the two mountain peaks in the photograph is thus for my resting eye indeed identical with that of real nature. Does that mean that I have to make the same eye-movement to go from the left to the right mountain in the landscape as in the picture? Of course, that would be so, the movement would be just as identical as the retinal images if the nodal point of the light-rays were identical with the rotation-point of the eyeball. But everybody knows that this is not at all the case. The light-rays cross in the lens. The angle of vision, and thus the size of the retinal image, are thus dependent upon the distance of the lens from the retina. But the movement of the eye is related to a rotation-point which lies about 13 mm. behind the cornea, roughly speaking 1 cm. behind the nodal point of the rays. This additional centimetre plays, of course, no rôle whatever, if I look at my mountains in the real landscape; following with my eyeball from the fixation-point of the left mountain to the fixation-point of the right mountain, I make a movement whose angle can be declared identical with the angle under which I saw the two mountains with the resting eye in the first position. This angle of vision was determined by the distance of the nodal point, which was in our case ten miles, while the angle of eye-movement was determined by the distance of the rotation-point, which would be ten miles plus one centimetre, and there is of course no possible difference for practical discrimination between these two distances.

"But the situation is completely changed if I turn to my little picture 10 cm. distant from my eye. The angle under which I see my two peaks is, of course, again the same under which I saw them in the real landscape. It is determined by the distance of the picture from the nodal point, which is in this case 10 cm. But the angle of the eye-movement necessary to fixate first the left and then the right peak is now a much smaller one because it is again determined by the distance from the rotation-point, and that is in this case 10 cm. plus 1 cm. With this short distance of the picture from the eye this one additional centimetre is not at all the negligible quantity which it was in addition to ten miles in the landscape. For the two real mountains the angle of the eye-movement had a tangent of one tenth; for the photograph mountains, in spite of their equal size of retinal image, the angle of necessary movement would of course have a tangent of one eleventh. Roughly speaking, we could say that the photograph, in order to produce the same eye-movement which the mountains in the landscape excited, would need a pictorial distance between the two photograph mountains of 11 mm. instead of 10 mm. Of course if the distance in the picture were made 11 mm. instead of 10, it would not cover any more the mountains of the landscape. The retinal image would thus be relatively too large and would not give us any longer the true landscape. On the other hand, if we tried to correct it by bringing the picture one centimetre nearer to the eye, then of course every retinal image would be enlarged by that necessary tenth, and yet there would be no help for the situation, as now again the eye-movement demanded by the retinal image would be relatively increased too.

"We can put it in this way: my real landscape demands a relation between retinal image and movement which my picture cannot produce under any circumstances whatever. That which would be needed to imitate the relations would be realized only if I had my retinal images from the picture at a distance of 10 cm., and at the same time the movements belonging to the same picture seen at a distance of 9 cm. That is of course unrealizable. We cannot see a picture without having our movements constantly controlled by the size of the real retinal images, as it is necessary that the distance seen in indirect vision is the distance covered by the fixation-point during the eye-movement. That demands, as we have seen, a different relation between retinal image and eye-movement for near and far, and no verant and no stereoscope can eliminate this factor. If a 10-mm. object in the photograph demands an 11-mm. movement to give the impression of real natural distance, then we have a condition which cannot be fulfilled.

"If we remember how extremely delicate is our normal sensitiveness for retinal distances and how the newer studies in stereoscopic vision have demonstrated an unsuspected delicacy of adjustment between retinal images and motor responses, it is evident that this so far always neglected relation must be an extremely important one. If we have one adjustment of central reaction in which a certain eye-movement corresponds to retinal images of one size, and another adjustment in which the same movements correspond to retinal images which are ten per cent larger, we can really not expect our judgment of distance to neglect the difference between these two systems of relations. Of course they represent two extreme cases. Every distance beyond 10 cm. demands its special adjustment up to the point where the distance becomes too large to be influenced by the distance from the nodal point to the rotation-point. We must thus presuppose a sliding scale of ever new adjustments for the different distances at which we see any object, and we have, in this relation, probably not the least important factor in the judgment of the third dimension for relatively near objects, and probably even more important than the irradiation circles which control the accommodation, as these circles must be the same for objects which lie before and behind the fixation-point. Of course the whole system of our localizing reactions becomes through these considerations more complex by far than the schematizations of the text-books propose. But physiological optics has shown at every point in its development that mere simplification has not always meant a deeper insight into the real relations."

It is evident that our studies in stereoscopic vision with lateral eye-position involve exactly the same principle and reaffirm completely Professor Münsterberg's theoretical views. In both cases, in the monocular of the verant as in the binocular of our experiments, the same retinal image has different psycho-physiological space-value on account of the different motor situation.

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BY E. B. HOLT

It is a familiar fact that when the head is passively turned about its vertical axis, the eyes do not move with the head but lag behind, keeping their fixation on that object toward which they were directed before the head moved. The eyes move in their sockets in a direction opposite to that in which the head has moved. Now it has been proved beyond a doubt by the experiments of Mach,[2] Crum Brown,[3] and Breuer,[4] that these lagging movements of the eyes are reflex and are governed by the semi-circular canals, which are stimulated directly by the motion of the head. Similar reflex eye-movements are found when the head is turned about some other than its vertical axis, the direction of such movements being always in confirmation of the theory. All these movements, together with the theory, are well described in the summaries of Peters[5] and Nagel.[6] The present paper deals solely with the eye-movements that occur after rotation of the head about its vertical axis.

The mechanism of these lagging, reflex movements is not, then, identical with that which enables us, when the head is at rest, to fix on and follow a luminous moving object,—the "pursuit movements" of Dodge.[7] It is, however, identical with that of Dodge's "fourth type"[8] and that of the compensatory eye-movements described by Brown,[9] Nagel,[10] and Delage,[11] and recently studied by Angier.[12] This function of the semi-circular canals was first suggested by Goltz in 1870. Now if the rotary movement of the head is prolonged, the eyes lag for a while on their first fixation-point, and then dart suddenly forward to a new fixation-point on which they rest for a while as before, until they dart forward again. Therefore if the head continues to rotate, the eyes fall into a regular and well-marked nystagmus. In this the lagging movements, or those opposite to the direction of the head, are called "compensatory," and are relatively slow and long. Their rate coincides closely if not exactly with that of the head-movement. But the movements forward, in the direction of the head-movement, are short and swift. Such are the facts during the rotation of the head.

But if this rotation has been somewhat prolonged, the ocular nystagmus continues after the head and body are brought to rest. But now its phases are reversed, and the slower eye-movements are in that direction in which the head has moved; while the swifter are in what before was the lagging direction. These observations are in accord with the semicircular canal theory, and are well established by various investigators.[13]

This paper presents the results of a photographic study of the reflex eye-movements following after rotation of the head (and body) about the vertical axis.

The subject whose eyes were to be photographed sat in a chair placed on a rotating platform, in such a position that the vertical axis of rotation passed through, or just posterior to the nose. Rays from an arc-lamp of 6 amperes, placed about 60 cm. from the subject's face, were so converged by a lens that when the subject came to rest, after the rotation, his two eyes were brightly illuminated. An adiathermal screen consisting of a dilute solution of copper ammonium sulphate kept the heat from being painfully intense on the eyes. The light fell slightly from one side on the subject's face, when he was brought to rest; and directly in front of him, at a distance of about 40 cm., was a camera of which the lens was on a level with his eyes. The ordinary ground-glass screen at the back of this camera was replaced by a light-proof box, in the front of which, and in the plane which should have been that of the ground glass, was a slit 55 mm. broad and 5 mm. high. Inside the box was a Ludwig kymograph of which the drum rotated on a horizontal axis: the circumference of the drum lay tangentially to the front of the box, and the line of tangency passed horizontally through the long axis of the slit. For each photograph a photographic film of sensitometer 40 was fixed to the drum, as paper is ordinarily fastened, and in moving, the drum carried this film upwards past the slit. It follows from this arrangement that 5 mm. along the length of this film were always exposed at once. The camera was so focused that the images of both eyes were sent through the slit, and fell on the film.

The subject's head was rigidly held by a rest: this rest was adjusted, and the camera focused, before the rotation. The adjustment of the head was greatly facilitated by fastening a fine black thread to pegs that projected forward from the head-rest, on either side; the thread was stretched horizontally, and at such a height that its image in the camera coincided with the middle of the long (horizontal) axis of the open slit. If then the subject, on seating himself in the chair, had his head so adjusted that each eye was directly behind the thread, each eye would certainly be imaged on the sensitive film. Neither the shadow of this thread on the subject's face, nor its image on the film, interfered in the least with the exposure that was made after rotation. This thread was further found very useful by the subject himself, who, after the rotation and just before the exposure was made, could make sure by sighting on the thread that his eyes had not slightly changed position during the rather protracted rotation. The subject was ordinarily turned twenty-five times at about the rate of one turn in two seconds. The kymograph was set in motion and the exposure commenced as soon as the whirling chair was brought to a dead stop. This stopping always took two or three seconds, at the very time when the nystagmus was most pronounced, so that the photographs do not show the maximum eye-movements. The exposure lasted through one rotation of the drum, nine seconds.

In the strongest negatives the movements of the eyes can be fairly well made out from the undulatory curve generated on the film by the dark image of the iris as it oscillated from side to side. But this is true only of the best negatives, and almost never of these if the eyes photographed had the iris blue. In order to obtain better definition in the photographs of the eye-movements, small flecks of Chinese white were tried, as invented and described by Judd.[14] A small square of white was laid with a brush on each cornea, on the side toward the lamp, so that its image on the film should be as bright as possible. The flecks were found to adhere to the eyeball even more perfectly than Judd himself has claimed; and they produced so little discomfort that the subject ordinarily forgot their presence on the eyes. Nevertheless their image as produced on the negatives, although much better than that of the iris, was generally not clearly readable, owing to the brief exposure and the illumination by electric light. This light seems not to be well reflected by the Chinese white: but in all cases where daylight can be employed the use of these flecks must be eminently satisfactory.

Thus it was found necessary to fall back on the image of the arc as reflected from the cornea. This corneal image invariably traced a clear, strong curve on the negative, and would have been appropriated at the outset, were it not that its movements are not, as is well known, a true register of the amplitude of the corresponding eye-movements; a fact that was shown clearly from a comparison in these negatives of the curves produced respectively by the flecks of Chinese white and by the corneal image. The former showed a much greater amplitude of movement. But the corneal reflection is a perfect register of the time and direction of the eye-movements; and in the following tables these features alone are studied. This reflection traced on the film a perfectly readable curve, although in some of the films, owing to a shifting of the carbons in the lamp taking place during the rotation, one of the eyes would be badly illuminated and a good record would be obtained from the other eye alone.

The arc ran on an alternating circuit of 60 phases per second, and owing to these interruptions of the illumination the curve of the corneal image showed on the negative as a dotted line in which the distance between any two dots represented one sixtieth of a second. Since the constancy of this alternation in the current has been measured in the Jefferson Physical Laboratory (of Harvard), and found to vary within a few tenths of one per cent only, the spacing of the dots on the negatives formed the most convenient possible means for determining the durations of the nystagmiform movements. These dots are shown in Figs. 3, 4, and 5 (Plates I and II).

Fig. 3 shows a portion of one of the films. The two curves are to be read from below upwards; but at the bottom is a photograph of the slit (showing a part of the subject's face) taken when the drum had made a little over one revolution and had come back to rest. Hence below the image of the slit, the curve of corneal reflection is doubled. "Right" and "Left" refer to the subject's right and left sides, so that the reader looks into the subject's face from in front. In the picture of the slit, the place on the cornea of the corneal reflection is shown; and also a minor reflection, which as may be seen traced no curve, from some other source of light. The fine line that crosses the slit horizontally is the image of the thread, above mentioned, which was used in adjusting the head. The time-dots are seen to be perfectly distinct, so that they could be accurately read with the help of a jeweller's eyeglass. Fig. 4 shows another part of the same negative, a portion subsequent to the single eye-curves of Fig. 3, that is, a continuation vertically upwards of Fig. 3. The rotation had been from the subject's left to his right, a direction that will be termed "clockwise" throughout this paper, and it can be seen that the quick eye-movements are toward the subject's left, while the slow are towards his right: had the photograph been taken during the rotation, the directions of the quick and slow movements would have been reversed. Two points may be observed in this figure which the tables will also bring out,—that the two eyes move together, and that as the nystagmus subsides the quick eye-movements become less frequent but endure no longer, or in other words, the slow movements alone increase in duration. The corneal reflection does not accurately show the amplitude of the movements; but direct inspection of a subject's eyes, as the nystagmus dies away, shows that generally (but perhaps not always) the amplitudes of both quick and slow movements decrease together. When this is the case, it follows that at the end of the nystagmus the rate of the slow movements decreases very much faster than that of the rapid movements.

Readable negatives were obtained from four, out of six subjects. Of such negatives there are fourteen, ten of which are of eye-movements after rotation clockwise, and four after rotation anti-clockwise. This distribution is accidental, for the rotations in each direction were about equal in number. With the exceptions to be noted later all the negatives exhibit the same features, so that of the fourteen only four examples are given in full in the tables; while for the others merely the averages of the duration of quick and slow eye-movements respectively are given.

TABLE I

Table I gives these averages for all the fourteen negatives. In four of these (H 2, H 3, Tu 1, Tu 2) simultaneous curves for both eyes were obtained. In every curve the slow eye-movements were in the same direction as the previous rotation; the rapid in the opposite direction. The very few single movements that are exceptions to this are noted under Table II. Had the photographs been taken during (instead of after) the rotation, the directions of rapid and slow movements would undoubtedly have been reversed. It is to be noted that when both eyes were recorded, their movements were generally identical, within the accuracy of measurement (one sixtieth of a second). There are a few exceptions to this. The averages of all slow and all rapid movements merely show that in general, and for that part of the nystagmus that was photographed, the slow eye-movements lasted six times as long as the rapid ones. This ratio varies considerably from one case to another, and at best throws little light on the whole nystagmiform series, since during the very first instants after the rotation the ratio of quick to slow movements would be less than one sixth, and at the very end of the series would be considerably more; this because toward the end the slow movements become much slower, while the rapid seem to change very little. The variations from case to case arise, at least partly, because in some cases the picture was taken more promptly, after the rotation stopped, than in others.

TABLE II

TABLE II, continued.

Parentheses indicate time during which the eye did not move at all.

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Table II gives in detail the data yielded by four of the most instructive films. C 3 is the longest record that was obtained; Tu 4 is among the shortest, though it is not the very shortest. H 2 and H 3 show how nearly alike are the simultaneous movements of the two eyes: .07 sec. is the greatest difference recorded on any film between simultaneous movements. All four records show how much less the duration of the slow movements is at the beginning of the record than at the end, and how little the fast movements vary in this respect.

H 2 is given because it is not typical; and about one half of the film itself is reproduced in Fig. 5 (Plate II). It will be seen that at four points there intervened between slow movements (toward the right) a rapid one that was also toward the right. This is the only record in which such a thing happened: and its explanation is problematical. With the subjects C and H, and only very rarely with these, a rapid movement sometimes took the place of a slow one, that is, occurred in the same direction as the slow movements (e. g., Table II, C 3). And a trifle more often, yet very seldom, a rapid movement was relatively slow (e. g., ibid.). With every subject there are a few cases in which the eyes stood still for a small part of a second (e. g., ibid.), and these moments of rest seem to come after a rapid or a slow movement indifferently.

McAllister[15] and others have shown that the eyes are seldom at rest even when voluntary fixation is attempted, and these anomalies in the nystagmiform series may well be the result of such random factors, which instead of being always inhibited by the afferent impulses from the semicircular canals, which govern the nystagmus, operate along with these latter, and sometimes even inhibit them. With the exception of these anomalies, the movements recorded in the photographs confirm the observations of Purkinje, Mach, Breuer, Delage, and other investigators.

In conclusion, the sensations of vertigo and of nausea seem not to be essentially connected with the nystagmus. Several subjects were so disagreeably affected by a preliminary rotation that it seemed best not to continue the experiment with them. With those, however, whose eyes were photographed, while they experienced a mild degree of vertigo and nausea during and after the first few rotations, these sensations soon wore off with further practice, while so far as could be observed their eye-movements were as ample and rapid as at first. The introspection of these subjects was that after the rotation the body seemed at rest and the stomach quite settled, while the visual field alone whirled rapidly in the direction opposite to that of the previous rotation.

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BY E. B. HOLT

During and after a prolonged rotation of the head, the visual field seems to spin around before one's eyes,—a phenomenon that is ordinarily called the "dizziness of Purkinje." Delage describes it as follows:[16] "In the experiment of Purkinje, while we are rotating in a positive sense, space seems possessed of a motion in the opposite direction.... This phenomenon is explained by the direction of the nystagmus."

"In the nystagmus," he continues, "the eyeballs execute two well-differentiated motions: one, a compensatory, relatively slow motion, during which images pass across the retina so as to give the appearance of a movement of space in the opposite direction; two, a swift motion opposite to the slow one, and so rapid that the images passing across the retina leave no sensation of their movement."

Now, in a previous paper[17] I have shown that there is a central anæsthesia, or central inhibition of visual sensations, during about the latter two thirds of the time occupied by every voluntary eye-jump; and in view of this I was led to enquire whether in fact, as Delage so confidently asserts, it is the speed of these more rapid movements, or some other factor, that causes them to leave no visual sensations. There can be no doubt that they do leave none, since, aside from the statement of Delage, in dizziness the visual field whirls always in only one direction; whereas it should otherwise appear to swing now to one side, now to the other, as the eyes move back and forth across the objects. I have found but one other mention of this point in the literature. In his Analyse,[18] Mach says, parenthetically, "(the jerky eye-movement leaves no optical impression)"; but he does not suggest that this is because of its greater speed.

In order to test this point, a 2 c. p. incandescent lamp was so arranged that it could be moved vertically in front of, and about four metres distant from, a rotating chair. Since after a rotation the eyes are oscillating from side to side, if the lamp is moved up and down an obliquely inclined after-image streak must be generated on the retina; and clearly there are four possible positions in which this may lie, as shown in Fig. 1.

The results were absolutely uniform (the author alone as subject); the after-image streak always lay on that side of the moving light toward which the slow eye-movements were directed, that is, the lamp appeared to drift obliquely up or down and in a lateral direction opposite to that of the slow eye-movements. Apart from its vertical displacement, then, the lamp behaved like the less intensely illumined parts of the visual field, seeming to be totally invisible during the swifter eye-movements. Now since the experiment was done in a partially darkened room and the eyes were partly adapted to darkness, the lamp should have been intense enough adequately to stimulate the retina even during the more rapid movements, and might be expected to leave an after-image streak on that side toward which these rapid movements were directed, and differing only from the streaks seen during the slow eye-movements in being inclined at a less angle from the horizontal. Yet no such streaks were visible.

These observations were made at about the same number of seconds after the rotation stopped, as the photographs were taken that are recorded in the preceding paper of this volume. The rapid movements were therefore about one sixth as long in duration as the slower ones. Since the respective amplitudes of rapid and slow must average very nearly the same, the rapid movements must have been about six times as swift as the slow movements. It needs therefore to be shown beyond a doubt that the 2 c. p. lamp was bright enough, in view of the briefness of stimulation of any one retinal element during the rapid eye-movement, to be above the threshold of perception. For this reason the experiment was not continued with other subjects.

The certainly adequate degree of illumination was realized during the photography of the eyes described in the preceding paper. Here during the post-rotary dizziness an arc lamp (of 6 amp.) was in front of the face and but a little to one side of the primary line of regard; it was 60 cm. distant from the eyes and on a level with them; a lens condensed the rays on the two eyes, and the light was diminished only just enough as not to be painful, by a dilute screen of copper ammonium sulphate about 3 cm. thick. Of course such an illumination must adequately stimulate each retinal element even during the most rapid eye-movements. Nevertheless with the four subjects that were photographed the arc lamp, like the rest of the visual field, seemed always to swim in one direction, and that opposite to the slower eye-movements. In one case where the eyes were photographed without the adiathermal screen, and the light was rather painfully intense, the lamp was still seen to drift in one and the same direction. There was never any trace of its moving to and fro, as there should have been had it been visible during both phases of the nystagmiform movements.

This absence of visual sensation during the more rapid eye-movements might conceivably depend on either peripheral or central inhibitory factors. But the anatomy and physiology of the eye offer no point of support for the supposition that during such movements the irritability of the rods and cones is momentarily reduced, or that the retinal layers posterior to the rods and cones suffer an interruption of function during a movement of the eyeball in its socket. Indeed, during some such movements, the "pursuit" movements (Dodge's second type), vision is unimpaired.[19] In view of these facts, and of the many known cases of the mutual inhibition of sensations where undoubtedly the process is a central one, it is by far most probable that this visual inhibition is also a central process; as was certainly the visual inhibition during voluntary eye-jumps, previously reported by me.[20]

The conclusion above reported that the visual inhibition during the more rapid phase of the nystagmus in no wise depends on an inadequate stimulation of the retina, due to the greater speed of the rapid movements, and that the inhibitory process is purely central, is further supported by the following phenomenon. If before the rotation has commenced, the eyes are so strongly stimulated that a lasting after-image is obtained, this after-image will, during the rotation, always be seen to swim in the direction opposite to the rotation, that is, with the slow eye-movements; but when the rate of rotation begins to decrease, and as Mach, Breuer, and Delage have shown, the slow eye-movements reverse their direction, the after-image also reverses its direction, and now swims in the direction of rotation, that is, still with the slow eye-movements. If the after-image persists long enough, it may still be observed, after the rotation has ceased, swimming in the same direction as the surviving slow eye-movements. If, for instance, the slow movements are from left to right, the after-image (best seen with the eyes closed) swims from the left to the right hand side of the field and disappears, reappears at the left and swims again toward the right, and continues to do this until the nystagmus entirely ceases.

This experiment was repeated several times, with four subjects, and with both clockwise and anti-clockwise rotations, and the results were uniformly as described above. In order to see whether this motion of the after-image really depended on the slower nystagmiform movements, the following variation was tried. It will be recalled that if the head is rotated not about a vertical (longitudinal) axis, but about a transverse axis, as, say, one passing through the ears, a nystagmus is produced in which during the rotation the slower eye-movements are opposite to the direction of rotation, while when the rotation is checked or stopped, the nystagmus, as before, reverses. The same is true if the rotation is about a sagittal axis. These conditions were approximately realized by having the subject sit as before on the rotary chair, but during the rotation hold his head horizontally to the right or left, forward or back. With any of these positions of the head, however, the rotation produced, on all of the subjects tried, extreme dizziness and a feeling of nausea that lasted in some cases for several hours. This fact made it impossible to ask for a set of the four possible positions of the head from any of the subjects. The following are the records that were obtained:

So far as these records go, they entirely confirm the results of other investigators as to the direction and the reversal of the nystagmus. In each of the cases the after-image moved with the slow eye-movements, reversing its direction with these slow movements, while the visual field whenever it was observed (the eyes were kept closed during the rotation) moved in the opposite direction to that of the after-image and the slow eye-movement. It is well known that after-images move with every involuntary eye-movement, and although they disappear during voluntary eye-jumps,[21] they reappear at the end of the jump in a position that is related to the new fixation-point exactly as the old position was to the former fixation-point. These after-images, then, are seen during the slow eye-movements whose direction they follow; but are not seen during the quick movements, when they must naturally move in the direction of these quick movements. And aside from this it is possible to observe introspectively that the after-image disappears at that side of the visual field toward which the slow eye-movements tend, and is for a moment invisible before it reappears on the other side of the field. As was shown above, the visual field always moves opposite to the direction of the slow eye-movements, as must of course be the case if there is no inhibition of vision during these movements. The simultaneous appearance of the after-image moving with, and the rest of the visual field moving contrary to, the direction of the slow eye-movements, with a uniform absence of the converse phenomena, seems to prove that vision is unimpaired during these slow movements, while it is completely inhibited during the rapid phases of the nystagmus.

Purkinje himself[22] called the slower phases "involuntary and unconscious," meaning by "unconscious" not that the visual field was not seen (for it just then is seen), but that the movement of the eyeball during the slow phases was not felt. I have observed, with the confirmation of several subjects, that this movement can also not voluntarily be inhibited; whereas the swift movement is so far voluntary that it can be inhibited at pleasure. It is possible, that is, to fix the eyes on that side of the field toward which the slow movements are directed, but not on any point at the other side of the field. The slow movements, then, during which vision is possible, are purely reflex. These slow movements, purely reflex and yielding clear vision, with the rapid movements, partly under voluntary control and attended by an inhibition of vision, present a parallelism, that may be not without significance, to the "pursuit" eye-movements (Dodge's "second type"), that are likewise relatively slow, are reflex, and yield remarkably clear vision, and the ordinary voluntary eye-jumps (Dodge's "first type"), that are relatively rapid, and are, like the rapid nystagmiform movements, attended by a central inhibition of vision.

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BY FOSTER PARTRIDGE BOSWELL

There are various kinds of visual irradiation, of which perhaps the best-known variety is that which appears as the enlargement of a brightly illuminated surface at the expense of a contiguous one of less intensity. This has been until recently the only form recognized, and until very lately the greater part of the literature has dealt with it alone.

The whole subject was carefully investigated by Plateau in 1831, and retinal irradiation extricated from phenomena which very often accompany it. He showed that the extent of irradiation varies with the intensity of the stimulating light and the time during which it is allowed to act. He was also the first to call attention to the phenomenon of so-called negative irradiation.

Somewhat later Volkman again called attention to negative irradiation, while Aubert, in opposing the explanation advanced by Volkman, first showed the relations existing between irradiation and contrast.

Dove was the first to investigate the influence of irradiation on stereoscopic pictures, thus calling attention to the question of binocular irradiation. Experiments in this direction, however, have in general given negative results in so far as any enlargement of the binocular portion is concerned.

Helmholtz examined the manner in which the stimulation at the border-line between a light and dark field changes in intensity, and drew a curve showing these modifications of intensity due to irradiation. Hering showed that the form of the Helmholtz intensity curve would be modified by the presence of other phenomena not strictly those of irradiation.

De Roux demonstrated the difference in the extent of a real induction on the foveal and the extra-foveal parts of the retina.

Charpentier has attempted to carry forward the general explanation by saying that this spreading of neural excitation, the existence of which he proves to be beyond question, takes the form of an undulatory excitation in the free nerve-endings of the retina. Bidwell has investigated more thoroughly in some respects than Charpentier the phenomena of the after-images of moving sources of light, which have bearing upon irradiation. The same is true with regard to McDougall, von Kries, Hess, and others. Burch has instituted investigations along these lines, especially concerning the inhibition of stimuli on contiguous portions of the retina. Hess has worked carefully upon the different phases of the stimulation derived from a moving source of light, the differences in functioning of the foveal and extra-foveal parts of the retina, the respective functions of the rods and cones, and in connection with this, made investigations in the visual perception of color-blind subjects. All these observations have important bearing on irradiation, contrast, and theories of color-vision.

In connection with some work which was being done upon the after-images of moving sources of light in the Harvard laboratory in the early winter of 1903, some phenomena were observed which I believe are due to one form or other of visual irradiation. They can be seen in various ways, perhaps most advantageously by observing with fixed eyes the passage of a luminous image over the retina. What one sees as such a figure moves by is a travelling band of light, its forefront somewhat like that of the stimulating source, the rest composed of a long train of after-images which differ very decidedly from one another in intensity and color. The advantage of this well-known method of observation lies in the fact that it enables one to translate the temporal relations between the different phases of the stimulation into spatial relations between the different portions of the moving band of light. For since the figure moves across in a plane before the observer, that which appears in his consciousness first in time will likewise appear as foremost on the plane in space. Thus by observing the train of images one practically sees the different phases of the stimulation spread out in order before one. The new phenomena we observed, however, have to do with but a single phase of the stimulation, the extreme front of the stimulating image.

The intensity of light used varied considerably with the differently colored images, and was regulated so as to give as well as possible the phenomena we wished to study. With white light the intensity was less than that of an eight-candle-power electric lamp placed about ten feet distant from the observer. When colored light was employed it was necessary to use a very much stronger source of illumination, since the colored glass which was used absorbed a great deal of light and in case of colors lying toward the violet end of the spectrum greater luminosity seemed demanded.

The apparatus used consisted of a three-foot pendulum with a screen attached. This screen swung with the pendulum. In the screen was an opening about four inches wide and three inches high, into which strips of cardboard or tin backed by a piece of ground glass could be slipped. In these strips differently shaped holes were made through which the light passed. In this manner an image of any desired form might be used. Behind the screen, between it and the lamp, was a frame in which other pieces of ground or colored glass were placed. These pieces of ground glass would reduce the intensity of the light and diffuse it evenly over the image. The observer sat ten feet away. When the pendulum was set in motion, the image would appear moving back and forth in an arc. In order to shorten this arc and to aid the observer in keeping his gaze perfectly fixed, a second screen was placed before and very close to the pendulum, between it and the observer. This screen was stationary. In it was a hole six inches long and two inches wide. The top and bottom of this hole were arcs of circles parallel with the arc in which the pendulum swung. The ends were radii.

The screen was so placed with reference to the observer that the moving image would pass directly across the middle of the opening, appearing from behind one side and disappearing behind the other. In the centre of the opening, directly in front of the place occupied by the moving image when the pendulum was at rest, were two luminous fixation-points, one above the other, below the path of the moving light. In order to measure apparent spatial differences between the phases of the stimulation, two wires were stretched vertically across the opening in the stationary screen. These wires could be moved nearer together or farther apart. Thus by measuring the apparent distances in space between the different parts of the moving figure a measure could be had of their temporal differences in coming into consciousness. The luminous image moved, during the time it was visible, at a velocity of about one and a quarter feet per second. Since the observer sat about ten feet from the instrument, this would be at an angular velocity of about seven degrees per second. In one experiment a higher and a lower velocity were also employed.

It was of course very easy to change the figures and vary them widely in form, color, and intensity. Most of those employed, however, were rather small, subtending an angular distance of not more than one degree. Since the whole opening did not subtend an angle of more than three degrees or so, nearly all the phases of the stimulation occurred at the fovea.

We noticed that the form of the stimulating images themselves seemed to suffer modification as the light swung by, not only because of the train of after-images which dragged behind them over the retina, but in other ways as well. For instance, a circular image (Plate III, Fig. 1) appeared crescent-shaped, and its forward edge possessed greater curvature than the segment of the circle which produced it. It was longer also from horn to horn than the diameter of the generating circle, and a faint haze surrounded the points extending outward and backward until lost in the blackness of the background. Von Kries remarks that a circular moving image appears cylindrical in form with a concave edge behind. By using a little higher speed we observed this phenomenon. At first we thought the crescent-shaped image to be due merely to an intensely black after-process, which Bidwell describes as following the positive image of a bright white light. This, taking place before the circular disc of light had gone forward a distance equal to its own diameter, would overlap the bright image from behind and a crescent-shaped figure would result, but the increase in width and convexity of the stimulating image as well as the laterally trailing clouds of light remained to be explained, and as this could not be done in terms of anything which might happen to the back of the image, another explanation had to be sought. In order to determine the effect of the form of the figure used as a source of light on the form of the apparent image, several differently shaped figures were employed. In place of the original circle, an oblong pointed at both ends was tried (Plate III, Fig. 2). The front of this figure appeared very convex indeed, while the ends, which, owing to the shape of the figure, were very much less effective as a stimulating source, trailed far behind the centre.

A crescent-shaped figure (Plate III, Fig. 4) gave rise to a very pretty phenomenon. When it moved toward its concave side, it appeared very much less concave on that side than the real figure, but when it moved the other way, toward its convex side, it seemed very much more curved than it was in reality.[23]

No. 3, a simple oblong figure, appeared curved like the others, almost as perfect a crescent as any of them.

The idea occurred to me that perhaps all these modifications in the curvature of the figures could be explained if we assumed two things: First: that there is a spreading of excitation from one portion of the retina to another. Each point will therefore be stimulated not only by the light falling directly upon it, but it will also derive a certain reënforcement of its stimulation from the points surrounding it. Thus a point lying toward the centre of one of these figures would be more favorably situated for receiving reënforcement than one located toward the periphery, where there are few neighboring points, and those lying mostly in one direction, namely, toward the centre.

This may be represented diagramatically, as in the illustration (Plate IV, Fig. 10), where the horizontal coördinates represent the spatial dimensions of an oblong image and the vertical coördinates the intensity of the excitation due to direct stimulation and its reënforcement by surrounding points at various portions of the figure.[24] Secondly I assumed that the stimulation at one part of the figure being thus rendered more intense, that part would appear in consciousness more quickly than the others and cause a modification in the form of the figure.[25] For example, in the case of the oblong figure, the light would be rendered most intense at the centre and less and less intense toward the ends, for the points in the centre of the figure will have their intensity increased by nervous excitation spreading to them from points lying toward the ends. Those toward the ends will be reënforced by light coming only from toward the centre. Thus the intensity of the centre of the figure will be increased, and as the figure moves across before the observer, the centre, appearing first in consciousness, would likewise appear foremost in space, the points near the centre a little later and so on, until finally, the ends being the last to appear, the whole front of the figure would take the form of a convex curve, after the manner in which it was observed. The back of the figure also appears curved, probably because of the fact that the front of the negative after-image, which closely follows it, is of the same shape as the front of the positive image, as was shown in the case of the circular figure.[26]

It is of course a well-known psychological fact that a light of greater intensity will take less time in coming into consciousness than one of less intensity. In this case, however, it was necessary to find some way of showing such differences between lights which were very little different in intensity. For one is practically unable to see any difference in intensity between the parts of a stationary image. So unless it could be shown that a difference in intensity between two sources of illumination, so small as to be imperceptible to the observer, will nevertheless make its presence known by the appearance of the brighter light in consciousness before the other, the explanation which I have suggested for the curvature of the images would have to be abandoned.

The following experiments do show, as I believe, that of two sources of light not perceptibly different in intensity, the brighter will appear in consciousness before the other, and that in the case of these figures the curvature of the image is due to a heightened intensity of the light in the centre through reënforcement of the excitation there present by stimulation spreading from the ends.

EXPERIMENT I

In the first of these experiments three dots of about three sixteenths of an inch were placed in a vertical row about three eighths of an inch apart (Plate IV, Fig. 1). No change was then observed in the form of the figure. The row of dots swung across the opening in a perfectly vertical line one directly above the other (Plate IV, Fig. 2). They were presumably too far apart for irradiation to take place between them. When, however, another dot was interposed between each end dot and the centre dot (Plate IV, Fig. 3), so that the excitement could extend from one dot to the next, the front of the line of dots no longer appeared vertical, but decidedly convex, the centre dot being perhaps three eighths of an inch before the dots on the ends (Plate IV, Fig. 4).

Absolutely the only difference between the two cases was that in the one, irradiation presumably could not occur, while in the other it conceivably could.

EXPERIMENT II

In the second of these experiments the curvature of a line of dots was observed and measured. Then the centre dots were slightly darkened (Plate IV, Fig. 5) by shading lightly with a lead pencil the ground glass which travelled with the pendulum and held in place the card from which the dots were cut, until the front of the image lost its curvature and appeared vertical (Plate IV, Fig. 6). The pendulum was then stopped and the row of dots observed closely, in order to see whether the dots in the centre were perceptibly of less intensity than those on the ends. No perceptible difference was found.

EXPERIMENT III

All the dots were covered, except the shaded central and the two unshaded end dots, in order that no irradiation might take place between them (Plate IV, Fig. 7). The pendulum was again set in motion, and the centre dot, instead of remaining co-linear with the dots on the ends, appeared considerably behind them (Plate IV, Fig. 3). This would show that irradiation must heighten the intensity of the excitation in the centre of the figure—for the two cases just mentioned are alike in every respect except that in the first (Fig. 6), where the dots were near enough together so that irradiation might occur between them, the intensity of the centre dot, which was objectively fainter than the end dots, was heightened enough by this induced excitation to appear in consciousness as soon as the two end dots, which were objectively of greater intensity; whereas in the second case (Fig. 7), where the dots were too far apart for irradiation to take place between them, the centre dot, being objectively of less intensity than the end dots, appeared behind them.

These experiments show that of two sources of light very little different in intensity the brighter will appear in consciousness before the other. Other things being equal, the difference in intensity may even be so small as to be imperceptible by direct comparison; it is able nevertheless to make its presence known by the order in which the lights appear. Exner made some experiments in 1868 to determine the time necessary for the perception of lights of different intensity. He used, however, stationary images of brief duration and tried to eliminate the effects of the after-image by flooding the visual field with light. This method has its disadvantages. It is incapable of measuring the minute temporal differences in latent perception of sources of light very slightly different in intensity.

While my method does not give the absolute time taken by any one light to enter consciousness, it is a very much more delicate method than Exner's for measuring differences in time of latent perception of sources of light very close to one another in intensity. It would be a very easy matter, having found the time of latent perception for a light of standard intensity, to determine by this method the time of lights of greater or less intensity.

These experiments also show that when irradiation is absent, the curvature of the images is absent; when irradiation is presumably present, curvature is present. For I find, not only in these, but also in a number of other experiments, that under all conditions in which the presence of irradiation is to be expected, the form of the images tends to be modified in precisely the manner that the assumption of its presence would lead one to anticipate. In all cases where irradiation is presumably absent, the contour of the front of the moving figure depends entirely on the amount of light proceeding from its different parts.

It is next in order to say something of the physiological causes of the phenomena we have been considering.

It is probable from what has been observed that in the case of the curved figures we are dealing with a form of visual irradiation which is due to the spreading of neural excitation over or through the layers of the retina. It is also evident from the close connection between irradiation and intensity that it must be of such a kind that the excitation produced in one part of the retina may communicate itself readily to another part. We have also seen in the case of the moving line of dots that the several dots could remain distinct from one another and yet could reënforce each other by means of communicated excitation. It must also be a very rapid form of irradiation, for the curvature of the figures does not increase very much during the time they are visible.

I think that the demands made by these different facts are best met by assuming that the spread of the nervous excitation which gives the reënforcement takes place in one of the interconnecting layers of nerve cells and fibres underlying the rods and cones. The line of dots which appeared curved and yet perfectly distinct from one another could very well communicate excitation to one another along these fibrils, and the intensity of one part be raised by the excitation of the near-lying parts. The fact that the dots remain distinct would not be contradictory. For in that case very near-lying parts might communicate excitation to one another without arousing to any very great activity the nerves that lead to the brain from the small unstimulated portions which lie between them. In this manner the intensity of the centre dots could be heightened enough to make the row appear convex, without any merging into one another on the part of the several dots. The fact that the dots do not fuse shows that the curvature is not due merely to a forward-spreading of the excitation in the retina. However, there is always a certain amount of light visible between the dots, with all the colors. This is especially noticeable with green light.

The fact that the elements of the retina form a kind of concatenated series from without inwards, a number of rods and cones corresponding to but one ganglion cell, furnishes a further bit of evidence in support of the explanation just advocated, since the irradiated excitation would tend to be "drained off" through the group of ganglion cells corresponding to the most highly stimulated portions and leave the intervening spaces comparatively free from centrally proceeding excitation. Thus also the individual dots in the five-dot figures may appear entirely distinct from one another and yet the centre ones be reënforced enough by irradiation to appear in consciousness in advance of the others.

SUBSIDIARY EXPERIMENTS

A number of other observations were made which present various exemplifications of the principles we have considered.

EXPERIMENT IV

An oblong figure, all its parts objectively of the same intensity, had its ends slightly darkened. When this was done the curvature had increased from twelve sixteenths to fourteen sixteenths of an inch.

The pendulum was stopped, and a very slight difference was perceived between the ends and the centre of the figure. This difference in intensity was greater than in the dot experiment, when the image had been darkened enough in the centre to make it appear vertical, because in this case, when the ends were darkened the centre would still be reënforced by irradiation from a considerable space which intervened between the shading and the centre.

EXPERIMENT V

The centre of the oblong figure was considerably darkened so as to counteract the effect of induction. By properly varying the amount of shading, one may make the front of the figure appear less convex, vertical, or even concave. This shows perfectly the effect of differences in intensity upon the curvature of the figure, but does not show so neatly as the similar experiments performed with the dots, the influence of the presence or absence of irradiation upon the intensity of the centre of the figure and so upon the curvature.

The illustration shows a case where the centre was too much darkened.

The two ends were comparatively free from shading. In each end-part irradiation took place. The points lying toward the centres of these ends received reënforcement, both from points lying toward the centre of the figure and from the extreme ends, and so the centres of the ends of the image were considerably brighter than either the extreme ends of the figure itself, or the sides of the end-parts toward the heavily shaded centre of the figure. Accordingly each end appeared convex for a short distance. The whole figure, however, being considerably brighter at the two ends than at the centre, on account of the heavy shading, the ends appeared in consciousness first and the centre afterwards, so that the figure as a whole seemed concave.

EXPERIMENT VI

An oblong figure was shaded rather heavily at one end, gradually becoming lighter toward the other, while about a third of the figure was free from shading. The shaded end always seemed to lag behind. The extreme front of the figure was at a point a little distance from the other end, before the shaded portion began. So that the front of the whole figure appeared, not like a segment of a circle, but like part of an oval with the bulge toward the brighter end.

Beyond the ends of all these images faint clouds of light were seen, as has been mentioned before, extending outward and backward, gradually decreasing in intensity, until lost in the surrounding blackness of the background.

Charpentier's bands, sometimes more and sometimes less in number, were observable in all of my figures and with all colors. Very often they appeared to be parallel to the forefront of the image, or even of a slightly greater degree of curvature.

EXPERIMENT VII

It is a well-known fact that a rotating color-disc, having colors which just fuse at a certain intensity, will show flicker at a slightly less intensity.

A color-disc was set in motion and the speed found where the colors were on the point of fusing. A piece of black cardboard, with a hole about an inch in diameter, was held close to the screen.

Around the periphery of the hole flickering appeared, while at the centre there was fusion. (The cardboard was held very close to the disc, so that there would be no shadows on the disc near its edges.) This fusion at the centre of the disc is probably due to the fact that the centre of the field is of slightly greater intensity than the edges, owing to irradiation. This difference in intensity makes the difference between the fusion at the centre and the slight flicker seen at the periphery.

Karl Marbe in a recent article mentions the difference in fusion between a point in the centre of the disc and a point near its border, and he thinks the increase of flickering in the latter is due to some influence on the part of the moving edge which separates the different parts of the disc. It would seem more probable from this last experiment that the fusion at the centre of the field of view was due to reënforcement of intensity by irradiation, and that the flicker about the periphery of the field was due to the lack of such reënforcement.

EXPERIMENT VIII

Three large dots were used and the centre one covered with tissue paper. The two end dots then appeared ahead of the centre dots. They were larger than the centre dot, due to irradiation over their borders. But this increase in size did not account for their position ahead in space. The centres of all the dots were not co-linear, but the middle dot was behind the others, thus, of course, showing the greater time necessary for the perception of the less luminous object.

EXPERIMENT IX

This was exactly similar to the preceding, except that the intensities of the various dots were reversed. The end dots were covered with tissue paper, instead of the centre one. Then the centre dot appeared first and the end dots after it.

EXPERIMENT X

Professor Hess finds that an image which, compared to those we used, was very long, subtending an angular distance of about thirty degrees, and which extends entirely across the fovea and overlaps the surrounding parts of the retina will appear curved backwards at the fovea, owing to the longer time of latent perception of the fovea and the macula. The accompanying illustration shows a modification of one of Hess's figures, in which the presence of this phenomenon and that of the convex image are both shown. The two phenomena were observed when a two-inch image was observed at a distance of about fourteen inches. The intensity of the light was that of an eight-candle-power lamp with three pieces of ground glass in front of it. (Very many of Hess's intensities are too great to give convex images.) Thus the image would be about 12° in height. About 5/12 of the figure would then fall on the macula and fovea and appear curved backwards in relation to the ends. The ends where they fell on the extra foveal parts of the retina appeared convex in front and concave at the rear as any small image of the right intensity does which falls on a homogeneous part of the retina.

EXPERIMENT XI

Charpentier, Bidwell, and others have made the observation that if a small source of light be exposed for a brief interval, excitation will proceed out in all directions over the retina, but if the light be exposed for a slightly longer period, the excitation will contract again and the light appear nearly its proper size and in its proper location at the stimulated portion of the retina. Using variously shaped figures we obtained analogous results, and the additional fact appeared that the outgoing excitation proceeds from the borders of the figures and that its form is somewhat determined by the form of the figure. An oblong image appeared vaguely elliptical, a diamond-shaped figure in the form of a more pointed ellipse, etc. These images were exposed for only a small fraction of a second, by means of a shutter. As the exposure grew longer the true form of the figures came out more and more clearly. There thus seems to be a general spreading of the stimulation in all directions over the retina from the borders of the images. Then, upon a slightly longer duration of the stimulus, this very rapid irradiation of excitation contracts and the irradiation becomes confined within the borders of the stimulated portion and affects the intensity of the different portions of the image. With strong intensities and certain colors it is, however, never wholly confined to the stimulated portion even of moving images. Charpentier speaks of "clouds of light accompanying his figures." With green light these clouds are especially noticeable. His "palm branch" phenomenon is a good instance of the irradiation of stimulation.

Besides these experiments which I have just described, several phenomena of a like sort were observed in connection with other experiments which were being performed in the laboratory at the same time. Dr. Holt was experimenting with a bright circular spot of light about one half inch in diameter, surrounded by a very faint ring about one half inch wide. When the whole image was moved about, the spot would seem to go back and forth across the less intense part so that the whole image looked like a jelly-fish swimming about in the water.

When the figure was allowed to remain stationary for a few moments it would resume its natural shape. Otherwise the bright part would seem to advance faster than the rest, sometimes even overlapping the border. This phenomenon was due to the fact that a bright light requires less time in coming into consciousness than a less intense one, and is, of course, the same in principle as those which were performed with dots when the bright dot moved ahead of the rest.

Another one of these phenomena occurred when an isosceles triangle was moved in a direction parallel to its base. The side toward which it moved appeared curved forward, with the apex bent backward. Toward the bottom, where there was the best chance for irradiation to have its effect, appeared the most advanced portion of the figure, while the bottom corner, although objectively the most advanced part of the figure, appeared rounded off and somewhat behind the part just above.

A narrow, vertical image with a large bulge behind the central part appeared with a large portion of this bulge in advance of the centre of the figure.

All these experiments show that a more intense object is, other things being equal, always located ahead of other objects co-linear with it. And I assume irradiation to account for the priority in localization of parts of the figure which are not objectively of greater intensity than others, but whose position makes them subject to reënforcement. The localization itself may be a function of more central organs, and not directly a question of the coming into consciousness more quickly of a more intense stimulation, although that seems to be the simplest explanation, but in any case priority of localization varies directly with the degree of intensity.

If the light is not bright enough to produce much irradiation the image will lose its curvature. If the light is too bright, although there may be a maximum of irradiation aroused and the absolute difference in intensity between the ends and centre of the image be at its greatest, yet this difference may not be great enough in proportion to the absolute intensity of the light to make the centre of the image appear in advance of the rest.

The curvature also varies with the angle subtended by the image and the portion of the retina upon which the image falls. If the image were too long, although all the processes which produce curvature be present, yet the front of the image would still appear vertical, because of the fact that each point in this long line would not derive reënforcement sensibly greater than that of the neighboring points. The best one could expect would be that these long figures should have their ends rounded off, which is usually the case. Most of the images which Professor Hess used in his experiments were too long to appear curved. All the images whose curvature we measured did not subtend an angle greater than 1° 10´, and were all seen on the fovea.

An image which subtends an angle of more than about 2° will hardly appear curved when it passes over the fovea.

We were sometimes able to see the curvature reversed. This happened in my own case about once in a hundred times, usually when my eyes were fatigued by the repeated passing of the moving light back and forth over the same portion of the retina. With other observers it occurred more often.

Slight vertical differences in fixation would cause the central part of the path taken by the moving light to become more fatigued than the edges and so to respond more slowly to the stimulation and reverse the curvature. It may be that some brain process which has to do with the apperception of the form and movement of visual objects becomes fatigued or does not always function properly, and so the curvature of the image may sometimes appear reversed. At any rate the more usual cases are those in which the convexity is present. The others, owing to the number of factors involved, and the vast majority of the opposite cases, may be regarded as due to temporary defects in the psycho-physical mechanism, which when properly working would give the more usual result.

QUANTITATIVE EXPERIMENTS

The object of the following experiments was to measure the amount of curvature produced by differing degrees of intensity of light at different speeds. An oblong figure was employed one fourth inch wide and two inches long. As has been mentioned, two vertical wires were stretched across the path in which the light moved. As the light swung by, it was attempted to get the wires at such a distance from one another that when one appeared tangent to the curve at the front of the figure the other would seem to cross the image at the point of intersection of the curve with the rest of the figure, as indicated in the diagram. (Plate IV, Fig. 9.)

The distance between the wires was then read off on a scale. Thus one was able to obtain a measure of the curvature of the figure when it was moving at different speeds and illuminated by different intensities of light, and to compare the observations of different subjects. The mean error in this work is surprisingly little, considering the difficulties in making the judgment as the light passed rapidly by the wires. Usually the moving light had to be observed several times before the curvature of the front of the moving image could be measured exactly. It would be perfectly obvious that the front was considerably curved, but it would often be wholly impossible to tell just how much it was curved, until the pendulum had swung back and forth four or five times. Fatigue and darkness adaptation modify the judgments considerably. If one's eyes were partially adapted to darkness some little difficulty was experienced in seeing clearly the curvature of the image. Fatigue comes on very rapidly indeed. Usually it was impossible to get more than four judgments without resting, and often only two could be made. It was sometimes impossible to measure the curvature at the exact point when the light passed under the cross-wires, so the curvature had to be observed carefully and compared with the distance between the wires, and a judgment made when the wires were not superimposed upon the image. With each intensity of light two judgments were taken, one when the cross-wires had to be brought nearer together, the other when they had to be moved farther apart. Several series of measurements were made by different observers, and the results averaged up and compared.

The following curves and tables give the different observations for the nine different intensities of white light,[27] and the three speeds which were used. In the case of the high speed the light moved across the opening in the screen placed before the pendulum at a velocity of about 1.5 ft. per sec. The middle speed was about 1.27 ft. per sec. and the low speed about .917 feet per sec. In all cases an oblong image was used, ¼ inch wide and 2 inches long. The numerals on the left of the plotted curves give the apparent curvature of the image in sixteenths of an inch, and were obtained by measuring the distance between the cross-wires when this distance measured the apparent curvature of the image in the way described above. The figures at the bottom designate the different intensities of light which were used. Number one is the greatest intensity, number nine the least; the others those in between.

High Speed. This curve shows very well indeed what seems to be typical of the relations between the intensity of the moving light and the apparent curvature of the front edge of the image. With the lowest degrees of intensity the amount of the curvature is very little. Sometimes it was difficult to measure it at all. The light was so faint and the speed so rapid that probably very little reënforcement or irradiation took place, although what did occur would show its presence most prominently, since, on account of the high speed at which the pendulum moved, any part of the image which should come into consciousness ahead of the rest, even by a very little time, would appear considerably in advance of the rest of the image in space. Of course a certain amount of time would be required for the stimulus to spread itself over the retina, since it has to overcome a certain amount of resistance in the nerve-layers, and if this time were not given, the curvature of the resulting image would be of course decreased. As the light brightened, however, the curvature increased rapidly, until finally, when the intensity of the light neared its highest point, the curvature ceased becoming greater, and finally decreased. The mean error in eight judgments taken by two people for each intensity of light was about .099 in.

The measurements with the middle speed were very similar. The curvature with the lowest intensity of light was somewhat greater than when this same light moved with the highest speed. The maximum point of curvature was reached with a light of less intensity, and the curvature was less. When yet higher intensities were used, the curve decreased rapidly. The amount of curvature was also much less with the brightest light than with the higher speed. The following table shows the judgments of three observers for this speed:

The low-speed measurements show the same general tendencies except that the curvature is smaller with this speed when the faintest lights were used than with any of the others. The maximum curvature is also less and occurs with a more intense light than with the middle speed. These modifications offer no special difficulties. Since the light moves slowly, although the centre of the image, which is reënforced by induced excitation from the ends, does appear in consciousness in time ahead of the rest of the image, yet it does not appear so far in advance of the rest of the figure in space as it would if the light moved with a higher speed. While the same difference in brightness between the centre and the ends of the image should make one part appear in consciousness just as far ahead of the rest in time with this speed as with any other, yet since the speed is slow it would not appear to be so far ahead in space. The fact that the maximum amount of apparent curvature is less would also be explained in the same manner.

When the high and middle speeds were used the results were surprisingly consistent and the variations between the observers not very great. With the low speed the individual differences are very much more prominent. Uncertainties and variations between observers and between different observations of the same observer became greater and greater as the intensity of the light decreased. One seemed to be approaching the lower limit of induction, below which, even if the spreading of light stimuli through the retina took place at all, it was to such a slight extent that it made no very marked difference in the appearance of the moving image. Individual differences are very great in this respect. For instance, my own average measurement was 15⁄64 in. of curvature for the image produced when a light of the lowest intensity moved at the lowest speed. Mr. Vaughan's measurements averaged 30⁄64 in., a measurement of just twice as much for the same light at the same speed.

So far there has been given only a general view of what happens when an oblong moving image appears convexly curved. It may be well to consider the different causes which determine a certain curvature of the image and see how they are related.

As we have seen, the curvature is a function of the difference in intensity of various excitations between the centre and the ends of the retinal area excited. This difference is modified both by the objective intensity of the light and by the speed at which the light moves. Its efficiency to produce curvature is also modified by both these factors, since it requires a certain small amount of time for the irradiation to take place. If this time is not given by the too rapid passage of the image over a certain part of the retina, the difference in intensity will be lessened and the curvature therefore be decreased. On the other hand, if the figure moves too slowly, although this difference in intensity may be as great as possible for the brightness of the light which is acting, yet the curvature may be lessened on account of the fact that, although the centre of the image does appear ahead of the ends in time, yet it does not appear so far ahead of the ends of the image in space as it would if the same difference in intensity were present and the image moved more rapidly.

It will be remembered that the intensity of the centre of the figure owes its increase to reënforcement by excitation irradiating from the surrounding points. It seems only reasonable to suppose that this added intensity due to irradiation does not increase without limit, or with exactly the required ratio to produce a curvature of the front of the image which becomes continuously greater as the intensity of the light increases, indefinitely.

If this be so, then, at a certain brightness, the difference in intensity between the ends and the centre of the image will have reached a maximum, and a further increase in brightness of the light will not serve to increase the apparent curvature of the image, but rather to decrease it.

COLOR IRRADIATION

The color of the image has a decided influence upon the amount of perceived curvature, independently of the intensity.

The following experiments were performed with lights of different colors, in order to investigate the relations between the kinds and amounts of irradiation of the different colors by comparing the amounts of the curvatures obtained. We encountered a good deal of difficulty in fixing upon a proper method of comparison between the different colors. Finally it was decided to use such intensities of light as would give a maximum amount of curvature with each of the four primary colors, to measure this amount in each case, and also to measure the amount obtained when the intensity of the light was greater and less than that required to give the maximum.

It was found that very different objective intensities of light were required to give a maximum amount of curvature with the different colors. The colored images were obtained by placing colored pieces of glass in a frame which stood before the source of light. The intensity of the light could be regulated by interposing or taking away pieces of ground glass which rested in the frame between the light and the colored glass.

The red glass gave a nearly saturated color, but its place in the spectrum was rather nearer the orange than I could have wished. It was a thick piece of glass and absorbed a great deal of light. A 32-candle-power light with four pieces of ground glass in front of it gave a maximum curve for most observers.

The yellow gave a very well-saturated color with light from the incandescent lamps which we used. The glass was thinner and absorbed less light than the red. A 32-candle-power lamp with three pieces of ground glass usually gave the maximum.

Two 32-candle-power lamps and one of 24-candle-power were required with the green.

The green glass was not quite so saturated in color as the red or yellow. It was a slightly yellowish green. Red and yellow rays were visible through it to some extent when it was examined through the spectroscope. It absorbed somewhat less light than the red and decidedly more than the yellow. The maximum curvature was obtained when the source of light was screened with four pieces of ground glass.

The blue glass was a bluish violet, very heavy, and absorbed a great deal of light; it allowed many red and violet rays to pass through. It was necessary to use with this glass two 32-candle-power lamps and one 100-candle-power. When the combined light of these lamps was reduced by interposing three thicknesses of ground glass, the maximum curvature was observed. The light which then appeared, however, seemed of greater intensity than any other which gave a maximum.

The curvature of the white light was measured again in order to compare it with the colored lights. This was necessary, since the work was done with a different set of subjects and the former work showed individual variations. An 8-candle-power light was used as before. This, reduced by four pieces of ground glass, gave the maximum in most cases.

The following curves and tables show the average of the observations of four subjects. In the table the figures under the columns numbered 1 and 2 represent the amount of curvature perceived when the intensity of light was greater than that required to give a maximum under 4 and 5, when the light was not strong enough to produce a maximum of curvature. The columns numbered 3 represent the greatest amount of curvature perceptible with each color.

The curves shown in the diagram represent these measurements plotted out.

The measurements were made in the same way as before, and are given in sixteenths of an inch.

In the diagram the abscissas represent the different intensities, the ordinates the amount of the curvature. To avoid confusion, the curve of the average of all the colors is left out of this diagram.

It will be noticed in these records that the different colors give very different measurements of curvature. Green gives by far the largest, being greater than any of the others at every point. Since the process of obtaining the curvature was the same with all the colors, these differences in curvature can only be due to inherent differences in the processes which give the sensations of the different colors. It cannot be due simply to one sort of intensity process, the same for all the colors, otherwise the curvature of all the colors would be the same. At the same time the curvature of the image is due to differences in intensity of excitation between one part of the image and another. There must be, therefore, a retinal excitation in some respects different for each color, capable of its different degrees of intensity. Of course these individual differences would have a decidedly limited range, for, as every one knows, if the intensity of a color be increased sufficiently its saturation vanishes and white appears in its place, while if the intensity be decreased without limit, black appears. It may be that different degrees of excitation in the different processes have different rates of time in coming into consciousness, so that an equal degree of difference in excitation between the ends and centre of the green image and the ends and centre of the red image would give decidedly different amounts of curvature, if it took a longer time after the centre of the green image had appeared in consciousness for the ends to appear than it did in the case of the red.

The time-differences might be greater with the same differences in intensities of excitation with one color and another. Or it may be that the excitation spreads in a different manner with each one of the colors, and therefore gives differing degrees of reënforcement with the different colors, and thus produces different amounts of curvature.

It is noticeable also that the amounts of curvature are related to one another in a peculiar way. Green has the greatest amount of curvature, yellow the next. Red is greater than blue with the higher intensities, they are equal at the maximum, and blue is greater than red when the lower intensities are used.

When a spectrum showing a fair degree of saturation is observed, it is seen that the point of greatest brightness lies in the yellows. As the intensity is heightened, this point moves toward the red end, and as it is lowered, it moves toward the blue.

It will be seen that the relation between the different amounts of curvature for the different colors is the same as that between the different degrees of apparent brightness when the intensity of the colors in the spectrum is decreased. It is not that of the extreme case of the phenomenon of Purkinje, but when the point of brightness has moved from the yellow into the yellowish greens or decidedly to the right of the place it occupies in the normal spectrum. In that case yellow would be the color second in brightness. In our measurements the amounts of curvature obtained from yellow images were next in size to those of green. The red and the violet-blue which we used would therefore be about equal. It is a noteworthy fact, however, that when the intensities of light (1) and (2) are too great to give a maximum of curvature the amount of curvature obtained with the red is greater than that of the blue, while with the intensities which are too small (4) and (5) to give a maximum the blue curve is greater than the red.

As yet it has not been possible for me to find an interpretation of these facts which would seem to meet all the requirements, and I should not wish to offer any explanation at present. The question of the possible connection of this phenomenon with Purkinje's is probably important for any explanation, though it is possible that the arrangement of the curves is merely a coincidence, yet this hardly seems likely, and it would seem as if an explanation of the connection would involve an attempt at explanation of Purkinje's phenomenon, and lead at once into the most doubtful problems of the theory of visual sensations.

It is also noticed that the series of numbers obtained when the amounts of curvature of the different colored images, at the different intensities given in the foregoing table, are averaged up, and the curve of the average of all colors is thus obtained, that this average curve is very like that obtained for the white light. These curves and the series of numbers which they represent are here given.

It will be noticed, however, that the curve for the white light, while nearly equal to that of the average for the colored lights at the maximum point, nevertheless falls considerably below it at each end. This may possibly be due to the fact that with white light it was only necessary to use an 8-candle-power lamp as a source of light, so that when pieces of ground glass were interposed in order to reduce the intensity of this light, very much greater reduction would occur with this comparatively weak source than would take place with an objective source of light of far greater brilliancy, as was the case with the colored lights. Hence there would be a greater difference in absolute intensity between intensities 1 and 3 with the white light than between intensities 1 and 3 with any of the colored lights, or that represented by their average. Thus the falling of the curve of the white light at each end may possibly be due to the fact that there is a greater difference in intensity represented by these parts of the curve in the case of the white light than is represented by the analogous portions of the curve of the average of the colored lights.

It will be remembered that these measurements were obtained when the image was upon the fovea, so that the white obtained was "cone white," and not due in any way to the functioning of the rods. It is interesting to note that the curve of the white is very near that of the average curve of all the colors, though I should hesitate to draw the conclusion from this that "cone white" is due to a mixture or fusion of all the excitations corresponding to the different colors.

In regard, however, to relations of the amounts of curvature of the images, there are several further considerations which ought to be noted. In the first place all three measurements were made when the images were entirely on the fovea. In the fovea there are no rods, so, whatever the connection of these facts with Purkinje's phenomenon, it is one which has to do with the functions of the cones alone.

Professor Hess, in his experiments upon totally color-blind subjects, found that exactly the same oscillatory processes in the course of the stimulation occurred with them as with normal subjects. He also found that the difference in the time of latent perception between the foveal and extra-foveal parts was the same for one set of subjects as for the other. The sole difference seemed to be in the one fact of not being able to perceive colors. From these facts it does not look as if the difference between seeing colors and color-blindness were by any means always due to the absence of cones in the color-blind eye. It may of course be true that an eye which is deficient in cones or which has a lesion of the fovea would have poor color perception. But it seems also true that an eye which, in so far as the rods and cones and their purely retinal processes were concerned, seems to be normal in every way, except perhaps that somewhat different intensities were required to give the same reactions (which might be explained by different central processes), may nevertheless belong to a person who is totally color-blind or totally unable to perceive colors with that eye.

If this should prove true, the cones would still be regarded as the end organs of color perception, but the cones would only give sensations of color when functioning in conjunction with some other more central process. The usual cases of color-blindness would be attributable, not to any deficiency in the cones or any other retinal process, but to a defect in this more central process, which, working in conjunction with the cones, gives us our sensations of color.

The usual views of the functions of the rods would not be affected by these considerations. They would continue to be regarded as end organs whose main business it is to deal with weak stimulations and to notice movement in objects whose images fall upon the periphery of the retina.

But the main difference would be that all cases of partial color-blindness and most cases of total color-blindness would be explained by lesions in the brain rather than abnormalities of retinal structure.

VARIOUS FORMS OF IRRADIATION

The endeavor to explain these phenomena of moving images which we have been considering and an examination of the literature of the subject have led me to conclude that there are five distinct types of irradiation. These are:

1. Irradiation α. The very rapid spreading of the nervous excitation over the retina, which extends far beyond the borders of the image and which occurs immediately upon stimulation. It is most distinctly observed with stationary sources of illumination of the briefest duration perceptible. This kind of irradiation has been discussed at length by Charpentier and Bidwell.

2. Irradiation β. As the apparent form of the moving image becomes distinctly perceptible, such irradiation takes place within the confines of the stimulated portion of the retina, so as to make the excitation present at favorably situated localities more intense than that of other places. The portions which are so situated as to receive this reënforcement are the first to enter consciousness. The various phenomena discussed earlier in this paper furnish examples of this process, as well as the phenomenon of the curved image.

3. Irradiation γ. After, and in part during, the rise and development of the reënforcing irradiation, emanations of nervous excitation of small intensity proceed from the borders of the stimulated portions and from the after-images, rapidly extending themselves over the retina and gradually decreasing in intensity.

4. Irradiation δ. When two fields of different intensities are brought into juxtaposition, the field having the greater intensity will enlarge itself at the expense of the other. This constitutes what has been usually termed irradiation, and is observable with stationary objects. This enlargement varies with the time during which it is observed, the absolute intensity of the light employed, and the relative differences in intensity of the two fields. Its angular extent under determinable conditions is constant, although it varies considerably from one observer to another, and with the same observer at different times. Its physiological explanation is probably similar to that of the other kinds of irradiation, viz., the spreading of nervous excitation over or through the layers of the retina, although various factors of accommodation, dispersion, achromatism, astigmatism, etc., enter in and modify the totality of the phenomenon. It will be noticed that reënforcement occurs in this kind of irradiation as well as in certain of the other forms. The sides of the dark fields upon which this form of irradiation shows itself appear curved inward at the centre, apparently showing the presence of a greater excitation in the lighter fields next to the centre of the darker ones.

5. Irradiation ε. When a luminous object has been observed for a long time (from thirty seconds to several minutes), the whole surrounding field will be flooded by a faint haze of light, which within certain limits increases in intensity the longer the stimulation is present. This phase has many characteristics of the first and most rapid kind of irradiation, and possibly represents a discontinuance of functioning, through fatigue of certain nervous mechanisms which prevent the spreading, or inhibit the perception, of this irradiatory excitation after the form of the object is distinguished clearly. It is probably largely due to such a mechanism that we are enabled to perceive as clearly and sharply as we do the outlines of objects which differ greatly in intensity from their backgrounds.

W. McDougall has developed a theory of inhibition[28] which he uses to explain the more usual kinds of irradiation. This explanation harmonizes very well with the results of my own experiments and helps to explain all the kinds of irradiation we have distinguished. Briefly stated the theory of inhibition is this: there is a transference of nervous excitation or energy through the nerves and from one neurone to another. This living nervous energy he calls neurin. The place where it crosses from one neurone to another he calls the synapse.

Of course these conceptions are not to be taken too literally. They seem to be rather, if they are to be of any value at all, a convenient way of handling certain neurological processes of which at present we know very little, but whose grosser modes of action are comprehended more easily by the use of such terms as "resistance," "neurin," etc. It is in this manner that I wish to be understood in the use I have made of Dr. McDougall's valuable contributions to the methodology of the subject.

Neurin is generated when a stimulus is applied to the afferent nerves. When a strong stimulus is applied, neurin is generated rapidly, and discharges across the synapse to the efferent neurone in a series of very rapid discharges like the multiple discharge of a Leyden jar. When the stimulus is weak the discharges take place more slowly. Consciousness occurs at the time of the discharges and occurs in pulses. When these pulses occur in very rapid succession we experience a continuous sensation, when the discharges take place at a lower rate we are conscious of a pulsative sensation, as for instance, in the visual phenomenon of Charpentier's bands.

Continuance of stimulation continues to produce neurin, but the multiple discharges caused by the incoming neurin cause fatigue in the synapses, and the neurin seeks new paths of discharge through unfatigued synapses.

The resistance of the synapses is first lowered by the incoming neurin, then raised again through fatigue. When the resistance is first lowered upon application of the stimulus, the neurin which might go through other channels of discharge is "drained off" through the synapses which have their resistance thus lowered, then as the resistance is again raised through fatigue, it again seeks discharge through synapses which are unfatigued.

Applying these conceptions to the different kinds of irradiation we have distinguished, we can bring them all under one category. One might remark in passing that, in so far as our purposes are concerned, it makes very little difference whether we regard consciousness as occurring upon the crossing of neurin from one neurone to another, or upon the charging and discharging of a cortical cell, so long as the conditions already referred to are maintained, viz., first, a lowering of resistance as the incoming nervous excitation finds its way through the cell or across the synapse, and then the gradual rise of resistance and its conduction into new channels by fatigue of the synapse, or exhaustion of the cell and a consequent turning of the excitation through fresh cells across fresh synapses before its passage into the efferent nerves.

When a light stimulation falls upon the retina, during the first one hundredth or one fiftieth of a second the nervous excitation of neurin will spread about generally through the retina for a considerable distance from the point immediately excited. Thus by means of the fibres of the retina faint excitations will go to the brain from all these different points, so that one will perceive a faint cloud of light, similar to that described under the first kind of irradiation. Moreover, since the portion of the retina directly stimulated by the light will have the most intense stimulation, this part will come to consciousness somewhat more quickly than the outlying parts, so that the cloud of light will first seem to spread outward from its source, and then, as the resistance in the synapses is lowered through the more intense stimulation of the part of the retina upon which the light directly falls, the outrunning excitation will be "drained off" from these portions of the retina outside of the borders of the image, and the halo or cloud of light will appear to contract again. This was observed by Charpentier and Bidwell, and in our own experiments.

Moreover, in case the synapses corresponding to the portions of the retina indirectly stimulated should have themselves periods of discharge and periods of charging, we might expect to see dark rings upon this halo, this was also first observed by Charpentier and Bidwell.

Secondly, as the resistance is lowered in the central organs corresponding to the end organs of the retina upon which the stimulation falls, the image tends to assume its true form, but irradiation has been, and probably still is, present through the layers of the retina, so that certain favorably located portions of the image secure reënforcement by means of this irradiation, in the manner described, and these portions appear in consciousness sooner than the others. This reënforcement, in the case of the travelling oblong image, will make it appear convex. Moreover, since the resistance of the synapses corresponding to the centre of the oblong images will be less than those corresponding to the ends, there will be a certain tendency to "drain off" the stimulation from the rest of the image, a sort of reënforcement of the reënforcement, which will also help in making the image appear curved. Of course all the conditions which we found to modify the curvature of the images will still hold good, these conceptions being used only to describe the course of events which causes the image to appear convex. Thus a very weak or a very intense or a very long or an excessively short image will not appear curved, owing to a lack of difference in intensity between the ends and the centre great enough to produce perceptible curvature.

As to the third kind of irradiation, that which proceeds from the ends of the moving image over the unstimulated portions of the retina, and which has the appearance of long streamers of light extending outward and backward from the moving image, this may be regarded as being in certain respects a form of the first and very rapid kind of excitation. It may well be that all the outrunning excitation which occurs immediately upon stimulation does not find its way to the central organs through those nerve-paths which correspond to the directly stimulated portions of the retina, even after the form of the image may be very clearly determined, but that some excitation proceeds outward from one retinal element to another, arousing fainter and fainter excitation as it proceeds. This being the case, we should expect to find these streamers of light from the ends of the image extending outward and backward over the retina. Of course the faster the image moved and the more intense it was, the longer then would be these streamers. For if the image moved very fast, very much less of the excitation would be "drained off" through the directly stimulated portion, and thus more of the excitation would be left behind, so to speak, by the image when it moved along rapidly, and this would appear to drag farther and farther behind. Of course these streamers being curved backward would appear more curved the faster the image moved, and if the pulsative processes occurred with these stimulations which occur in the course of other retinal stimulations, we should have Charpentier's "palm-branch" phenomenon.

The fourth kind of irradiation which we have defined is of course the best-known form, and is that which has been the most discussed by the many writers on the subject. It will be remembered that this form appears in stationary objects which have been observed for some little time (from four to ten seconds), and consists in the apparent enlargement of a more intensely illuminated portion at the expense of a less illuminated one. This enlargement occurs after all trace of the first kind of irradiation has vanished, and of course no trace of the third kind comes in, since the object is stationary. The course of events may then be somewhat as follows. In the first perception of the object we have the wide-spreading irradiation described. Then way is made through the synapses corresponding to the stimulated portion of the retina, and the wide-spreading irradiation is drained off through these open channels, so that the image contracts again to its proper size. But at the same time it is not likely that there will not be a slight irradiatory enlargement of the borders of the image. For irradiation is present within the confines of the image. This is shown not only in the case of moving images, but also in the fact that the edges of the less intensely illuminated portions of the field are curved inward, this being most probably due to the fact that the centres of the contiguous luminous portions are reënforced by irradiation proceeding from the direction of both the ends.

Not all of the excitation proceeds to the brain from the directly stimulated portions of the image merely, but a little irradiates over the borders and causes an apparent enlargement of the brighter field. It has also been shown by Plateau and others that the amount of irradiation increases both with the intensity of the stimulation and with the time during which it acts. Of course, as to the intensity there is no question. As to the time-element, it may be that the excitation at the border spreads rather slowly outward after the previous contraction of the image to its proper dimensions, which takes place within a very short time after stimulation, until a sort of balance is reached between the tendency of the image to enlarge itself through irradiation and the tendency for this irradiatory excitation to be drained off through the nerves corresponding to the stimulated portion of the retina, after which no further apparent enlargement takes place.

In some of our experiments with dots we found that after a dot of the proper intensity of illumination had been steadily gazed at for some time the centre would appear dark. This seems to be due to the fact that the centre of such an image was reënforced by irradiation, so that the nervous mechanism corresponding to it became fatigued more quickly and the stimulation at the centre no longer gave such intense sensations as the rest of the figure, but appeared darker.

Passing to the fifth and last variety of irradiation, this seems due to fatigue in the inhibiting apparatus which reduced the spread of the first kind of irradiation. Following out the scheme we have applied, it would seem as if the channels which were first opened by the direct stimulation became blocked through fatigue, and, therefore, the excitation produced in the retina were forced to seek new paths through to the brain by means of the nerves which proceed from the unstimulated portions of the retina. Thus if the resistance through fatigue occurs slowly, the excitation which spreads may increase in intensity and in extent. So, as the resistance increased, a portion corresponding to the directly stimulated portion and its slight irradiatory enlargement of the borders would be surrounded by a cloud of light growing in size and intensity.

Of course the limiting case would be when the external cloud of light attained as great or even greater intensity as the stimulated portion, but such a case would probably be impossible to realize because of other conditions which would prevent.

It may be that this fifth variety is caused partly at least by a cortical spreading of the excitation. But it seems to me more likely, in view of the fact that we could find no irradiatory enlargement of the binocular portions of stereoscopic images and for a number of other reasons, that the induction is retinal in character and that after the resistance through fatigue has arisen in the central organs the stimulation spreads out over the retina to the unstimulated portions of the field and proceeds from thence to the brain. This seems more probable than that the stimulation continues to be confined merely to the stimulated portion of the retina, but seeks passage from one portion to another of the brain through fresh neurones which branch only from those nerve-tracts which proceed from the directly stimulated portions of the retina.

To conclude; we have seen that there are various forms of irradiation which take place during the perception of stationary and moving sources of illumination.

That there are certain modifications in the form of a moving image which are probably due to one of these processes.

Concerning color irradiation it was found that the curvature of the images varied with the color of the light, so that a figure illuminated by a colored light of one intensity would not have the same curvature as one illuminated by a light of the same intensity but of another color. Green gives the greatest curvature, yellow the next, red and blue about the same. In other words the differences in curvature of the images follow the order of the brightness of the colors in a spectrum the intensity of which is much reduced.

From a consideration of these phenomena we were led to discuss the functions of the rods and cones in the retina of the eye, and the suggestion was made that differences in color-vision were due to central rather than retinal processes, and that in many cases of partial or total color-blindness the retina would be found normal and the defect in vision due to a lesion in some more central structure.

The various forms of visual irradiation which have been described by a number of different writers we found to be all forms of one rather simple process. Resistance, removal of resistance upon further stimulation, and recurrence of resistance through fatigue in some part of the optic tract, together with the spreading of stimulation over the retina (probably through the molecular layers) from one afferent nerve to another are assumed as the minimal requirements which are sufficient to explain the five forms of irradiation which have been considered in this paper.

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BY F. M. URBAN

The material of this paper was obtained by an experimental investigation which was carried on in the Harvard laboratory from February, 1904, till June, 1905. The immediate purpose of these experiments was a study in the expression of the feeling-tone of simple sense-stimuli. Breathing and circulation were the functions the changes of which were observed by tracing the curves of thoracal and of abdominal breathing and the sphygmographic curves simultaneously. Acoustical, tactual, pain, and smell sensations were studied in this way, special attention being devoted to the smell and pain sensations. These stimuli have the advantage that the physiological reactions of the subject are more uniform than the reactions to other stimuli. The number of experiments performed in this investigation was large, although a subject was never experimented on for more than forty minutes, because the facilities of the laboratory allowed a continuous experimenting for several hours a day on different subjects. All the experiments were performed on trained subjects. Only the changes in the form of the sphygmographic curve will be discussed in this paper. The results of this observation confirm the observations of previous investigators in so far as the same changes in the curves were observed and the introspections of the subject were, on the whole, similar to those obtained by other observers. It does not seem probable, however, that a satisfactory discussion of the results can be given on the basis of merely mechanical measurements of the curves, and it, therefore, seemed necessary to reconsider the principles of the theory of the sphygmographic curves.

There are two methods which can be applied to the study of the psychology of feelings. They are called the method of impression and the method of expression. The first is a purely psychological method, while the latter is confined by its definition to the study of the physiological changes which are the accompaniments of feelings. The method of expression is never used as a pure method in investigations which are carried on for psychological purposes, because the introspections of the subject must be compared with the physiological results. It therefore has the character of a mixed method. The first experimental investigations into the psychology of feelings were started by Fechner, who employed the pure method of impression. At this time, however, the apparatus for studying the circulation had been greatly improved and sooner or later these instruments were sure to be used for a more exact study of the influence of feelings on circulation. It was to be hoped that the crude observations on the changes of the heart-beats and of the circulation under the influence of feelings might be followed up in detail.

Darwin laid stress on the importance of certain bodily accompaniments of feelings, and he inaugurated the genetic explanation in this field. But even if the genetic explanation is successfully carried through, human psychology remains unexplained, and, furthermore, those emotional expressions which Darwin described form only a part of the physiological accompaniments which may be observed with the instruments now in use. The invention or at least the great improvement of these instruments is due to the investigators in the middle of the last century, and a more thorough understanding of the delicate changes of respiration, circulation, and of temperature was not possible before the construction of these sensitive recorders. It seems that Mosso was the first to observe these small changes under the influence of mental activity in general, and feelings in special; in this sense it may be said that Mosso started the experimental physiology of feelings. The discovery of the influence of feelings on circulation is very important, and it is to be appreciated that Mosso saw these slight changes which escaped an observer like Marey. In the Mémoire offered to the Academy[29] on March 26, 1860, Marey gives a great number of circumstances which influence the sphygmographic curve, but feelings or mental phenomena are not mentioned in this list. It is true that he speaks in a later publication[30] of the influence of "moral ideas" on the circulation and makes the hypothesis that these ideas influence the circulation in the same way as other disturbing influences, i. e., by changing the peripheral resistance. At this time Marey was already in possession of his sphygmograph, but nothing in this passage indicates that he saw the influence of feelings on the tracings. On the contrary, the words "Sans rien livrer à l'hypothèse" seem to indicate that Marey had no other facts in mind than those commonly known. He certainly did not follow up his observation, and his statement at this point does not differ very much from the observations of the old psychologists, that emotions change certain physiological functions, of which a more or less complete list is frequently given.[31]

It certainly is a long step from this vague statement to Mosso's experimental investigations. His new instruments, the plethysmograph, and the balance, enabled him to study the distribution of the blood,[32] and he observed the influence of mental phenomena on the circulation,[33] on the bladder,[34] and on the temperature of the brain.[35] His work, "La Paura," describes the physiological effects of emotions somewhat in detail.

The way toward applying the method of expression to the study of emotions was shown by the results of previous physiological investigations. Casual observations of the influence of certain sense-stimuli on respiration and circulation were made by Naumann, Couty and Charpentier, Thanhoffer, Dogiel, Gley, Mays, Istomanow and Tarchanoff, Féré, Delabarre, and others.[36] The changes of breathing seem to be of greater importance, and some writers account breathing the most delicate physiological index of feelings.[37] It seems, however, that a satisfactory treatment can be obtained only by direct comparison of the respiration and circulation, and it now but seldom occurs that circulation is observed exclusively.

There are three different instruments for observing the circulation: the plethysmograph, the sphygmomanometer, and the sphygmograph. Each of these instruments allows one to observe a different feature of the circulation. The sphygmomanometer records the pressure in the artery; the plethysmograph records the volume of a certain part of the body; and the sphygmograph records the movement of a certain part of the arterial wall. The curves traced with the sphygmograph indicate to a certain extent the pressure of the blood, and sometimes they are called curves of blood-pressure to distinguish them from the plethysmographic curves which are called curves of pulse-volume.

The invention of these instruments is due to physiological investigations of the pulse. The problem of studying the pulse by graphic, or at least experimental methods, begins with the investigations of Hales and Poiseuille. The first great success in this line was the construction of the "Kymographion" of Ludwig, but this instrument had the disadvantage that it could be applied only by scission of an artery. This circumstance, of course, confined the application of the instrument to the study of the pulse of animals. After several attempts by Hérisson, Chelius, and others, Vierordt succeeded in constructing his sphygmograph, by which curves of the normal human pulse could be obtained. Some years afterwards Marey constructed his much more sensitive instrument, which was made still handier by the use of air transmission. Buisson was the first to use air transmission for sphygmography, but Upham had used it before for similar purposes. A considerable number of sphygmographs has been constructed since, and though they may show some improvements in detail, the technique of the sphygmograph has made no marked progress since Marey, and his instrument has been found by experimental tests remarkably exact.

The curves traced with the sphygmograph are extremely variable in shape and size. In almost every normal curve, however, a steep ascent may be seen; it is called the up-stroke or percussion stroke, and this part of the sphygmographic curve has the name of the anacrotic phase. This line of ascent ends abruptly and within the limits of the usual speed of the recording drum it goes over into the descent by a sharp angle. The descending part of the curve is called the catacrotic phase. The descent is not so abrupt and is not a more or less straight line, but is interrupted by secondary elevations. The first secondary elevation is the largest and is called the dicrotic.[38]

These secondary elevations were seen first by Chelius and Vierordt, and from the beginning they aroused considerable interest. It was known that sometimes during fever the pulse takes an abnormal form, where two beats of the pulse, a strong one and a weaker one, may be felt for every heart-beat (pulsus bis feriens). This form of the pulse was thought to be entirely abnormal and it was therefore a great surprise for the first modern investigators to find these secondary elevations in tracings of the normal pulse curve. The conviction of the abnormality of the dicrotic pulse form was so firm that Vierordt always applied his instrument in such a way that it did not trace the dicrotic elevation, although it was sensitive enough to trace the exact form of the pulse curve. Marey, however, used his much more delicate instrument and found the dicrotic elevation in most of the normal pulse curves.[39] For this reason Marey's sphygmograph met at first with considerable criticism (Meissner), but the critical examinations by v. Wittich, Buisson, and Mach showed that the dicrotic elevation could not be due to an error of the instrument, for so great an error was out of question, and there no longer remained a doubt as to the genuine existence of the dicrotic elevation in the normal pulse curve. The sphygmograph, thus, had revealed two new and surprising features of the pulse; (1) The ascent and the descent do not take place with equal rapidity, the ascent being steep, the descent gradual;

(2) the descent is interrupted by secondary elevations. Neither of these facts could be observed by applying the finger and it seemed important to explain them. The explanation of the dicrotic promised to be of special interest, as it was shown that abnormal dicrotism is in close relation to the normal form of the pulse curve.

This caused considerable interest in the observation of the pulse, and the sphygmograph was supposed to be of the greatest importance for medical diagnosis. Burdon Sanderson,[40] Landois, Lorain,[41] Ozanam,[42] Pfungen,[43] Riegel,[44] Roy and Adami,[45] and others have studied the sphygmographic curve under abnormal conditions, and wellnigh all diseases have been studied by these observers with the sphygmograph. The results were ambiguous and did not seem to justify the amount of work spent on these observations. The enthusiasm for the sphygmograph subsided, and it was no longer expected to obtain a diagnosis, or even, indeed, a prognosis of a disease from mere inspection of a pulse curve. Later investigators, in fact, confined their research to the proof of the ambiguity of the sphygmograms, which could be valuable only in connection with other observations. It could not be hoped that an explanation of the abnormalities of the pulse curve would be found before an understanding of the normal form was attained. It, therefore, seemed necessary to decide between two theories of the origin of the normal pulse curve, which had opposed each other almost since the discovery of the existence of the dicrotic elevation. Both theories chiefly refer to the origin of the dicrotic, and they agree on this, that the dicrotic elevation is due to a wave travelling in the blood, but they disagree on the direction in which this wave is moving. These two theories may be called the theory of the peripheral, and the theory of the central origin of the dicrotic wave.

The theory of the peripheral origin of the dicrotic wave assumes that the change of pressure which is indicated by the dicrotic elevation originates somewhere at the periphery and travels through the arteries towards the heart.[46] Commonly it is assumed that the dicrotic originates in the arterioles. This theory has been mentioned first, because it is the simpler in every respect, though the less probable. The origin of the dicrotic wave according to this theory is similar to the origin of the echo.

Buisson was the first who gave an explanation of the dicrotic elevation by assuming a central origin of this wave. His theory was adopted by Marey, who stated it in this way. The action of the heart causes the blood to be pumped into the aorta with considerable strength. The blood leaves the aorta by its inertia and expands the arterial system. In the arterioles it finds an obstacle and being reflected it flows back to the aorta. But there it finds the semilunar valves closed and a new wave is produced by reflection. This wave has an effect similar to the first, and this reflection of waves lasts until the valves are thrown open again. The existence of several secondary waves is explained by the great velocity with which the blood travels through the arterial system.[47]

This theory is open to many objections. First, there is no reason why the blood wave should not produce a dicrotic elevation when it flows back to the aorta. Second, the narrow lumen of the arterioles cannot be an obstacle to the flowing blood, because if an artery splits up into small branches, the sum of the lumina of the branches is greater than the lumen of the artery. Lack of space, therefore, cannot be the cause of the reflection of the pulse wave. Marey, finally, is mistaken in his conception of the effect of the blood pumped into the aorta by the action of the left ventricle. He supposes that the entering blood pushes before it the whole column of blood in the arteries. This view is refuted by the actual measurements of the velocity of the pulse wave, because if it were true the pulse would appear at the same moment in every part of the body.[48]

These are the more obvious of the arguments against Marey's theory. Other investigators have tried to state a more correct theory of the central origin of the dicrotic wave. Landois's theory belongs to this type of improved theories of the central origin. The action of the left ventricle, according to Landois, causes the primary pulse wave which travels down the arterial system, until it is extinguished in the arterioles. The walls of the arteries are expanded by the arriving blood wave, and, when the valves close, they force the blood onward by their elasticity. There is a free way to the periphery, but the blood pushed towards the heart finds the semilunar valves closed and is reflected. In this way a new positive wave originates which may produce in the same way a secondary or tertiary wave.[49]

It seemed necessary first to decide between the theories of the central and of the peripheral origin of the dicrotic wave. Many investigations have been carried on for this purpose, and some of them bear witness to the high ability of the investigators. It is, however, remarkable that the arguments which have been brought forward in favor of one hypothesis chiefly consist in reasons why the other hypothesis should not be accepted. These experiments can be divided into two classes. The first class comprises all the experiments which study the relation of the pulse curve to other functions, or its dependence on various conditions. The above mentioned observations of the pathological changes of the pulse curve belong to this class. The object of frequent studies of this type has been the relation of the sphygmographic curve to the curve of the apex beat. The papers of Otto and Haas,[50] Garrod,[51] Traube,[52] Rosenstein,[53]

Maurer,[54] Gibson,[55] François Frank,[56] and Edgren[57] deal with this problem. The curve of intraventricular pressure cannot be studied in man for obvious reasons, and only in some cases has an attempt been made to compare the sphygmographic curve with the curve of intraventricular pressure obtained from animals. One of the most interesting attempts in this line will be mentioned later.

To the second class belong all those investigations, by which experimental evidence in favor of one or the other hypothesis has been collected. The experiments which belong to this class are in so far more decisive as the conditions of the experiments are better known and, therefore, easier to interpret. Von Kries proved the existence of the dicrotic in the femoral artery of an animal after having replaced the heart by a bag filled with liquid.[58] Grashey[59] and Hoorweg[60] have demonstrated the existence of secondary waves in models, on which peripheral reflection was impossible. To the same type of experiments belong Marey's[61] and Grashey's registration of the waves in elastic tubes, and Mach's[62] tracings from a mechanical model on which the resulting movement of two simple components could be registered. Without giving any physiological theory Mach showed how curves similar to the pulse curves can be obtained by the registration of a movement, the mechanical conditions of which are known.

As the results of these investigations, we may state the following facts as arguments against any hypothesis of the peripheral origin of the dicrotic elevation.

(1) Automatic registration of the pulse wave shows that the dicrotic appears sooner in the regions nearer to the heart than in regions which are more distant. The opposite would be the case if the dicrotic elevation were due to a wave travelling from the periphery to the heart.

(2) The dicrotic appears at the same time after the primary wave in a dwarf as in a tall man. This would be impossible if the wave had to travel so much farther.

(3) Inhalation of amyl nitrite makes the dicrotic almost disappear. The adherents of the theory of the peripheral origin of the dicrotic wave explain this fact by supposing that this drug dilates the arterioles and makes little reflection possible. Their opponents say that the action of the heart and the resistance of the system are so enfeebled that the backward flow is slight and gives rise only to a small wave.

(4) If an artery is opened and the blood allowed to spurt on a revolving drum of white paper a curve is obtained which shows the dicrotic elevation (the hemautographic curve of Landois). The resistance of the periphery is totally lacking in this case and the dicrotic elevation could not appear if it were due to a wave reflected at the periphery.

(5) The appearance of the dicrotic is not retarded if an elastic tube is placed between the periphery and the place where the instrument is adjusted. If the dicrotic were due to a wave reflected at the periphery it would be retarded because the wave would have to travel a distance so much greater.

These arguments prove the impossibility of the theory of the peripheral origin of the dicrotic wave. Also the other hypothesis meets with a number of serious difficulties, and we mention the following facts which are arguments not against any special form of this theory, but against any hypothesis which starts from the assumption that the dicrotic elevation is due to a wave travelling from the heart to the periphery.

(1) The descent of the catacrotic phase ought to be a succession of diminishing waves, but not a slow descent with merely small elevations.

(2) This hypothesis accounts for none of the abnormal pulse forms.

(3) The blood ought to push against the semilunar valves with a force not less than 1/2 - 2/3 of the force of the contraction of the ventricle, because this is about the relative height of the first secondary elevation with regard to the primary wave, which is due to the contraction of the ventricle.

(4) It does not account for the disappearance of the dicrotic elevation through lack of elasticity of the arterial wall: for the dicrotic elevation is most marked in youth, becomes lower in old age, and disappears in diseases like atheroma and arteriosclerosis, which impair the elasticity of the arterial wall. Landois's theory overcomes this theory only apparently, although the dicrotic would be absent, yet in that case the descent of the primary wave ought to be as steep as its ascent.

(5) This theory is refuted by the experiment of v. Kries, who proved the existence of the dicrotic if the heart is replaced by a valveless bag.

The obvious impossibility of making the theories agree with the facts does not permit one to accept any of them. All of them are based on the supposition that the dicrotic elevation is due to a wave travelling in the blood, and this belief is founded on the following argument: If a wave travels in the blood the sphygmographic curve shows an elevation; the dicrotic elevation is an elevation in the sphygmographic curve. Therefore, the dicrotic elevation is due to a wave travelling in the blood. This fallacy is responsible for the astonishing fact that the refutation of one of two apparently contradictory statements does not prove the other. It is characteristic of the present state of the problem concerning the origin of the dicrotic elevation, that a modern writer[63] calls it "inextricably complicated."

The contradiction between the theories of the peripheral and of the central origin of the dicrotic, however, is only apparent, and neither may be true, because it might be that this elevation is not due to a wave which travels in the blood. The experiments of the previous investigators seem to point in this direction. The disappearance of the secondary elevations when the arterial wall has lost the properties of an elastic body, the above-mentioned experiments of v. Kries, and the observations of Grashey and Marey on the movements of the walls of an elastic tube indicate clearly that nothing but elasticity is needed to produce these secondary or dicrotic elevations, for, in the different experiments, they are produced as well when the heart and its valves are replaced by a valveless bag as when the function of the valves is unimpaired; as well with resistance at the periphery as without, the only condition being that the walls are elastic. This proves the importance of the elasticity of the arterial wall. The experiments of the graphic registration of the movements of the walls of an elastic tube, furthermore, indicate that the conditions of this experiment are a close imitation of the mechanical conditions which prevail in the arteries. It may be expected that the analysis of the conditions of the experiment will give an insight into the origin of the sphygmographic curves, because the tracings which Grashey and Marey took from the walls of a rubber tube resemble closely the tracings of the human pulse. This experiment, first, proves that the form of the curve depends merely on physical conditions. The movement of a point of the wall of the tube depends on the following four factors: (1) The elasticity of the wall; (2) the incompressibility of the liquid; (3) the form of the original wave, i. e., the way in which the liquid is pumped into the tube; (4) the rate of outflow. If the process of pressing liquid into the tube is repeated regularly, a stationary form of movement will be obtained eventually; the amount of outflow for one interval is constant in this case. This means that eventually a state is attained where the same quantity of liquid which is pumped into the tube at one end flows out from the tube at the other. The physiological bearing of this result is that the turgor of an artery does not change without a cause. Such a change would be indicated by the going up or down of the base-line of the tracing.

The first two factors are, in physiology, studied with relative ease. The elastic qualities of the arteries have been studied since Poiseuille and John Hunter by Wertheim, Zwardemaaker, Marey, and others, and they are more or less well known. The physical properties of the blood are very nearly those of an incompressible liquid, and this is certainly true for the small pressure to which the blood is exposed in the arteries.

As to the initial form of the wave which the action of the left ventricle produces in the arterial system, we get a hint from the experiments of Grashey, v. Kries, and Marey, where the sudden compression of a bag furnished the initial shock.[64] These changes of pressure can be represented by a curve like that in Fig. 1.

So long as the contraction of the left ventricle lasts and the valves are open, the action of the heart produces a certain pressure in the aorta, but the influence of the intraventricular pressure is zero when the valves are closed. The second phase of the curve Fig. 1, where the pressure is zero, certainly gives the influence of the intraventricular pressure during the diastole, because there is no communication between the ventricle and the arterial system when the valves are closed. The question is whether the rest of the curve can represent the changes of the intraventricular pressure when the valves are open.

The first curves of intraventricular pressure were traced by Chauveau and Marey. These experiments were made on a horse, and they have been repeated since it was discovered that they can be performed also on smaller animals. Besides Chauveau and Marey may be mentioned the names of Fick, Huerthle, v. Frey, Rolleston, Bayliss and Starling. The curves obtained by various observers belong to two types; one shows the so called "plateau," the other does not. Recent experiments have proved that this difference of results is due to a difference in methods. This also is suggested by the fact that different curves have been obtained from animals of the same species. Two methods have been applied lately for testing these curves of intraventricular pressure. The first was devised by Bayliss and Starling. It consisted chiefly in the photographic registration of the movement of the liquid in a manometer tube. The photographic registration is frictionless, and the mass of the moving liquid was so small that vibrations by inertia were fairly excluded for pressures which are not greater than the intraventricular pressure.[65] The second method was used by Porter. The idea of this method was to trace only a part of the curve, not the whole. The writing lever, thus, has in the beginning of the tracing no inertia at all, and the tracing may be overdrawn but is certainly correct in form up to the next point of inflexion of the curve.[66] These tests and the repeated experiments of Chauveau leave no doubt as to the existence of the plateau.

The varying pressure from the heart which produces the pulse wave may be described in this way: The pressure suddenly rises to a maximum and maintains it for a certain time; when the semilunar valves close, the pressure drops as suddenly as it rose, and remains at zero until the valves open again. Such a function can be represented by a curve like Fig. 1, and this is the reason why the complicated action of the heart can be superseded by the compression of a bag without changing the mechanical conditions of the problem. Of course it can not be expected that a schematic curve will show all the details of the real tracing. It is suggested, however, by Frank[67] that many of the small irregularities of the curve of intraventricular pressure are due to vibrations caused by the inertia of the apparatus and that the true form of the curve of intraventricular pressure is very simple. This remark is supported by Huerthle,[68] who tested the apparatus of Marey, Knoll and Grunmach. Marey's tambour was found to be the most exact, but even this instrument produces deformities in the tracings, though the general outlines are exact. This would indicate that the schematic representation of Fig. 1 is a very close imitation of the real form of the curve of intraventricular pressure, although empirical tracings do not show right angles and straight lines. It seems, however, that the undulations of the plateau are genuine, since they are found in the most reliable tracings, and it may be possible to explain them merely on the basis of the physical conditions of the experiment.

The fourth factor of importance is the rate of outflow. We may introduce the following assumption as to the rate of outflow of the blood through the capillaries: The outflow through the capillaries is uniform in the short time of one heart-beat. The fact has been mentioned above that the quantity of outflowing blood must be equal to the quantity of incoming, for any stationary form of the pulse movement; this new hypothesis means that the velocity of the outflow is constant. One might think that this assumption is warranted by the law of Poiseuille that the amount of outflow through a horizontal capillary filled with liquid under constant pressure depends on the fourth power of the radius and on the difference of pressure at the two ends of the tube, and is inversely proportional to the constant of friction and to the length of the tube. This law has been proved mathematically and tested physically only for horizontal tubes and constant pressure. Neither of these suppositions holds for the capillaries of the arterial system. The connection between the hypothesis in question and Poiseuille's law is this. Let us suppose that an artery splits up in a great number of arterioles which go off in every direction. The amount of outflow is then a complicated function, because the law of Poiseuille does not hold for every direction of the capillaries; but it will be equal to the outflow through a tube of certain radius and certain direction in the same time. Our assumption says that the law of Poiseuille holds for this typical but imaginary tube. The essential point of this hypothesis is merely the supposition that the outflow of blood through the capillaries follows α law.[69]

It is possible to show that the graphic registration of a movement under these four conditions must give curves which correspond to the pulse curves in every respect. The action of the left ventricle causes the pulse wave which travels through the arterial system with considerable velocity. This wave expands the arteries and the whole system is filled with blood because the wave arrives by its great velocity at the periphery before the contraction of the ventricle is finished. The increased pressure forces the blood to enter the arterioles, through which it passes at a constant rate. When the valves are closed, the amount of blood decreases uniformly and the volume of the blood contained in an artery can be represented graphically by a straight line of more or less steep descent, as is shown in Fig. 2. Now the walls of an artery have to a high degree the qualities of an elastic body, and, therefore, they are forced back by elasticity after being displaced from the position of equilibrium by the shock of the arriving pulse wave. The movement of a point of the arterial wall, therefore, results from two components: (1) From the movement which it would perform if it were merely forced to remain on the surface of the blood in the artery, and (2) from the movement due to the elasticity of the arterial wall. Both movements have the same direction, because the column of blood is enclosed in a cylinder the radius of which decreases regularly, and the elastic force of the arterial wall is directed towards the centre. The direction of both forces is in the line of the radius, and the resulting movement of these two components, therefore, can be found by simple superposition. Of the first component we know that it can be represented graphically by a straight line.

An elastic force tends always to bring the body back to the position of equilibrium; if the distance is not too great, the force is proportional to the elongation. A physical body is always under the influence of friction, the acceleration of which is opposite to the direction of the movement, and therefore diminishes the velocity. The form of the resulting movement depends on the amount of friction, and, roughly speaking, we may distinguish two types of elastic movements:[70] the first type is a periodic movement, the second an aperiodic. Let us suppose that a body is carried from its position of equilibrium by a sudden impulse, which transmits a certain velocity to the body. Friction and elasticity diminish this velocity, and after a certain time the body attains a maximum elongation, where the velocity is zero. Then the body returns under the influence of elasticity and under the retardation of friction. There are two cases possible, either the elastic force is strong enough to overcome friction and to carry the body over the position of equilibrium, or it is not strong enough. In the first case, it is easy to see, the body repeats the same form of movement on the other side of the position of equilibrium, and the conditions being constant a vibratory movement results as the stationary form. In the second case the body approaches the position of equilibrium asymptotically. The first case may be illustrated by the vibrations of a magnet needle suspended with little friction, the second by the movement of a door which is regulated by a well-working shutter.

These forms of the movement of a body under the influence of elasticity and friction are illustrated in Fig. 3.

Curve 1 shows a movement where friction is so small that it can be neglected; it is, of course, a simple sine curve. Curve 2 shows the effect of friction on vibrations. The period of damped vibrations is greater than in the frictionless movement, but the amplitudes are smaller. The amplitudes of a damped vibration decrease constantly and there is a simple relation between two subsequent amplitudes. The ratio between them is constant, and, therefore, if one amplitude and this constant ratio are known, all the other amplitudes can be calculated. The amplitudes of such a movement decrease as the terms of a geometric series. The dotted line in Fig. 4 represents the rapidity of this decrease. It is obvious that the smaller the constant ratio of two subsequent terms is, the more rapidly will the amplitudes decrease. This ratio depends on friction, and becomes smaller when friction becomes greater. A vibration under heavy friction dies out quickly. Curve 3 shows a movement where friction is too great to allow any vibrations. The body does not acquire a velocity which can carry it over the position of equilibrium, but it approaches this position with ever diminishing velocity.

These are the types of movement which the arterial wall can perform by its elasticity in consequence of the shock of the arriving pulse wave. The mechanical nature of the components on which depends the form of the sphygmographic curve is, therefore, known. The constructions in Fig. 5 show how the resulting movement can be found.

These curves are constructed in this way. The lines AB represent the time of the interval of one heart-beat. The straight line EB represents the decreasing volume of the artery and the curves on AB represent the elastic movement of the arterial wall. Both are synchronous movements, and a line perpendicular to AB gives the corresponding points. The points of the resulting movement are found by arithmetical addition of the two ordinates. The results of these constructions prove that the curves show the dicrotic elevation only if the elastic force is great enough to make a vibratory movement possible. Aperiodic movements do not produce this elevation. The friction is always great for the movement of the walls of an artery, and there are only the two possibilities, of a vibratory movement which dies out quickly, and of an aperiodic movement. This accounts for the fact that the dicrotic elevation may be missing sometimes, and that in other cases several secondary elevations may be seen, the number of which, however, is always limited, and their relative height rapidly diminishing. It may be remarked that the length of the lines AB seems essential to the form of the resulting curve. Curves I and III differ very much in the length of the lines AB, while the lines AE are equal and the vibratory movements are only slightly different. The resultants, nevertheless, seem to differ very much. It is easy to see that a different speed of the recording drum will have an effect on the tracings which is similar to that of a change in the length of the lines AB in the constructions. This is one more reason why mere inspection of the curves cannot give a satisfactory result.

These constructions show that the sphygmographic curves must show great variations, since the amount of blood pumped into the system, the elasticity of the arteries and friction of the surrounding tissues are subjected very likely not only to individual but also to local and temporal variations. But under given conditions only a certain form of the pulse wave is possible, and this form does not change so long as these conditions do not change. The sphygmograms in Fig. 6 show some of the typical forms of the pulse curve.

No. I shows the influence of high arterial tension, and No. II of low tension. The first corresponds to No. II in Fig. 5, the second to Nos. I and III. Nos. IV and V of Fig. 5 show the effect of great friction and small elasticity. The constructions differ in the form of the elastic movement; the position of equilibrium is reached with different velocity in both cases. The resulting movements differ slightly in the form of the catacrotic phase. Both forms may be seen in No. III of Fig. 6. This sphygmogram was taken from an artery with low tension, and this form of the sphygmographic curve is well known as characteristic of the "soft" pulse. If the artery has lost to a large extent the qualities of an elastic body, and if the outflow is very rapid, the pulse curve shows nothing but the slight elevation of the travelling wave; No. IV in Fig. 6 shows a curve of this character.

This theory explains many surprising facts which resisted every attempt at explanation. The anacrotic part shows a steep ascent, because it is due to the sudden arrival of the blood wave. It seems that an interruption in the descent may be seen only in abnormal cases. The sphygmograms of twelve normal individuals were observed regularly by me during more than a year without once discovering an anacrotic elevation.

The hemautographic curve of Landois is produced in this way. The form of this curve depends on the velocity of the escaping jet of blood. The velocity of the blood flow depends on the resistance of the arterial system in the sense that the velocity decreases when the resistance increases. When the arterial wall is in the negative phase of vibration the lumen of the artery is smaller, and, therefore, the velocity smaller. This is confirmed by the actual tracings of the velocity of the circulation by Marey.

It is also obvious that the dicrotic elevation never can arrive before the primary wave, because the arterial wall cannot perform elastic vibrations before it is expanded by the impulse of the arriving blood wave. Neither is it surprising that the "dicrotic wave" seems to travel in the same direction and with a velocity equal or almost equal to the velocity of the pulse wave. Such a difference can be produced only by a difference in the time of the vibrations of the arteries at different points of the body. The time of one vibration is necessarily very short, and the length of this interval depends on the circumstances which determine the elasticity of the arterial wall and the friction. These conditions may be subjected to local variations. If, therefore, the time-interval between the primary and the secondary elevation is measured at two different points (e. g., at the carotid and at the radialis) a difference of time may be found. Starting from the supposition that the dicrotic elevation is due to a wave travelling in the blood, one could attribute this difference of time to a velocity of the "dicrotic wave" which is slightly different from the velocity of the primary wave. The fact that the dicrotic elevation appears later in places farther from the heart was interpreted as a proof that the wave travelled out from the heart. No theory which assumes that the dicrotic elevation is due to a wave travelling in the blood can give a reason why two waves of the same form and origin should travel through the same liquid at different velocities.

At this point a theory must be mentioned, which was brought forward recently, because it is based on measurements of the velocity of propagation of the dicrotic wave. This theory is connected with Krehl's theory of the function of the valves. The blood, according to Krehl, enters the aorta through a small opening, and expanding in a large space it produces fluctuations and eddies, which would close the valves if they were not kept open by the blood which streams through under high pressure. They must, therefore, close at the moment when the aortic pressure is equal to the intraventricular pressure. This occurs shortly after the moment indicated by the beginning of the decline of the intraventricular pressure curve. Now the second sound of the heart is heard somewhere in the descending part of the cardiogram[71] and the measurements of Huerthle[72] have shown that the second sound is heard 0.02" after the beginning of the descent of the cardiogram. This seems to indicate that the second sound of the heart is in a temporal relation to the closure of the valves. Many theories of the origin of the sounds of the heart agree on this one point that the second sound is due to a noise in the muscles. It therefore may be supposed that the second sound is due to the tension of the valves when they close or shortly afterwards. The problem now would seem to be to find an elevation in the descending branch of the curve of intraventricular pressure, or in the tracings of the apex beat, which could be attributed to the closure of the valves. It was taken for granted that the curves of intraventricular pressure and those of the apex beat were identical. In many of these tracings an elevation was found which may be called "the wave f." This elevation is not found in all the tracings, and its position seems to be rather variable. Edgren[73] remarks that the wave f was always found near the abscissa no matter whether the preceding decline of the curve was great or small. In some of Chauveau's tracings the wave f is missing or indistinct,[74] in others it is very well marked and approximately in the middle of the descending branch of the curve.[75]

Edgren made experiments on the temporal relation of the wave f and of the dicrotic wave, which to avoid misunderstandings he calls the "wave f´." His experiments were made as follows. A sphygmogram from the carotid and a cardiogram were taken simultaneously, the points of the writing-levers being in the same vertical line. The wave f´ appeared a little after the wave f. The length of this interval could be calculated by measuring the distance between these waves, as the speed of the drum was known. From this was subtracted the time of propagation of the dicrotic from the heart to the point where the instrument was fixed. In this way it was found that the time between the appearance of the wave f and of the wave f´ was equal to the time of propagation of the dicrotic wave from the heart. Edgren concluded that the dicrotic wave is in close temporal relation to the closure of the valves.[76] To this comes the supposition that the wave f´ is due to a change of pressure proceeding from the heart. The wave f´, therefore, could be attributed to the tension of the valves.[77] Edgren and Tigerstedt are the chief exponents of this theory.

In so far as this theory assumes that the dicrotic elevation is due to a wave travelling from the heart to the periphery,[78] it is open to all the arguments against a theory of the central origin of the dicrotic wave. Against the more special assertion that the dicrotic elevation is in connection with the closure of the valves, the following facts must be mentioned. We grant that the tracings of the apex beat may be directly substituted for the curves of intraventricular pressure, although this is by no means obvious, since one tracing gives the form of the pressure changes and the other the effect of the shock of the heart against the wall of the chest. It is, furthermore, not proved that the wave f is due to the closure of the valves and that the waves f and f´ correspond to each other so closely as Edgren's experiments seem to indicate. His measurements of the length of lines were made with an exactitude of 0.1 mm., but his computations were carried to the third decimal place of a second. The third decimal is generally inexact and the second in a large number of cases. Experimental evidence, furthermore, directly contradicts the statement that the dicrotic elevation corresponds to the wave f. Fredericq[79] traced pressure curves in the ventricle and in the aorta, and determined the points of equal pressure in both curves. He thus found that a point near the beginning of the descent of the curve of intraventricular pressure corresponds to the dicrotic. His experiments are rather conclusive against the theory in question, since the wave f is very well marked in these tracings of Fredericq. The following facts, however, are fatal for the theory that the closure of the valves causes the dicrotic elevation: The dicrotic wave disappears in diseases like atheroma and arteriosclerosis which do not impair the function of the valves, but affect the elasticity of the arterial wall, and it is not affected by valvular insufficiency. The independence of the dicrotic from the function of the valves is conclusively proved by v. Kries, who found the dicrotic elevation in the femoral artery of an animal whose heart was replaced by a valveless bag.

All these facts, on the contrary, can be understood easily in the light of the theory that the sphygmographic curve gives the movements of the arterial wall, which movement is conditioned by the decreasing amount of blood in the artery, and the elastic vibrations of the wall around a variable position of equilibrium. In some cases the conditions of the problem are rather simple, and admit an analytic treatment, the results of which fit closely to the experimental facts. This part of the theory, however, has merely physiological interest, and therefore is discussed in a separate paper. It may be mentioned at this point that this theory of normal dicrotism is essentially identical with the theory of abnormal dicrotism as stated by Galen. He believed that the second beat of the pulsus bis feriens was due to an elastic vibration of the arterial wall. "Ex eodem genere sunt dicroti; nam arteria in occursu quasi repellitur, moxque redit.... Neque enim tum arteria contrahitur, sed quasi concuteretur, occidit; cuius delapsum a primae distentionis termino nulla dirimit manifesta quies, ut animadvertitur in contractione: sed simulatque attolli destitit, recidit atque ita paulisper vibrata, mox occurrit iterum."[80] Galen, however, is mistaken in his view, and in his observation that sometimes three or more pulse-beats may be felt with the finger. No form of the pulse is known where three or more beats may be felt for every heart-beat, and the actual tracings exclude the possibility of this observation for the pulsus bis feriens. The pulsus bis feriens is due to an increase of the frequency of the heart-beats. If the new pulse wave arrives before the vibrations of the arterial wall have had time to subside, the new wave and the already existing vibration may interfere in such a way as to produce this abnormal pulse form.

The form of a single wave of the sphygmographic curve may be influenced by changes in the following conditions:

(1) The pulse wave may have an initial form which cannot be represented by the schematic curve in Fig. 1. This may be due to an irregularity of the function of the ventricle. The action of the heart has an influence on the length of the waves, which length is determined by the rapidity of the heart-beats. This influence has been mentioned before. A change in the rapidity of the heart-beats has no great influence on the form of the catacrotic part of the curve so long as the impact of the new pulse wave does not arrive before the vibrations of the arterial wall have had time to subside.

(2) Differences of the elasticity of the arterial wall affect materially the form of the catacrotic part of the sphygmographic curve. It has also some influence on the height of the curves, because the amplitude of elastic vibrations depends on the elastic force for a given force of the shock. The degree of elasticity of the arterial wall is subjected to individual variations, and it depends in a given subject on the state of innervation of the wall.

(3) The surrounding tissues have a certain influence, since their resistance determines the friction opposing the vibration. This accounts for the fact that merely local conditions, such as a change of the position of the arm or the adjustment of the instrument, may change the form of the pulse curve. For instance, if the sphygmogram is taken from the a. radialis the instrument is placed between the styloid process of the radius and the tendon of the flexor carpi radialis. In the neighborhood of this place are two venae comites and a superficial branch of the median or radial vein. A change in the position of the arm will have a certain influence on the circulation in the veins, and influence the turgor of these vessels. Increased turgor increases the friction, and thus produces the different forms of the tracings.

(4) The changes of the turgor of the artery, moreover, cause a general rise or lowering of the curve. This symptom is essentially ambiguous for the turgor of the artery may be changed as well by an increase or decrease of the amount of blood pumped into the arterial system as by a decrease or increase of the amount of blood which passes through the capillaries.

The influences mentioned under (1) may be seen in tracings taken from cases of cardiac insufficiency, and have merely pathological interest. All the other influences, however, can be observed in the curves which are traced for psychological purposes. The changes in the general rise or fall of the curves are not so very hard to observe,[81] and for the observation of the rapidity of the heart-beats it is only necessary to trace a time-curve and count the number of beats or measure the length of every single beat. Also the changes of the height of the waves can easily be measured. This has been done conscientiously by several observers. It is by far harder to see the changes in the form of the catacrotic branch, and only a few keen observers have seen them. These changes of the pulse curve under the influence of feelings were proved as facts by experiments, but their interpretation was doubtful. With the exception of the rapidity of the heart-beats, which could easily be observed in some other way, all the symptoms of the influence of feelings on circulation are ambiguous. A difference in height of the single waves may be due to a change in the amount of blood which is pumped into the artery, but it also may be due to a change in the amplitude of the vibrations of the artery. The form of the catacrotic part of the sphygmographic curve may be changed by a different state of innervation of the arterial wall, but it also may be due to an increase or decrease of the friction of the surrounding tissues. The general rise or fall of the curve may indicate a change in the amount of blood which leaves the left ventricle, but it also may indicate a change in the amount of the capillary outflow.

The problem, nevertheless, is fully determined, and a solution is suggested by the constructions in Fig. 5. The form of the resulting movement depends, first, on the length of the line AB, secondly, on the length of the line AE, and thirdly, on the nature of the elastic movement. An elastic movement is determined if three constants are known, one of which is the amplitude, the second the friction, and the third the elasticity. Only AB can be measured directly, and there remain four unknown quantities to be determined. Four measurements must be sufficient for this purpose. It is obvious, however, that not any four measurements will do, but a method can be devised by which it is possible to determine each one of these four quantities. The problem can be solved in every case provided that the sphygmogram is trustworthy enough to justify the work. The length AB is proportional to the time of one heart-beat, and the length of the line AE is proportional to the amount of blood pumped into the arteries. The successful analysis of the pulse curves, therefore, shows changes of the action of the heart and makes it possible to distinguish them from the changes at the periphery.

Besides the length of the heart-beats there are invariably these four quantities which must be determined by the analysis of the pulse curves: Amount of incoming blood, amount of outflowing blood, elasticity of the artery, and friction of the tissues. These quantities depend on the action of the heart, the peripheral resistance, and the state of innervation of the artery. It is not possible to discuss here the bearing of this theory and of the facts which may be connected with it, on the different views of the localization and operation of the centres which control these functions. Anatomical and physiological evidence, however, leaves no doubt that the function of the heart and the innervation of the arteries and capillaries are under the control of nervous centres. It may be supposed, therefore, that changes of the pulse curve like those due to the influence of feelings are the effect of the function of these centres. It is to be expected that the detailed analysis of the pulse curves may give some indications as to the nature of this influence, for it may be observed how the function of these centres changes under the influence of mental processes.

A complete analysis of the physiological accompaniments of a feeling process must give a description of the changes in the function of the heart and the system, besides a description or at least enumeration of the other changes which can be observed. By a number of such investigations material for a general theory of physiological accompaniments of feelings may be obtained, which would not be void of interest for the psychology of feelings. Such a theory must contain the answers to the following questions: (1) How do the physiological reactions depend on the sense-stimulus? (2) How many possible circulatory reactions are there? (3) What is the location and interdependence of the respective physiological centres? The first question cannot so far be answered in general, but it will be possible to give a general answer when a greater number of systematic investigations on the effect of sense-stimuli have been carried on. Papers like those of Mentz may settle the question for certain sense-stimuli. From the results which have been obtained so far it comes out clearly that the reaction does not depend merely on the nature of the stimulus, but that it depends largely on the psychical and physiological state of the subject. The answer to the second question may be given readily, but it seems advisable to give it in connection with an experimental investigation. It may be said, nevertheless, that the number of typical reactions is rather limited. The third problem, by its nature, cannot be definitely answered before the location of the respective centres is ascertained and their interdependence explained.

It is, finally, a merit of this theory of the pulse curves that it shows how the form of this curve may depend on central processes. The problem of the mysterious influence of mental processes is thus reduced to the analysis of merely physiological conditions. The theories on the nature of this influence are so numerous that they may well be called innumerable, and they vary from accepting a direct influence of ideas on the circulation to considering the body as a sounding-board which by every sensation is shaken in all its parts. Each one of these theories is also a theory of feelings, and a more or less exact description of these changes has been often taken for a descriptive psychology of feelings. The example of the sounding-board is taken from one of those papers which expound the theory that bodily changes follow directly on the perception, and that our sensation of these facts is the emotion. Every one of these bodily changes, whatsoever, is perceived, acutely or obscurely, the moment it occurs. This theory is defended by the argument that if we try to abstract from consciousness all the sensations of our bodily symptoms, we find we have nothing left behind. This argument, which may be found in almost every paper that deals with this theory, is remarkable, because it sometimes is referred to processes of every description, and thus comes into contradiction with psychophysical parallelism which excludes the acceptance of psychical states which have no physical correlate. This theory, as will have been noticed, is the theory of feelings expounded by James, Lange, Ribot, and others. It is widely accepted, and may be found also in books of popular or semi-popular nature. Two observations must be made against this view:

First, a perception of a bodily change which is felt in the moment the change occurs exists only in the theory, every real process needing a certain time. This point of the theory may be improved by admitting that the afferent process lasts as long as any other of the physiological processes of this kind. Either assumption, however, is contradicted by the experimental evidence supplied by Lehmann that the physiological changes occur after the beginning of an emotional state.

Secondly, if the theory refers only to those bodily changes which we know, it certainly is not true, for emotional states are sometimes observed without it being possible to find with modern instruments any bodily accompaniments. If the theory refers to bodily changes of every description, it is certainly true, or, better, it is beyond all attack because it becomes identical with psychophysical parallelism. In this general form this theory of feelings is as good as no theory at all, because it refers to mental states of every description.[82]

This conception of emotional states of mind as perceptions of bodily sensations would hardly have been promulgated, if the authors had tried to base it on experiments performed in the laboratory. An emotion but not the feeling-tone of a simple sensation may be mistaken for the sum of bodily sensations. It is, furthermore, remarkable that the promoters of this theory do not make a clear distinction between sensation and feeling. They introduce an emotional element by calling the perception of bodily changes a feeling of these changes. Only in this way do they succeed in building up emotional states of mind out of elements which are seemingly sensational. This does not succeed if the word feeling is replaced by the word sensation. The failure of this theory is due to two facts, first to the starting from a philosophical doctrine, and second to the lack of a precise distinction between feeling and sensation. It cannot be doubted after the above discussion how a definition of this difference may be given which holds for every empirical investigation.

A sense-stimulus produces a complex of nervous and central processes. Among these is a certain group of processes which manifest themselves by changing the innervation of the heart, the blood-vessels, the lungs, and certain muscles. Another group is formed by those nervous and central processes which are more or less immediate effects of the sense-stimulation. The first group of processes is referred subjectively to an emotional state of mind, and the second to a cognitive process; the first group of processes is the physiological accompaniment of feelings, the second that of sensations. The relative independence of the first group from the second group is warranted by the fact that the same processes are observed as accompaniments of ideational processes. A strict limit between these two groups of processes can be drawn when the central processes are better known, because to the first group belong all those processes which are found to be accompaniments as well of sensational as of ideational processes. In different sensations the emotional process may be more or less marked, and in others the cognitive process may be prominent, but it seems that feelings are an invariable accompaniment of the sensation. This suggests the definition of feelings as psychic processes, the physiological accompaniment of which are central processes which depend largely on the state of the organism, and which manifest themselves by changes in the innervation of the heart, the blood-vessels, the lungs, and muscles. The impossibility of directly comparing the sensations of different subjects is recognized, and it is also impossible to compare feelings, because in either case we are dealing with psychic processes.

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BY JOHN A. H. KEITH

The object of this investigation was to ascertain the mutual influence of simultaneous stimuli that appealed to different senses with regard to the intensity of their feeling values. The investigation covers combinations: (1) of colors and active touches, (2) of colors and passive touches, (3) of tones and active touches, (4) of tones and passive touches, (5) of colors and tones.

The basis of appreciation was a numerical scale[83] as follows:

• 1. Very disagreeable.
• 2. Disagreeable.
• 3. Slightly disagreeable.
• 4. Indifferent.
• 5. Slightly agreeable.
• 6. Agreeable.
• 7. Very agreeable.

The color series began with the one hundred thirty-six colors as put out by the Milton Bradley Co. This series consists of ninety pure spectrum colors, ten whites, blacks, and grays, and thirty-six broken spectrum colors. The colors were exposed at the back of a semicircular black-lined box for about two seconds. The subject was seated at a convenient distance, about three and a half feet, from the colors. In order to have a constant light, all experiments were conducted in a dark room with an electric light suspended over the subject's head. The whole series was used for ten times in order to get the range of judgments. Then twenty-eight colors, covering as fully as possible the range from 1 to 7, were selected for further experiment in combination.

At the same time a series of thirty-six touches, from velvet to sandpaper, was being employed as the colors were. From this number fourteen were finally selected.

Similarly, by using a reed box, with reeds ranging from 128 to 1024 vibrations per second and separated from each other by four vibrations, from a much larger series twenty-seven tone-combinations were finally selected.

Moreover, from time to time, each selected series was given alone; and on the basis of these readings, averaging from thirty to forty, the "standard" for each stimulus was made. Tables I to III give a brief description of the stimuli and also the "standards" for each of two subjects, F. and M.

In connection with this table it may be noted that the subjects agree regarding 2, 19, and 22, red tint no. 1, black, and A-red light; that M. estimates the colors higher than F. in seventeen cases; and that F. estimates 1, 3, 11, 13, 14, 15, 17, 18, higher than M. does. These individual differences are probably explicable on grounds of association; they are not, however, connected with the problem under consideration here, for we are concerned with the effect of combinations with other stimuli.

The various articles were fastened to small pieces of wood and placed in small cardboard boxes. In active touch, the subjects were allowed to stroke the object gently twice, always with a contracting movement of the forefinger of the right hand. In passive touch, the operator stroked the subject's forefinger with the object, twice as before. The table explains itself.

The table of tone-combinations shows that some are simply repetitions of the same chord in a higher octave, as 1 and 9, 5 and 13, 15 and 14. The first sixteen are harmonious; so also the twenty-sixth; the others introduce beats and discords, some of which are agreeable, as 20 and 21, while others are disagreeable, as 19 and 27. Individual differences appear in this as in the previous series. The totals introduced into the tables simply go to show that in each series the total judgments are not widely diverse.

The following tables deal with the combined series.

Table IV shows that the appreciation of the colors was, in general, lowered slightly by the combinations with the tones; and, also, that the appreciation of the tones was lowered more than one point by the combinations with the colors. By referring to Tables I and III, M.'s average for the colors is 4.44 and for the tones 4.20.

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Under combination influences, the average is reduced to 4.14+ for colors, and 3.12 for tones.

The next Table, V, shows the color-tone results for F. as Table IV showed them for M. F.'s average for colors (Table I) alone was 4.35, and was reduced in combination with tones by .26, or to 4.19. So, also, F.'s average for tones alone (Table III), 4.33, was reduced by .74+ to 3.59. The averages in both cases show the same general tendency to a lowering of the appreciation in both series when the series are combined, but the tones are lowered more than the colors.

Table VI shows the effect of combining tones and active touches as reported by M.

The effect of this combination is clear and unmistakeable. The appreciation of the tones is raised 1.71+ points; and of the active touches, .31+ points. This result is the opposite of that shown in Table IV, where colors and tones were combined. There is this agreement, however, that the appreciation of the tones is changed more than that of the other stimuli. Relatively, the appreciation of the touches changes least.

Table VII shows the effects on F. of combining tones and active touches. The same general tendencies appear as in the case of M.; but the changes in appreciation are not so marked. This is not easy to explain, for F. estimated both the active touches and the tones higher when taken alone than did M.

Table VIII shows the effect on F. of combining tones and passive touches. The same general tendency to increased appreciation appears, but the tones are raised more and the touches raised less than in Table VII. This may be explained, perhaps, on the basis of increasing appreciation with increased participation.

Tables IX and X show the effect on M. and F., respectively, of combining colors and active touches. M. estimates both slightly higher, while F. estimates both slightly lower. This difference cannot be explained by the standards for each subject. From Tables I and III we get:

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Evidently no dynamic explanation of this difference is possible. It has been impossible to reveal the cause of this particular difference; in all other respects, however, the subjects agree in the general tendencies revealed.

There remain the combinations of colors and passive touches. These combinations are shown in Tables XI and XII.

In these tables the same general tendency to estimate both higher when colors and passive touches are combined appears. M. raises the colors more than F., and F. raises the touches more than M. This is perfectly regular, as the following table shows:

The whole results are recapitulated for both M. and F. in Table XIII.

From this last table certain conclusions may be drawn.

(1) When Colors and Tones were combined, both were lowered in the appreciation of both subjects. The percentages show: (a) That about the same number of colors was lowered, 59.6% for F. and 59% for M. (b) That about the same average displacement of colors occurred, .26+ for F. and .29+ for M. (c) That about the same number of tones was lowered, 77% for F. and 81% for M. (d) That the tones were lowered more for M., -.74+ for F., and 1.08+ for M.

(2) When Colors and Active Touches were combined, for F. both are lowered; for M. both are raised. The colors are lowered only very slightly, .13 for F, while for M they are raised .43+; and conversely, the active touches are lowered .38+ for F, and raised only .17+ for M. Still, it appears clear that with F. there was an interference,—and both colors and active touches are lowered while the same combinations with M. are mutually reënforcing.

(3) When Colors and Passive Touches were combined, the appreciation of both was raised for both F. and M. The result shows: (a) That the percentages of colors and passive touches raised are practically the same for both F. and M. (b) That the color displacement is greater for M. than for F., being +.72+ for M. and only .40+ for F. (c) That there is only a slight difference in the displacement of the passive touches.

(4) When Tones and Active Touches were combined, the appreciation of both was raised for both F. and M. It appears (a) that the displacement for F. is very slight, .45+ and .23+, when compared with the displacement for M., 1.71+ and .31+. But, (b) that the displacement of tones is greater than that of the active touches for both F. and M. (c) That this same relatively great displacement of tones occurred in the opposite direction when colors and tones were combined.

(5) When Tones and Passive Touches were combined, the appreciation of both was raised for F,—the tones being raised more than the passive touches. This combination was not tried with M. for lack of time.

From time to time some special tests of these general tendencies were applied. From the results already set forth, one could predict that there was a strong probability that when the tone-series was combined with a constant touch-series, active or passive, the appreciation of the tones would be raised. This was tried by allowing the subject M. to rub his hand over the somewhat rough pillow of a tilting board. The results showed that the appreciation of twenty-five out of twenty-seven tones was raised. Other predictions were similarly verified.

Of course, any experiment of this nature is exposed to a great many chances of error. The subjects may be fatigued, or depressed generally. But the wide range of different readings taken is a reasonable assurance that the chances of error are minimized. And it is to be noted, also, that individual differences of appreciation do not vitiate the results. In getting the "standards" no less than 2000 judgments were given by each subject, while for the tables each subject gave no less than 4000 judgments. The curves made from these data show the effect of each separate combination by their variation from the standard. The tables and analyses and conclusions already introduced show, in general, that our appreciation of each of several stimuli in combination is different from our appreciation of the same stimuli when taken separately. The results show that this appreciation may be either raised or lowered; that is to say, our feeling of values is not constant for a given stimulus under all conditions.

From Table XIII I have found the average displacement of each series to be as follows:

• Tones, .90.
• Colors, .37.
• A. Touches, .28.
• P. Touches, .23.

This shows that passive touches are subject to the least displacement, while active touches, colors, and tones are respectively subject to a greater variation. The sight-touch world is more stable than the auditory world. With M. the tones go over a full point both below and above the standard.

This report does not treat of the particular effects of a qualitative nature that follow from the possible combinations of series of stimuli of different feeling values,—such as the effect of an agreeable touch upon a slightly disagreeable tone, or upon an indifferent color, and so on. All such effects can be traced by rearranging the data already collected, and this may be done in a subsequent paper which may enter also into the theoretical discussion of the whole problem.

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