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Title: Harvard Psychological Studies, Volume 1
       Containing Sixteen Experimental Investigations from the
              Harvard Psychological Laboratory.

Author: Various

Editor: Hugo Münsterberg

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Language: English

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The Psychological Review

EDITED BY

WITH THE CO-OPERATION OF

ALFRED BINET, ÉCOLE DES HAUTES-ÉTUDES, PARIS;
JOHN DEWEY, H.H. DONALDSON, UNIVERSITY OF CHICAGO;
G.S. FULLERTON, UNIVERSITY OF PENNSYLVANIA;
G.H. HOWISON, UNIVERSITY OF CALIFORNIA;
JOSEPH JASTROW, UNIVERSITY OF WISCONSIN;
G.T. LADD, YALE UNIVERSITY;
HUGO MÜNSTERBERG, HARVARD UNIVERSITY;
M. ALLEN STARR, COLLEGE OF PHYSICIANS AND SURGEONS, NEW YORK;
CARL STUMPF, UNIVERSITY, BERLIN;
JAMES SULLY, UNIVERSITY COLLEGE, LONDON.

H.C. WARREN, PRINCETON UNIVERSITY, Associate Editor and Business Manager.

Series of Monograph Supplements, Vol. IV. (Whole No. 17), January, 1903.

Click here for Table of Contents

Harvard Psychological Studies,

Volume I

CONTAINING

Sixteen Experimental Investigations from the Harvard Psychological Laboratory.

EDITED BY

HUGO MÜNSTERBERG.

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PREFACE.

The appearance of the HARVARD PSYCHOLOGICAL STUDIES does not indicate an internal change in the work of the Harvard Psychological Laboratory. But while up to this time the results of our investigations have been scattered in various places, and have often remained unpublished through lack of space, henceforth, we hope to have in these STUDIES the opportunity to publish the researches of the Harvard Laboratory more fully and in one place. Only contributions from members of the Harvard Psychological Laboratory will be printed in these volumes, which will appear at irregular intervals, and the contributions will represent only our experimental work; non-experimental papers will form an exception, as with the present volume, wherein only the last one of the sixteen papers belongs to theoretical psychology.

This first volume does not give account of all sides of our laboratory work. An essential part of the investigations every year has been the study of the active processes, such as attention, apperception, and volition. During the last year several papers from these fields have been completed, but we were unable to include them in this volume on account of the space limits; they are kept back for the second volume, in which accordingly the essays on the active functions will prevail, as those on perception, memory, and feeling prevail in this volume. It is thus clear that we aim to extend our experimental work over the whole field of psychology and to avoid one-sideness. Nevertheless there is no absence of unity in our work; it is not scattered work as might appear at a first glance; for while the choice of subjects is always made with relation to the special interests of the students, there is after all one central interest which unifies the work and has influenced the development of the whole laboratory during the years of my direction.

I have always believed—a view I have fully discussed in my 'Grundzüge der Psychologie'—that of the two great contending theories of modern psychology, neither the association theory nor the apperception theory is a satisfactory expression of facts, and that a synthesis of both which combines the advantages without the defects of either can be attained as soon as a psychophysical theory is developed which shall consider the central process in its dependence, not only upon the sensory, but also upon the motor excitement. This I call the action theory. In the service of this theory it is essential to study more fully the rôle of the centrifugal processes in mental life, and, although perhaps no single paper of this first volume appears to offer a direct discussion of this motor problem, it was my interest in this most general question which controlled the selection of all the particular problems.

This relation to the central problem of the rôle of centrifugal processes involves hardly any limitation as to the subject matter; plenty of problems offer themselves in almost every chapter of psychology, since no mental function is without relation to the centrifugal actions. Yet, it is unavoidable that certain groups of questions should predominate for a while. This volume indicates, for instance, that the æsthetic processes have attracted our attention in an especially high degree. But even if we abstract from their important relation to the motor functions, we have good reasons for turning to them, as the æsthetic feelings are of all feeling processes decidedly those which can be produced in the laboratory most purely; their disinterested character makes them more satisfactory for experimental study than any other feelings.

Another group of researches which predominates in our laboratory is that on comparative psychology. Three rooms of the laboratory are reserved for psychological experiments on animals, under the special charge of Dr. Yerkes. The work is strictly psychological, not vivisectional; and it is our special purpose to bring animal psychology more in contact with those methods which have found their development in the laboratories for human psychology. The use of the reaction-time method for the study of the frog, as described in the fifteenth paper, may stand as a typical illustration of our aim.

All the work of this volume has been done by well-trained post-graduate students, and, above all, such advanced students were not only the experimenters but also the only subjects. It is the rule of the laboratory that everyone who carries on a special research has to be a subject in several other investigations. The reporting experimenters take the responsibility for the theoretical views which they express. While I have proposed the subjects and methods for all the investigations, and while I can take the responsibility for the experiments which were carried on under my daily supervision, I have left fullest freedom to the authors in the expression of their views. My own views and my own conclusions from the experiments would not seldom be in contradiction with theirs, as the authors are sometimes also in contradiction with one another; but while I, of course, have taken part in frequent discussions during the work, in the completed papers my rôle has been merely that of editor, and I have nowhere added further comments.

In this work of editing I am under great obligation to Dr. Holt, the assistant of the laboratory, for his helpful coöperation.

CONTENTS.

PLATES.

STUDIES IN PERCEPTION.

EYE-MOVEMENT AND CENTRAL ANÆSTHESIA.

BY EDWIN B. HOLT.

I. THE PROBLEM OF ANÆSTHESIA DURING EYE-MOVEMENT.

A first suggestion of the possible presence of anæsthesia during eye-movement is given by a very simple observation. All near objects seen from a fairly rapidly moving car appear fused. No further suggestion of their various contour is distinguishable than blurred streaks of color arranged parallel, in a hazy stream which flows rapidly past toward the rear of the train. Whereas if the eye is kept constantly moving from object to object scarcely a suggestion of this blurred appearance can be detected. The phenomenon is striking, since, if the eye moves in the same direction as the train, it is certain that the images on the retina succeed one another even more rapidly than when the eye is at rest. A supposition which occurs to one at once as a possible explanation is that perchance during eye-movement the retinal stimulations do not affect consciousness.

On the other hand, if one fixates a fly which happens to be crawling across the window-pane and follows its movements continuously, the objects outside swim past as confusedly as ever, and the image of the fly remains always distinct. Here the eye is moving, and it may be rapidly, yet both the fly and the blurred landscape testify to a thorough awareness of the retinal stimulations. There seems to be no anæsthesia here. It may be, however, that the eye-movement which follows a moving object is different from that which strikes out independently across the visual field; and while in the former case there is no anæsthesia, perhaps in the latter case there is anæsthesia.

[4]Cattell,1 in considering a similar experience, gives his opinion that not the absence of fusion for the moving eye, but its presence for the resting eye, needs explanation. "More than a thousand interruptions per second," he believes, "give a series of sharply defined retinal processes." But as for the fusion of moving objects seen when the eyes are at rest, Cattell says, "It is not necessary and would probably be disadvantageous for us to see the separate phases." Even where distinct vision would be 'disadvantageous' he half doubts if fusion comes to the rescue, or if even the color-wheel ever produces complete fusion. "I have never been able," he writes, "to make gray in a color-wheel from red and green (with the necessary correction of blue), but when it is as nearly gray as it can be got I see both red and green with an appearance of translucence."

That the retina can hold apart more than one thousand stimulations per second, that there is, in fact, no such thing as fusion, is a supposition which is in such striking contrast to all previous explanations of optical phenomena, that it should be accepted only if no other theory can do justice to them. It is hoped that the following pages will show that the facts do not demand such a theory.

Another simple observation is interesting in this connection. If at any time, except when the eyes are quite fresh, one closes one's eyes and attends to the after-images, some will be found which are so faint as to be just barely distinguishable from the idioretinal light. If the attention is then fixed on one such after-image, and the eyes are moved, the image will suddenly disappear and slowly emerge again after the eyes have come to rest. This disappearance during eye-movements can be observed also on after-images of considerable intensity; these, however, flash back instantly into view, so that the observation is somewhat more difficult. Exner,2 in speaking of this phenomenon, adds that in general "subjective visual phenomena whose origin lies in the retina, as for instance after-images,[5] Purkinje's vessel-figure, or the phenomena of circulation under discussion, are almost exclusively to be seen when the eye is rigidly fixed on a certain spot: as soon as a movement of the eye is made, the subjective phenomena disappear."

The facts here mentioned in no wise contradict a phenomenon recently discussed by McDougall,3 wherein eye-movements revive sensations which had already faded. Thus an eye-movement will bring back an after-image which was no longer visible. This return to vividness takes place after the movement has been completed, and there is no contention that the image is seen just during the movement.

The disappearance of after-images during eye-movements is mentioned by Fick and Gürber,4 who seek to explain the phenomenon by ascribing it to a momentary period of recovery which the retina perhaps undergoes, and which would for the moment prevent further stimulations from being transmitted to the optic nerve. Exner observes that this explanation would not, however, apply to the disappearance of the vessel-figure, the circulation phenomenon, the foveal figure, the polarization-sheaf of Haidinger, Maxwell's spot, or the ring of Löwe; for these phenomena disappear in a similar manner during movement. Exner offers another and a highly suggestive explanation. He says of the phenomenon (op. citat., S. 47), "This is obviously related to the following fact, that objective and subjective impressions are not to be distinguished as such, so long as the eye is at rest, but that they are immediately distinguished if an eye-movement is executed; for then the subjective phenomena move with the eye, whereas the objective phenomena are not displaced.... This neglect of the subjective phenomena is effected, however, not by means of an act of will, but rather by some central mechanism which, perhaps in the manner of a reflex inhibition, withholds the stimulation in question from consciousness, without our assistance and indeed without our knowledge." The suggestion of a central mechanism which brings about a reflex inhibition is the significant point.

[6]It is furthermore worth noting that movements of the eyelid and changes in the accommodation also cause the after-images to disappear (Fick and Gürber), whereas artificial displacement of the eye, as by means of pressure from the finger, does not interfere with the images (Exner).

Another motive for suspecting anæsthesia during eye-movement is found by Dodge,5 in the fact that, "One may watch one's eyes as closely as possible, even with the aid of a concave reflector, whether one looks from one eye to the other, or from some more distant object to one's own eyes, the eyes may be seen now in one position and now in another, but never in motion." This phenomenon was described by Graefe,6 who believed it was to be explained in the same way as the illusion which one experiences in a railway coach when another train is moving parallel with the coach in which one sits, in the same direction and at the same speed. The second train, of course, appears motionless.

This explanation of Graefe is not to be admitted, however, since in the case of eye-movement there are muscular sensations of one's own activity, which are not present when one merely sits in a coach. These sensations of eye-movement are in all cases so intimately connected with our perception of the movement of objects, that they may not be in this case simply neglected. The case of the eye trying to watch its own movement in a mirror is more nearly comparable with the case in which the eye follows the movement of some independent object, as a race-horse or a shooting-star. In both cases the image remains on virtually the same point of the retina, and in both cases muscular sensations afford the knowledge that the eye is moving. The shooting-star, however, is perceived to move, and the question remains, why is not the eye in the mirror also seen to move?

F. Ostwald7 refutes the explanation of Graefe from quite different considerations, and gives one of his own, which depends on the geometrical relations subsisting between the axes [7]of vision of the real eye and its reflected image. His explanation is too long to be here considered, an undertaking which indeed the following circumstance renders unnecessary. While it is true that the eye cannot observe the full sweep of its own movement, yet nothing is easier than to observe its movement through the very last part of the arc. If one eye is closed, and the other is brought to within about six inches of an ordinary mirror, and made to describe little movements from some adjacent part of the mirror to its own reflected image, this image can almost without exception be observed as just coming to rest. That is, the very last part of the movement can be seen. The explanation of Ostwald can therefore not be correct, for according to it not alone some parts of the movement, but absolutely all parts alike must remain invisible. It still remains, therefore, to ask why the greater part of the movement eludes observation. The correct explanation will account not only for the impossibility of seeing the first part of the movement but also for the possibility of seeing the remainder.

Apart from the experience of the eye watching itself in a glass, Dodge (loc. citat.) found another fact which strongly suggested anæsthesia. In the course of some experiments on reading, conducted by Erdmann and Dodge, the question came up, how "to explain the meaning of those strangely rhythmic pauses of the eye in reading every page of printed matter." It was demonstrated (ibid., p. 457) "that the rhythmic pauses in reading are the moments of significant stimulation.... If a simple letter or figure is placed between two fixation-points so as to be irrecognizable from both, no eye-movement is found to make it clear, which does not show a full stop between them."

With these facts in view Dodge made an experiment to test the hypothesis of anæsthesia. He proceeded as follows (ibid., p. 458): "A disc of black cardboard thirteen inches in diameter, in which a circle of one-eighth inch round holes, one half inch apart, had been punched close to the periphery all around, was made to revolve at such a velocity that, while the light from the holes fused to a bright circle when the eye was at rest, when the eye moved in the direction of the disc's rotation from one fixation point, seen through the fused circle of [8]light, to another one inch distant, three clear-cut round holes were seen much brighter than the band of light out of which they seemed to emerge. This was only possible when the velocity of the holes was sufficient to keep their images at exactly the same spot on the retina during the movement of the eye. The significant thing is that the individual round spots of light thus seen were much more intense than the fused line of light seen while the eyes were at rest. Neither my assistant nor I was able to detect any difference in brightness between them and the background when altogether unobstructed." Dodge finds that this experiment 'disproves' the hypothesis of anæsthesia.

If by 'anæsthesia' is meant a condition of the retinal end-organs in which they should be momentarily indifferent to excitation by light-waves, the hypothesis is indeed disproved, for obviously the 'three clear-cut round holes' which appeared as bright as the unobstructed background were due to a summation of the light which reached the retina during the movement, through three holes of the disc, and which fell on the same three spots of the retina as long as the disc and the eyeball were moving at the same angular rate. But such a momentary anæsthesia of the retina itself would in any case, from our knowledge of its physiological and chemical structure, be utterly inconceivable.

On the other hand, there seems to be nothing in the experiment which shows that the images of the three holes were present to consciousness just during the movement, rather than immediately thereafter. A central mechanism of inhibition, such as Exner mentions, might condition a central anæsthesia during movement, although the functioning of the retina should remain unaltered. Such a central anæsthesia would just as well account for the phenomena which have been enumerated. The three luminous images could be supposed to remain unmodified for a finite interval as positive after-images, and as such first to appear in consciousness. Inasmuch as 'the arc of eye movements was 4.7°' only, the time would be too brief to make possible any reliable judgment as to whether the three holes were seen during or just after the eye-movement. With [9]this point in view, the writer repeated the experiment of Dodge, and found indeed nothing which gave a hint as to the exact time when the images emerged in consciousness. The results of Dodge were otherwise entirely confirmed.

II. THE PHENOMENON OF 'FALSELY LOCALIZED AFTER-IMAGES.'

A further fact suggestive of anæsthesia during movement comes from an unexpected source. While walking in the street of an evening, if one fixates for a moment some bright light and then quickly turns the eye away, one will observe that a luminous streak seems to dart out from the light and to shoot away in either of two directions, either in the same direction as that in which the eye moved, or in just the opposite. If the eye makes only a slight movement, say of 5°, the streak jumps with the eye; but if the eye sweeps through a rather large arc, say of 40°, the luminous streak darts away in the opposite direction. In the latter case, moreover, a faint streak of light appears later, lying in the direction of the eye-movement.

This phenomenon was probably first described by Mach, in 1886.8 His view is essentially as follows: It is clear that in whatever direction the eye moves, away from its luminous fixation point, the streak described on the retina by the luminous image will lie on the same part of the retina as it would have lain on had the eye remained at rest but the object moved in the opposite direction. Thus, if the eye moves to the right, we should expect the streak to appear to dart to the left. If, however, the streak has not faded by the time the eye has come to rest on a new fixation point (by supposition to the right of the old), we should expect the streak to be localized to the left of this, that is, to the right of the former fixation-point. In order to be projected, a retinal image has to be localized with reference to some point, generally the fixation-point of the eyes; and it is therefore clear that when two such fixation-points are involved, the localization will be ambiguous if for any reason the central apparatus does not clearly determine which shall be the point of reference. With regard to the oppositely moving [10]streak Mach says:9 "The streak is, of course, an after-image, which comes to consciousness only on, or shortly before, the completion of the eye-movement, nevertheless with positional values which correspond, remarkably enough, not to the later but to the earlier position and innervation of the eyes." Mach does not further attempt to explain the phenomenon.

It is brought up again by Lipps,10 who assumes that the streak ought to dart with the eyes and calls therefore the oppositely moving streak the 'falsely localized image.' For sake of brevity we may call this the 'false image.' The explanation of Lipps can be pieced together as follows (ibid., S. 64): "The explanation presupposes that sensations of eye-movements have nothing to do with the projection of retinal impressions into the visual field, that is, with the perception of the mutual relations as to direction and distance, of objects which are viewed simultaneously.... Undoubtedly, however, sensations of eye-movements, and of head-and body-movements as well, afford us a scale for measuring the displacements which our entire visual field and every point in it undergo within the surrounding totality of space, which we conceive of as fixed. We estimate according to the length of such movements, or at least we deduce therefrom, the distance through fixed space which our view by virtue of these movements has traversed.... They themselves are nothing for our consciousness but a series of purely intensive states. But in experience they can come to indicate distance traversed." Now in turning the eye from a luminous object, O, to some other fixation-point, P, the distance as simply contemplated is more or less subdivided or filled in by the objects which are seen to lie between O and P, or if no such objects are visible the distance is still felt to consist of an infinity of points; whereas the muscular innervation which is to carry the eye over this very distance is an undivided unit. But it is this which gives us our estimate of the arc we move through, and being thus uninterrupted it will appear shorter than the contemplated, [11]much subdivided distance OP, just as a continuous line appears shorter than a broken line. "After such analogies, now, the movement of the eye from O to P, that is, the arc which I traverse, must be underestimated" (ibid., S. 67). There is thus a discrepancy between our two estimates of the distance OP. This discrepancy is felt during the movement, and can be harmonized only if we seem to see the two fixation-points move apart, until the arc between them, in terms of innervation-feeling, feels equal to the distance OP in terms of its visual subdivisions. Now either O and P can both seem to move apart from each other, or else one can seem fixed while the other moves. But the eye has for its goal P, which ought therefore to have a definite position. "P appears fixed because, as goal, I hold it fast in my thought" (loc. citat.). It must be O, therefore, which appears to move; that is, O must dart backward as the eye moves forward toward P. Thus Lipps explains the illusion.

Such an explanation involves many doubtful presuppositions, but if we were to grant to Lipps those, the following consideration would invalidate his account. Whether the feeling of innervation which he speaks of as being the underestimated factor is supposed to be a true innervation-feeling in the narrower sense, or a muscular sensation remembered from past movements, it would in the course of experience certainly come to be so closely associated with the corresponding objective distance as not to feel less than this. So far as an innervation-feeling might allow us to estimate distance, it could have no other meaning than to represent just that distance through which the innervation will move the organ in question. If OP is a distance and i is the feeling of such an innervation as will move the eye through that distance, it is inconceivable that i, if it represent any distance at all, should represent any other distance than just OP.

Cornelius11 brought up the matter a year later than Lipps. Cornelius criticises the unwarranted presuppositions of Lipps, and himself suggests that the falsely localized streak is due to [12]a slight rebound which the eye, having overshot its intended goal, may make in the opposite direction to regain the mark. This would undoubtedly explain the phenomenon if such movements of rebound actually took place. Cornelius himself does not adduce any experiments to corroborate this account.

The writer, therefore, undertook to find out if such movements actually are made. The observations were made by watching the eyes of several subjects, who looked repeatedly from one fixation-point to another. Although sometimes such backward movements seemed indeed to be made, they were very rare and always very slight. Inasmuch as the 'false' streak is often one third as long as the distance moved through, a movement of rebound, such as Cornelius means, would have to be one third of the arc intended, and could therefore easily have been noticed. Furthermore, the researches of Lamansky,12 Guillery,13 Huey,14 Dodge and Cline,15 which are particularly concerned with the movements of the eyes, make no mention of such rebounds. Schwarz16 above all has made careful investigations on this very point, in which a screen was so placed between the observer and the luminous spot that it intervened between the pupil and the light, just before the end of the movement. Thus the retina was not stimulated during the latter part of its movement, just when Cornelius assumed the rebound to take place. This arrangement, however, did not in the least modify the appearance of the false streak.

This work of Schwarz certainly proves that the explanation of Cornelius is not correct. Schwarz found that the phenomenon takes place as well when the head moves and the eyes are fixed relatively to the head, as when the eyes alone move. He furthermore made this observation. Meaning by a the point of departure and by b the goal of either the eye-or the head-movement, [13]movement, he says (ibid., S. 400-2): "While oftentimes the streak of the after-image extended uninterruptedly to the point b, or better seemed to proceed from this point,—as Lipps also reported—yet generally, under the experimental conditions which I have indicated, two streaks could be seen, separated by a dark space between; firstly the anomalous one" (the false streak) "rather brilliant, and secondly a fainter one of about equal or perhaps greater length, which began at the new fixation-point b and was manifestly an after-image correctly localized with regard to the situation of this point. This last after-image streak did not always appear; but it appeared regularly if the light at a was bright enough and the background dark.... It was impossible for this second after-image streak to originate in the point b, because it appeared equally when b was only an imaginary fixation-point.... This consideration makes it already conceivable that the two parts of the total after-image are two manifestations of the one identical retinal stimulation, which are differently localized.... Therefore we must probably picture to ourselves that the sensation from the strip of the retina stimulated during the quick eye-movement is, during the interval of movement or at least during the greater part of it, localized as if the axis of vision were still directed toward the original fixation-point. And when the new position of rest is reached and the disturbance on the retinal strip has not wholly died away, then the strip comes once more into consciousness, but this time correctly localized with reference to the new position of the axis of vision. By attending closely to the behavior as regards time of both after-image streaks, I can generally see the normal after-image develop a moment later than the anomalous one" (that is, the false streak). Schwarz finally suggests (S. 404) that probably between the first and second appearances of the streak an 'innervation-feeling' intervenes which affords the basis for localizing the second streak ('correctly') with reference to the new position of the eye.

After this digression we return to consider how this phenomenon is related to the hypothesis of anæsthesia during eye-movements. If we accept the interpretation of Schwarz, there is one retinal process which is perceived as two luminous streaks [14]in space, localized differently and referred to different moments of time. It is surprising, then, that a continuous retinal process is subjectively interpreted as two quite different objects, that is, as something discontinuous. Where does the factor of discontinuity come in? If we suppose the retinal disturbance to produce a continuous sensation in consciousness, we should expect, according to every analogy, that this sensation would be referred to one continuously existing object. And if this object is to be localized in two places successively, we should expect it to appear to move continuously through all intervening positions. Such an interpretation is all the more to be expected, since, as the strobic phenomena show, even discontinuous retinal processes tend to be interpreted as continuously existing objects.

On the other hand, if there were a central anæsthesia during eye-movement, the continuous process in the retina could not produce a continuous sensation, and if the interval were long enough the image might well be referred to two objects; since also, in the strobic appearances, the stimulations must succeed at a certain minimal rate in order to produce the illusion of continuous existence and movement.

This consideration seemed to make it worth while to perform some experiments with the falsely localized after-images. The phenomenon had also by chance been noted in the case of the eye moving past a luminous dot which was being regularly covered and uncovered. The appearance is of a row of luminous spots side by side in space, which under conditions may be either falsely or correctly localized. Since these dots seemed likely to afford every phenomenon exhibited by the streaks, with the bare chance of bringing out new facts, apparatus was arranged as in Fig. 1, which is a horizontal section.

DD is a disc which revolves in a vertical plane, 56 cm. in diameter and bearing near its periphery one-centimeter holes punched 3 cm. apart. E is an eye-rest, and L an electric lamp. SS is a screen pierced at H by a one-centimeter hole. The distance EH is 34 cm. The disc DD is so pivoted that the highest point of the circle of holes lies in a straight line between the eye E and the lamp L. The hole H lies also in [15]this straight line. A piece of milk-glass M intervenes between L and H, to temper the illumination. The disc DD is geared to a wheel W, which can be turned by the hand of the observer at E, or by a second person. As the disc revolves, each hole in turn crosses the line EL. Thus the luminous hole H is successively covered and uncovered to the eye E; and if the eye moves, a succession of points on the retina is stimulated by the successive uncovering of the luminous spot. No fixation-points are provided for the eye, since such points, if bright enough to be of use in the otherwise dark room, might themselves produce confusing streaks, and also since an exact determination of the arc of eye-movement would be superfluous.

Fig. 1.

The eye was first fixated on the light-spot, and then moved horizontally away toward either the right or the left. In the first few trials (with eye-sweeps of medium length), the observations did not agree, for some subjects saw both the false and the correct streaks, while others saw only the latter. It was [16]found later that all the subjects saw both streaks if the arc of movement was large, say 40°, and all saw only the correctly localized streak if the arc was small, say 5°. Arcs of medium length revealed individual differences between the persons, and these differences, though modified, persisted throughout the experiments. After the subjects had become somewhat trained in observation, the falsely localized streak never appeared without the correctly localized one as well. For the sake of brevity the word 'streak' is retained, although the appearance now referred to is that of a series of separate spots of light arranged in a nearly straight line.

The phenomena are as follows.—(1) If the arc of movement is small, a short, correctly localized streak is seen extending from the final fixation-point to the light-spot. It is brightest at the end nearer the light. (2) If the eye-movement is 40° or more, a streak having a length of about one third the distance moved through is seen on the other side of the light from the final fixation-point; while another streak is seen of the length of the distance moved through, and extending from the final fixation-point to the light. The first is the falsely, the second the correctly localized streak. The second, which is paler than the first, feels as if it appeared a moment later than this. The brighter end of each streak is the end which adjoins the luminous spot. (3) Owing to this last fact, it sometimes happens, when the eye-movement is 40° or a trifle less, that both streaks are seen, but that the feeling of succession is absent, so that the two streaks look like one streak which lies (unequally parted) on both sides of the spot of light. It was observed, in agreement with Schwarz, that the phenomenon was the same whether the head or the eyes moved. Only one other point need be noted. It is that the false streak, which appears in the beginning to dart from the luminous hole, does not fade, but seems to suffer a sudden and total eclipse; whereas the second streak flashes out suddenly in situ, but at a lesser brilliancy than the other, and very slowly fades away.

These observations thoroughly confirmed those of Schwarz. And one could not avoid the conviction that Schwarz's suggestion of the two streaks being separate localizations of the same [17]retinal stimulation was an extremely shrewd conjecture. The facts speak strongly in its favor; first, that when the arc of movement is rather long, there is a distinct feeling of succession between the appearances of the falsely and the correctly localized images; second, that when both streaks are seen, the correct streak is always noticeably dimmer than the false streak.

It is of course perfectly conceivable that the feeling of succession is an illusion (which will itself then need to be explained), and that the streak is seen continuously, its spacial reference only undergoing an instantaneous substitution. If this is the case, it is singular that the correctly seen streak seems to enter consciousness so much reduced as to intensity below that of the false streak when it was eclipsed. Whereas, if a momentary anæsthesia could be demonstrated, both the feeling of succession and the discontinuity of the intensities would be explained (since during the anæsthesia the after-image on the retina would have faded). This last interpretation would be entirely in accordance with the observations of McDougall,17 who reports some cases in which after-images are intermittently present to consciousness, and fade during their eclipse, so that they reappear always noticeably dimmer than when they disappeared.

Now if the event of such an anæsthesia could be established, we should know at once that it is not a retinal but a central phenomenon. We should strongly suspect, moreover, that the anæsthesia is not present during the very first part of the movement. This must be so if the interpretation of Schwarz is correct, for certainly no part of the streak could be made before the eye had begun to move; and yet approximately the first third was seen at once in its original intensity, before indeed the 'innervation-feelings' had reached consciousness. Apparently the anæsthesia commences, it at all, after the eye has accomplished about the first third of its sweep. And finally, we shall expect to find that movements of the head no less than movements of the eyes condition the anæsthesia, since neither by Schwarz nor by the present writer was any difference observed in the phenomena of falsely localized after-images, [18]between the cases when the head, and those when the eyes moved.

III. THE PERIMETER-TEST OF DODGE, AND THE LAW OF THE LOCALIZATION OF AFTER-IMAGES.

We have seen (above, p. 8) how the evidence which Dodge adduces to disprove the hypothesis of anæsthesia is not conclusive, since, although an image imprinted on the retina during its movement was seen, yet nothing showed that it was seen before the eye had come to rest.

Having convinced himself that there is after all no anæsthesia, Dodge devised a very ingenious attachment for a perimeter 'to determine just what is seen during the eye-movement.'18 The eye was made to move through a known arc, and during its movement to pass by a very narrow slit. Behind this slit was an illuminated field which stimulated the retina. And since only during its movement was the pupil opposite the slit, so only during the movement could the stimulation be given. In the first experiments nothing at all of the illuminated field was seen, and Dodge admits (ibid., p. 461) that this fact 'is certainly suggestive of a central explanation for the absence of bands of fusion under ordinary conditions.' But "these failures suggested an increase of the illumination of the field of exposure.... Under these conditions a long band of light was immediately evident at each movement of the eye." This and similar observations were believed 'to show experimentally that when a complex field of vision is perceived during eye-movement it is seen fused' (p. 462).

Between the 'failures' and the cases when a band of light was seen, no change in the conditions had been introduced except 'an increase of the illumination.' Suppose now this change made just the difference between a stimulation which left no appreciable after-image, and one which left a distinct one. And is it even possible, in view of the extreme rapidity of eye-movements, that a retinal stimulation of any considerable intensity should not endure after the movement, to be then perceived, whether or not it had been first 'perceived during the movement'?[19]

Both of Dodge's experiments are open to the same objection. They do not admit of distinguishing between consciousness of a retinal process during the moment of stimulation, and consciousness of the same process just afterward. In both his cases the stimulation was given during the eye-movement, but there was nothing to prove that it was perceived at just the same moment. Whatever the difficulties of demonstrating an anæsthesia during movement, an experiment which does not observe the mentioned distinction can never disprove the hypothesis.

Fig. 2.

For the sake of a better understanding of these bands of light of Dodge, a perimeter was equipped in as nearly the manner described by him (ibid., p. 460) as possible. Experiments with the eye moving past a very narrow illuminated slit confirmed his observations. If the light behind the slit was feeble, no band was seen; if moderately bright, a band was always seen. The most striking fact, however, was that the band was not localized behind the slit, but was projected on to that point where the eye came to rest. The band seemed to [20]appear at this point and there to hover until it faded away. This apparent anomaly of localization, which Dodge does not mention, suggests the localization which Schwarz describes of his streaks. Hereupon the apparatus was further modified so that, whereas Dodge had let the stimulation take place only during the movement of the eye across a narrow slit between two walls, now either one of these walls could be taken away, allowing the stimulation to last for one half of the time of movement, and this could be either the first or the second half at pleasure. A plan of the perimeter so arranged is given in Fig. 2.

PBCDB'P is the horizontal section of a semicircular perimeter of 30 cm. radius. E is an eye-rest fixed at the centre of the semicircle; CD is a square hole which is closed by the screen S fitted into the front pair of the grooves GG. In the center of S and on a level with the eye E is a hole A, 2 cm. in diameter, which contains a 'jewel' of red glass. The other two pairs of grooves are made to hold pieces of milk-or ground-glass, as M, which may be needed to temper the illumination down to the proper intensity. L is an electric lamp. B and B' are two white beads fixed to the perimeter at the same level as E and A, and used as fixation-points. Although the room is darkened, these beads catch enough light to be just visible against the black perimeter, and the eye is able to move from one to the other, or from A to either one, with considerable accuracy. They leave a slight after-image streak, which is, however, incomparably fainter than that left by A (the streak to be studied), and which is furthermore white while that of A is bright red. B and B' are adjustable along a scale of degrees, which is not shown in the figure, so that the arc of eye-movement is variable at will. W is a thin, opaque, perpendicular wall extending from E to C, that is, standing on a radius of the perimeter. At E this wall comes to within about 4 mm. of the cornea, and when the eye is directed toward B the wall conceals the red spot A from the pupil. W can at will be transferred to the position ED. A is then hidden if the eye looks toward B'.

The four conditions of eye-movement to be studied are indicated in Fig. 3 (Plate I.). The location of the retinal stimulation is also shown for each case, as well as the corresponding appearance of the streaks, their approximate length, and above all their localization. For the sake of simplicity the refractive effect of the lens and humors of the eye is not shown, the path of the light-rays being in each case drawn straight. In all four cases the eye moved without stopping, through an arc of 40°.

Psychological Review. Monograph Supplement, 17. Plate I.

Fig 3.
HOLT ON EYE-MOVEMENT.

[21]To take the first case, Fig. 3:1. The eye fixates the light L, then sweeps 40° toward the right to the point B'. The retina is stimulated throughout the movement, l-l'. These conditions yield the phenomenon of both streaks, appearing as shown on the black rectangle.

In the second case (Fig. 3:2) the wall W is in position and the eye so adjusted in the eye-rest that the light L is not seen until the eye has moved about 10° to the right, that is, until the axis of vision is at Ex. Clearly, then, the image of L falls at first a little to the right of the fovea, and continues in indirect vision to the end of the movement. The stimulated part of the retina is l-l' (Fig. 3:2). Here, then, we have no stimulation of the eye during the first part of its movement. The corresponding appearance of the streak is also shown. Only the correctly localized streak is seen, extending from the light L toward the right but not quite reaching B'. Thus by cutting out that portion of the stimulation which was given during the first part of the movement, we have eliminated the whole of the false image, and the right-hand (foveal) part of the correct image.

Fig. 3:3 shows the reverse case, in which the stimulation is given only during the first part of the movement. The wall is fixed on the right of L, and the eye so adjusted that L remains in sight until the axis of vision reaches position Ex, that is, until it has moved about 10°. A short strip of the retina next the fovea is here stimulated, just the part which in case 2 was not stimulated; and the part which in case 2 was, is here not stimulated. Now here the false streak is seen, together with just that portion of the correct streak which in the previous case was not seen. The latter is relatively dim.

Thus it looks indeed as if the streak given during the first [22]part of an eye-movement is seen twice and differently localized. But one may say: The twice-seen portion was in both cases on the fovea; this may have been the conditioning circumstance, and not the fact of being given in the early part of the movement.

We must then consider Fig. 3, case 4. Here the eye moves from B to B', through the same arc of 40°. The wall W is placed so that L cannot be seen until the axis of vision has moved from EB to EL, but then L is seen in direct vision. Its image falls full on the fovea. But one streak, and that the correctly localized one, is seen. This is like case 2, except that here the streak extending from L to the right quite reaches the final fixation-point B'. It is therefore not the fact of a stimulation being foveal which conditions its being seen in two places.

It should be added that this experiment involves no particular difficulties of observation, except that in case 4 the eye tends to stop midway in its movement when the spot of light L comes in view. Otherwise no particular training of the subject is necessary beyond that needed for the observing of any after-image. Ten persons made the foregoing observations and were unanimous in their reports.

This experiment leaves it impossible to doubt that the conjecture of Schwarz, that the correct image is only the false one seen over again, is perfectly true. It would be interesting to enquire what it is that conditions the length of the false streak. It is never more than one third that of the correct streak (Fig. 3:1; except of course under the artificial conditions of Fig. 3:3) and may be less. The false streak seems originally to dart out from the light, as described by Lipps, visibly growing in length for a certain distance, and then to be suddenly eclipsed or blotted out simultaneously in all its parts. Whereas the fainter, correct streak flashes into consciousness all parts at once, but disappears by fading gradually from one end, the end which lies farther from the light.

Certain it is that when the false streak stops growing and is eclipsed, some new central process has intervened. One has next to ask, Is the image continuously conscious, suffering only an instantaneous relocalization, or is there a moment of central [23]anæsthesia between the disappearance of the false streak and the appearance of the other? The relative dimness of the second streak in the first moment of its appearance speaks for such a brief period of anæsthesia, during which the retinal process may have partly subsided.

We have now to seek some experimental test which shall demonstrate definitely either the presence or the absence of a central anæsthesia during eye-movements. The question of head-movements will be deferred, although, as we have seen above, these afford equally the phenomenon of twice-localized after-images.

IV. THE PENDULUM-TEST FOR ANÆSTHESIA.

A. Apparatus must be devised to fulfil the following conditions. A retinal stimulation must be given during an eye-movement. The moment of excitation must be so brief and its intensity so low that the process shall be finished before the eye comes to rest, that is, so that no after-image shall be left to come into consciousness after the movement is over. Yet, on the other hand, it must be positively demonstrated that a stimulation of this very same brief duration and low intensity is amply strong enough to force its way into consciousness if no eye-movement is taking place. If such a stimulation, distinctly perceived when the eye is at rest, should not be perceptible if given while the eye is moving, we should have a valid proof that some central process has intervened during the movement, to shut out the stimulation-image during that brief moment when it might otherwise have been perceived.

Obviously enough, with the perimeter arrangement devised by Dodge, where the eye moves past a narrow, illuminated slit, the light within the slit can be reduced to any degree of faintness. But on the other hand, it is clearly impossible to find out how long the moment of excitation lasts, and therefore impossible to find out whether an excitation of the same duration and intensity is yet sufficient to affect consciousness if given when the eye is not moving. Unless the stimulation is proved to be thus sufficient, a failure to see it when given during an eye-movement would of course prove nothing at all.[24]

Perhaps the most exact way to measure the duration of a light-stimulus is to let it be controlled by the passing of a shutter which is affixed to a pendulum. Furthermore, by means of a pendulum a stimulation of exactly the same duration and intensity can be given to the moving, as to the resting eye. Let us consider Fig. 4:1. If P is a pendulum bearing an opaque shield SS pierced by the hole tt, and BB an opaque background pierced by the hole i behind which is a lamp, it is clear that if the eye is fixed on i, a swing of the pendulum will allow i to stimulate the retina during such a time as it takes the opening tt to move past i. The shape of i will determine the shape of the image on the retina, and the intensity of the stimulation can be regulated by ground-or milk-glass interposed between the hole i and the lamp behind it. The duration of the exposure can be regulated by the width of tt, by the length of the pendulum, and by the arc through which it swings.

If now the conditions are altered, as in Fig. 4:2, so that the opening tt (indicated by the dotted line) lies not in SS, but in the fixed background BB, while the small hole i now moves with the shield SS, it necessarily follows that if the eye can move at just the rate of the pendulum, it will receive a stimulation of exactly the same size, shape, duration, and intensity as in the previous case where the eye was at rest. Furthermore, it will always be possible to tell whether the eye does move at the same rate as the pendulum, since if it moves either more rapidly or more slowly, the image of i on the retina will be horizontally elongated, and this fact will be given by a judgment as to the proportions of the image seen.

It may be said that since the eye does not rotate like the pendulum, from a fulcrum above, the image of i in the case of the moving eye will be distorted as is indicated in Fig. 4, a. This is true, but the distortion will be so minute as to be negligible if the pendulum is rather long (say a meter and a half) and the opening tt rather narrow (say not more than ten degrees wide). A merely horizontal movement of the eye will then give a practically exact superposition of the image of i at all moments of the exposure.

Psychological Review. Monograph Supplement, 17. Plate II.

Fig. 4.

Fig. 6
HOLT ON EYE-MOVEMENT.

[25]Thus much of preliminary discussion to show how, by means of a pendulum, identical stimulations can be given to the moving and to the resting eye. We return to the problem. It is to find out whether a stimulation given during an eye-movement can be perceived if its after-image is so brief as wholly to elapse before the end of the movement. If a period of anæsthesia is to be demonstrated, two observations must be made. First, that the stimulation is bright enough to be unmistakably visible when given to the eye at rest; second, that it is not visible when given to the moving eye. Hence, we shall have three cases.

Fig. 5 shows the exact arrangement of the experiment. The figure represents a horizontal section at the eye-level of the pendulum of Fig. 4, with accessories. E is the eye which moves between the two fixation-points P and P'. WONW is a wall which conceals the mechanism of the pendulum from the subject. ON is a rectangular hole 9 cm. wide and 7 cm. high, in this wall. SS is the shield which swings with the pendulum, and BB is the background (cf. Fig. 4). When the pendulum is not swinging, a hole in the shield lies behind ON and exactly corresponds with it. Another in the background does the same. The eye can thus see straight through to the light L.

Each of these three holes has grooves to take an opaque card, x, y, or z; there are two cards for the three grooves, and they are pierced with holes to correspond to i and tt of Fig. 4. The background BB has a second groove to take a piece of milk-glass M. These cards are shown in Fig. 6 (Plate II.) Card I bears a hole 5 cm. high and shaped like a dumb-bell. The diameter of the end-circles (e, e) is 1.3 cm., and the width of the handle h is 0.2 cm. Card T is pierced by two slits[26] EE, EE, each 9 cm. long and 1.3 cm. high, which correspond to the two ends of the dumb-bell. These slits are connected by a perforation H, 1.5 cm. wide, which corresponds to the handle of the dumb-bell. This opening EEHEE is covered by a piece of ground-glass which serves as a radiating surface for the light.

Fig. 5.

The distance EA (Fig. 5) is 56 cm., and PP' is 40 cm.; so that the arc of eye-movement, that is, the angle PEP', is [27]very nearly 40°, of which the 9-cm. opening ON 9° 11'. SS is 2 cm. behind ON, and BB 2 cm. behind SS; these distances being left to allow the pendulum to swing freely.

It is found under these conditions that the natural speed made by the eye in passing the 9-cm. opening ON is very well approximated by the pendulum if the latter is allowed to fall through 23.5° of its arc, the complete swing being therefore 47°. The middle point of the pendulum is then found to move from O to N in 110σ19. If the eye sweeps from O to N in the same time, it will be moving at an angular velocity of 1° in 11.98σ (since the 9 cm. are 9° 11' of eye-movement). This rate is much less than that found by Dodge and Cline (op. cit., p. 155), who give the time for an eye-movement of 40° as 99.9σ, which is an average of only 2.49σ to the degree. Voluntary eye-movements, like other voluntary movements, can of course be slow or fast according to conditions. After the pendulum has been swinging for some time, so that its amplitude of movement has fallen below the initial 47° and therewith its speed past the middle point has been diminished, the eye in its movements back and forth between the fixation-points can still catch the after-image of i perfectly distinct and not at all horizontally elongated, as it would have to be if eye and pendulum had not moved just together. It appears from this that certain motives are able to retard the rate of voluntary movements of the eye, even when the distance traversed is constant.

The experiment is now as follows. The room is darkened. Card T is dropped into groove z, while I is put in groove y and swings with the pendulum. One eye alone is used.

Case 1. The eye is fixed in the direction EA. The pendulum is allowed to swing through its 47°. The resulting visual image is shown in Fig. 7:1. Its shape is of course like T, Fig. 6, but the part H is less bright than the rest because it is exposed a shorter time, owing to the narrowness of the handle of the dumb-bell, which swings by and mediates the exposure. Sheets of milk-glass are now dropped into the back groove of[28] BB, until the light is so tempered that part H (Fig. 7:1) is barely but unmistakably visible as luminous. The intensity actually used by the writer, relative to that of EE, is fairly shown in the figure. (See Plate III.)

It is clear, if the eye were now to move with the pendulum, that the same amount of light would reach the retina, but that it would be concentrated on a horizontally narrower area. And if the eye moves exactly with the pendulum, the visual image will be no longer like 1 but like 2 (Fig. 7). We do not as yet know how the intensities of e, e and h will relatively appear. To ascertain this we must put card I into groove x, and let card T swing with the pendulum in groove y. If the eye is again fixed in the direction EA (Fig. 5), the retina receives exactly the same stimulation that it would have received before the cards were shifted if it had moved exactly at the rate of the pendulum. In the experiments described, the handle h of this image (Fig. 7:2) curiously enough appears of the same brightness as the two ends e, e, although, as we know, it is stimulated for a briefer interval. Nor can any difference between e, e and h be detected in the time of disappearance of their after-images. These conditions are therefore generous. The danger is that h of the figure, the only part of the stimulation which could possibly quite elapse during the movement, is still too bright to do so.

Case 2. The cards are replaced in their first positions, T in groove z, I in groove y which swings. The subject is now asked to make voluntary eye-sweeps from P to P' and back, timing his moment of starting so as to bring his axis of vision on to the near side of opening ON at approximately the same time as the pendulum brings I on the same point. This is a delicate matter and requires practice. Even then it would be impossible, if the subject were not allowed to get the rhythm of the pendulum before passing judgment on the after-images. The pendulum used gives a slight click at each end of its swing, and from the rhythm of this the subject is soon able to time the innervation of his eye so that the exposure coincides with the middle of the eye-movement.

Psychological Review. Monograph Supplement, 17. Plate III.

Fig. 7.
HOLT ON EYE-MOVEMENT.

[29]It is true that with every swing the pendulum moves more slowly past ON, and the period of exposure is lengthened. This, however, only tends to make the retinal image brighter, so that its disappearance during an anæsthesia would be so much the less likely. The pendulum may therefore be allowed to 'run down' until its swing is too slow for the eye to move with it, that is, too slow for a distinct, non-elongated image of i to be caught in transit on the retina.

With these eye-movements, the possible appearances are of two classes, according to the localization of the after-image. The image is localized either at A (Fig. 5), or at the final fixation-point (P or P', according to the direction of the movement). Localized at A, the image may be seen in either of two shapes. First, it may be identical with 1, Fig. 7. It is seen somewhat peripherally, judgment of indirect vision, and is correctly localized at A. When the subject's eye is watched, it is found that in this case it moved either too soon or too late, so that when the exposure was made, the eye was resting quietly on one of the fixation-points and so naturally received the same image as in case 1, except that now it lies in indirect vision, the eye being directed not toward A (as in case 1) but towards either P or P'.

Second, the image correctly localized may be like 2 (Fig. 7), and then it is seen to move past the opening ON. The handle h looks as bright as e, e. This appearance once obtained generally recurs with each successive swing of the pendulum, and scrutiny of the subject's eye shows it to be moving, not by separate voluntary innervations from P to P' and then from P' to P, but continuously back and forth with the swing of the pendulum, much as the eye of a child passively follows a moving candle. This movement is purely reflex,20 governed probably by cerebellar centers. It seems to consist in a rapid succession of small reflex innervations, and is very different from the type of movement in which one definite innervation carries the eye through its 42°, and which yielded the phenomena with the perimeter. A subject under the spell of this reflex must be exercised in innervating his eye to move from P to P'[30] and back in single, rapid leaps. For this, the pendulum is to be motionless and the eye is not to be stimulated during its movement.

These two cases in which the image is localized midway between P and P' interest us no further. Localized on the final fixation-point, the image is always felt to flash out suddenly in situ, just as in the case of the 'correctly localized' after-image streaks in the experiments with the perimeter. The image appears in one of four shapes, Fig. 7: 2 or 3, 4 or 5.

First, the plain or elongated outline of the dumb-bell appears with its handle on the final fixation-point (2 or 3). The image is plain and undistorted if the eye moves at just the rate of the pendulum, elongated if the eye moves more rapidly or more slowly. The point that concerns us is that the image appears with its handle. Two precautions must here be observed.

The eye does not perhaps move through its whole 42°, but stops instead just when the exposure is complete, that is, stops on either O or N and considerably short of P or P'. It then follows that the exposure is given at the very last part of the movement, so that the after-image of even the handle h has not had time to subside. The experiment is planned so that the after-image of h shall totally elapse during that part of the movement which occurs after the exposure, that is, while the eye is completing its sweep of 42°, from O to P, or else from N to P'. If the arc is curtailed at point O or N, the handle of the dumb-bell will of course appear. The fact can always be ascertained by asking the subject to notice very carefully where the image is localized. If the eye does in fact stop short at O or N, the image will be there localized, although the subject may have thoughtlessly said before that it was at P or P', the points he had nominally had in mind.

But the image 2 or 3 may indeed be localized quite over the final fixation-point. In this case the light is to be looked to. It is too bright, as it probably was in the case of Dodge's experiments. It must be further reduced; and with the eye at rest, the control (case I) must be repeated. In the experiments here described it was always found possible so to reduce the [31]light that the distinct, entire image of the dumb-bell (2, Fig. 7) never appeared localized on the final fixation-point, although in the control, H, of Fig. 7:1, was always distinctly visible.

With these two precautions taken, the image on the final fixation-point is like either 3, 4, or 5. Shape 5 very rarely appears, while the trained subject sees 4 and 3 each about one half the times; and either may be seen for as many as fifteen times in succession.

Shape 4 is of course exactly the appearance which this experiment takes to be crucial evidence of a moment of central anæsthesia, before the image is perceived and during which the stimulation of the handle h completely elapses. Eight subjects saw this phenomenon distinctly and, after some training in timing their eye-movements, habitually. The first appearance of the handleless image was always a decided surprise to the subject (as also to the writer), and with some eagerness each hastened to verify the phenomenon by new trials.

The two ends (e, e) of the dumb-bell seem to be of the same intensity as in shape 2 when seen in reflex movement. But there is no vestige whatsoever of a handle. Two of the subjects stated that for them the place where the handle should have been, appeared of a velvety blackness more intense than the rest of the background. The writer was not able to make this observation. It coincides interestingly with that of von Kries,21 who reports as to the phases of fading after-images, that between the disappearance of the primary image and the appearance of the 'ghost,' a moment of the most intense blackness intervenes. The experiments with the pendulum, however, brought out no ghost.

We must now enquire why in about half the cases shape 3 is still seen, whereas shape 5 occurs very rarely. Some of the subjects, among whom is the writer, never saw 5 at all. We should expect that with the intensity of H sufficiently reduced 4 and 5 would appear with equal frequency, whereas 3 would be seen no oftener than 2; shape 5 appearing when the eye did not, and 4 when it did, move at just the rate of the pendulum.[32] It is certain that when 4 is seen, the eye has caught just the rate of the pendulum, and that for 3 or 5 it has moved at some other rate. We have seen above (p. 27) that to move with the pendulum the eye must already move decidedly more slowly than Dodge and Cline find the eye generally to move. Nothing so reliable in regard to the rate of voluntary eye-movements as these measurements of Dodge and Cline had been published at the time when the experiments on anæsthesia were carried on, and it is perhaps regrettable that in the 'empirical' approximation of the natural rate of the eye through 40° the pendulum was set to move so slowly.

In any case it is highly probable that whenever the eye did not move at just the rate of the pendulum, it moved more rapidly rather than more slowly. The image is thus horizontally elongated, by an amount which varies from the least possible up to 9 cm. (the width of the opening in T), or even more. And while the last of the movement (O to P, or N to P'), in which the stimulation of H' is supposed to subside, is indeed executed, it may yet be done so rapidly that after all H' cannot subside, not even although it is now less intense by being horizontally spread out (that is, less concentrated than the vanished h of shape 4). This explanation is rendered more probable by the very rare appearance of shape 5, which must certainly emerge if ever the eye were to move more slowly than the pendulum.

The critical fact is, however, that shape 4 does appear to a trained subject in about one half the trials—a very satisfactory ratio when one considers the difficulty of timing the beginning of the movement and its rate exactly to the pendulum.

Lastly, in some cases no image appears at all. This was at first a source of perplexity, until it was discovered that the image of the dumb-bell, made specially small so as to be contained within the area of distinct vision, could also be contained on the blind-spot. With the pendulum at rest the eye could be so fixed as to see not even the slight halo which diffuses in the eye and seems to lie about the dumb-bell. It may well occur, then, that in a movement the image happens to fall on the blind-spot and not on the fovea. That this accounts for the cases where no [33]image appears, is proved by the fact that if both eyes are used, some image is always seen. A binocular image under normal convergence can of course not fall on both blind-spots. It may be further said that the shape 4 appears as well when both eyes are used as with only one. The experiment may indeed as well be carried on with both eyes.

Some objections must be answered. It may be said that the image of h happens to fall on the blind-spot, e and e being above and below the same. This is impossible, since the entire image and its halo as well may lie within the blind-spot. If now h is to be on the blind-spot, at least one of the end-circles e, e will be there also, whereas shape 4 shows both end-circles of the dumb-bell with perfect distinctness.

Again, it cannot properly be urged that during the movement the attention was distracted so as not to 'notice' the handle. The shape of a dumb-bell was specially chosen for the image so that the weaker part of the stimulation should lie between two points which should be clearly noticed. Indeed, if anything, one might expect this central, connecting link in the image to be apperceptively filled in, even when it did not come to consciousness as immediate sensation. And it remains to ask what it is which should distract the attention.

In this connection the appearance under reflex eye-movement compares interestingly with that under voluntary. If the wall WONW (Fig. 5) is taken from before the pendulum, and the eye allowed to move reflexly with the swinging dumb-bell, the entire image is seen at each exposure, the handle seeming no less bright than the end-circles. Moreover, as the dumb-bell opening swings past the place of exposure and the image fades, although the handle must fade more quickly than the ends, yet this is not discernible, and the entire image disappears without having at any time presented the handleless appearance.

B. Another test for this anæsthesia during movement is offered in the following experiment. It is clear that, just as a light-stimulation is not perceived if the whole retinal process begins and ends during a movement, so also a particular phase of it should not be perceived if that phase can be given complete within the time of the movement. The same pendulum which [34]was used in the previous experiment makes such a thing possible. If in place of the perforated dumb-bell the pendulum exposes two pieces of glass of nearly complementary colors, one after the other coming opposite the place of exposure, the sensations will fuse or will not fuse according as the pendulum swings rapidly or slowly. But now a mean rate of succession can be found such as to let the first color be seen pure before the second is exposed, and then to show the second fused with the after-image of the first. Under some conditions the second will persist after the first has faded, and will then itself be seen pure. Thus there may be three phases in consciousness. If the first color exposed is green and the second red, the phases of sensation will be green, white, and perhaps red. These phases are felt to be not simultaneous but successive. A modification of this method is used in the following experiment. (See Fig. 8, Plate IV.)

T and I here correspond to the cards T and I of Fig. 6. T consists of a rectangular opening, 9×5 cm., which contains three pieces of glass, two pieces of green at the ends, each 2.8 cm. wide and 7 cm. high, and a piece of red glass in the middle 3.4 cm. wide and only 1.5 cm. high, the space above and below this width being filled with opaque material. The shape of the image is determined as before by the hole in I, which now, instead of being a dumb-bell, is merely a rectangular hole 2 cm. wide and 5 cm. high. Exactly as before, T is fixed in the background and I swings with the pendulum, the eye moving with it.

The speed of the pendulum must be determined, such that if I lies in the front groove (Fig. 5, x) and the eye is at rest, the image will clearly show two phases of color when T swings past on the pendulum. With T and I as described above, a very slow pendulum shows the image green, red (narrow), and green, in succession. A very fast pendulum shows only a horizontal straw-yellow band on a green field (Fig. 8:5). There is but one phase and no feeling of succession. Between these two rates is one which shows two phases—the first a green field with a horizontal, reddish-orange band (Fig. 8:3), the second quickly following, in which the band is straw-yellow (5). It might be expected that this first phase would be preceded by an entirely green phase, since green is at first exposed. Such is however not the case. The straw-yellow of the last phase is of course the fusion-color of the red and green glasses. It would be gray but that the two colors are not perfectly complementary. Since the arrangement of colors in T is bilaterally symmetrical, the successive phases are the same in whichever direction the pendulum swings.

Psychological Review. Monograph Supplement, 17. Plate IV.

Fig. 8.
HOLT ON EYE-MOVEMENT.

[35]It is desirable to employ the maximum rate of pendulum which will give the two phases. For this the illumination should be very moderate, since the brighter it is, the slower must be the pendulum. With the degree of illumination used in the experiments described, it was found that the pendulum must fall from a height of only 9.5° of its arc: a total swing of 19°. The opening of T, which is 9 cm. wide, then swings past the middle point of I in 275σ.

Now when the eye moves it must move at this rate. If the eye is 56 cm. distant from the opening, as in the previous case, the 9 cm. of exposure are 9° 11' of eye-movement, and we saw above that 9° 11' in 110σ is a very slow rate of movement, according to the best measurements. Now it is impossible for the eye to move so slowly as 9° 11' in 275σ. If, however, the eye is brought nearer to the opening, it is clear that the 9 cm. of exposure become more than 9° 11' of eye-movement. Therefore the eye and the fixation-points are so placed that EA (Fig. 5) = 26 cm. and PP' = 18 cm. The total eye-movement is thus 38° 11', of which the nine-centimeter distance of exposure is 19° 38'. Now the eye is found to move very well through 19° 38' in 275σ, although, again, this is much more than a proportionate part of the total time (99.9σ) given by Dodge and Cline for a movement of the eye through 40°. The eye is in this case also moving slowly. As before, it is permissible to let the pendulum run down till it swings too slowly for the eye to move with it; since any lessened speed of the pendulum only makes the reddish-orange phase more prominent.

As in the experiment with the dumb-bell, we have also here three cases: the control, the case of the eye moving, and again a control.[36]

Case 1. T swings with the pendulum. I is placed in the front groove, and the eye looks straight forward without moving. The pendulum falls from 9.5° at one side, and the illumination is so adjusted that the phase in which the band is reddish-orange, is unmistakably perceived before that in which it is straw-yellow. The appearance must be 3 followed by 5 (Fig. 8).

Case 2. T is fixed in the background, I on the pendulum, and the phenomena are observed with the eye moving.

Case 3. A repetition of case 1, to make sure that no different adaptation or fatigue condition of the eye has come in to modify the appearance of the two successive phases as at first seen.

The possible appearances to the moving eye are closely analogous to those in the dumb-bell experiment. If the eye moves too soon or too late, so that it is at rest during the exposure, the image is like T itself (Fig. 8) but somewhat fainter and localized midway between the points P and P'. If the eye moves reflexly at the rate of the pendulum, the image is of the shape i and shows the two phases (3 followed by 5). It is localized in the middle and appears to move across the nine-centimeter opening.

A difficulty is met here which was not found in the case of the dumb-bell. The eye is very liable to come to a full stop on one of the colored surfaces, and then to move quickly on again to the final fixation-point. And this happens contrary to the intention of the subject, and indeed usually without his knowledge. This stopping is undoubtedly a reflex process, in which the cerebellar mechanism which tends to hold the fixation on any bright object, asserts itself over the voluntary movement and arrests the eye on the not moving red or green surface as the exposure takes place. A comparable phenomenon was found sometimes in the experiment with the dumb-bell, where an eye-movement commenced as voluntary would end as a reflex following of the pendulum. In the present experiment, until the subject is well trained, the stopping of the eye must be [37]watched by a second person who looks directly at the eye-ball of the subject during each movement. The appearances are very varied when the eye stops, but the typical one is shown in Fig. 8:1. The red strip AB is seldom longer and often shorter than in the figure. That part of it which is superposed on the green seldom shows the orange phase, being almost always of a pure straw-yellow. The localization of these images is variable. All observations made during movements in which the eye stops, are of course to be excluded.

If now the eye does not stop midway, and the image is not localized in the center, the appearance is like either 2, 4, or 5, and is localized over the final fixation-point. 2 is in all probability the case of the eye moving very much faster than the pendulum, so that if the movement is from left to right, the right-hand side of the image is the part first exposed (by the uncovering of the left-hand side of T), which is carried ahead by the too swift eye-movement and projected in perception on the right of the later portion. 3 is the case of the eye moving at very nearly but not quite the rate of the pendulum. The image which should appear 2 cm. wide (like the opening i) appears about 3 cm. wide. The middle band is regularly straw-yellow, extremely seldom reddish, and if we could be sure that the eye moves more slowly than the pendulum, so that the succession of the stimuli is even slower than in the control, and the red phase is surely given, this appearance (3) would be good evidence of anæsthesia during which the reddish-orange phase elapses. It is more likely, however, that the eye is moving faster than the pendulum, but whether or not so inconsiderably faster as still to let the disappearance of the reddish phase be significant of anæsthesia, is not certain until one shall have made some possible but tedious measurements of the apparent width of the after-image. Both here and in the following case the feeling of succession, noticeable between the two phases when the eye is at rest, has disappeared with the sensation of redness.

The cases in which 5 is seen are, however, indisputably significant. The image is apparently of just the height and width of i, and there is not the slightest trace of the reddish-orange phase. The image flashes out over the final fixation-point, [38]green and straw-yellow, just as the end-circles of the dumb-bell appeared without their handle. The rate of succession of the stimuli, green—red—green, on the retina, is identical with that rate which showed the two phases to the resting eye: for the pendulum is here moving at the very same rate, and the eye is moving exactly with the pendulum, as is shown by the absence of any horizontal elongation of the image seen. The trained subject seldom sees any other images than 4 and 5, and these with about equal frequency, although either is often seen in ten or fifteen consecutive trials. As in the cases of the falsely localized images and of the handleless dumb-bell, movements of both eyes, as well as of the head but not the eyes, yield the same phenomena. It is interesting again to compare the appearance under reflex movement. If at any time during the experiments the eye is allowed to follow the pendulum reflexly, the image is at once and invariably seen to pass through its two phases as it swings past the nine-centimeter opening.

The frequent and unmistakable appearance of this band of straw-yellow on a non-elongated green field without the previous phase in which the band is reddish-orange, although this latter was unmistakable when the same stimulation was given to the eye at rest, is authenticated by eight subjects. This appearance, together with that of the handleless dumb-bell, is submitted as a demonstration that during voluntary movements of the eyes, and probably of the head as well, there is a moment in which stimulations are not transmitted from the retina to the cerebral cortex, that is, a moment of central anæsthesia. The reason for saying 'and probably of the head as well,' is that although the phenomena described are gotten equally well from movements of the head, yet it is not perfectly certain that when the head moves the eyes do not also move slightly within the head, even when the attempt is made to keep them fixed.

Most of the criticisms which apply to this last experiment apply to that with the dumb-bell and have already been answered. There is one however which, while applying to that other, more particularly applies here. It would be, that [39]these after-images are too brief and indistinct to be carefully observed, so that judgments as to their shape, size, and color are not valid evidence. This is a perfectly sensible criticism, and a person thoroughly convinced of its force should repeat the experiments and decide for himself what reliance he will place on the judgments he is able to make. The writer and those of the subjects who are most trained in optical experiments find the judgments so simple and easily made as not to be open to doubt.

In the first place, it should be remembered that only those cases are counted in which the movement was so timed that the image was seen in direct vision, that is, was given on or very near the fovea. In such cases a nice discrimination of the shape and color of the images is easily possible.

Secondly, the judgments are in no case quantitative, that is, they in no case depend on an estimate of the absolute size of any part of the image. At most the proportions are estimated. In the case of the dumb-bell the question is, Has the figure a handle? The other question, Are the end-circles horizontally elongated? has not to be answered with mathematical accuracy. It is enough if the end-circles are approximately round, or indeed are narrower than 9 cm. horizontally, for at even that low degree of concentration the handle was still visible to the resting eye. Again, in the experiment with the color-phases, only two questions are essential to identify the appearance 5: Does the horizontal yellow band extend quite to both edges of the image? and, Is there certainly no trace of red or orange to be seen? The first question does not require a quantitative judgment, but merely one as to whether there is any green visible to the right or left of the yellow strip. Both are therefore strictly questions of quality. And the two are sufficient to identify appearance 5, for if no red or orange is visible, images 1, 2, and 3 are excluded; and if no green lies to the right or left of the yellow band, image 4 is excluded. Thus if one is to make the somewhat superficial distinction between qualitative and quantitative judgments, the judgments here required are qualitative. Moreover, the subjects make these judgments unhesitatingly.[40]

Finally, the method of making judgments on after-images is not new in psychology. Lamansky's well-known determination of the rate of eye-movements22 depends on the possibility of counting accurately the number of dots in a row of after-images. A very much bolder assumption is made by Guillery23 in another measurement of the rate of eye-movements. A trapezoidal image was generated on the moving retina, and the after-image of this was projected on to a plane bearing a scale of lines inclining at various angles. On this the degree of inclination of one side of the after-image was read off, and thence the speed of the eye-movement was calculated. In spite of the boldness of this method, a careful reading of Guillery's first article cited above will leave no doubt as to its reliability, and the accuracy of discrimination possible on these after-images.

As to judgments on the color and color-phases of after-images, there is ample precedent in the researches of von Helmholtz, Hering, Hess, von Kries, Hamaker, and Munk. It is therefore justifiable to assume the possibility of making accurately the four simple judgments of shape and color described above, which are essential to the two proofs of anæsthesia.

V. SUMMARY AND COROLLARIES OF THE EXPERIMENTS, AND A PARTIAL, PHYSIOLOGICAL INTERPRETATION OF THE CENTRAL ANÆSTHESIA.

We have now to sum up the facts given by the experiments. The fact of central anæsthesia during voluntary movement is supported by two experimental proofs, aside from a number of random observations which seem to require this anæsthesia for their explanation. The first proof is that if an image of the shape of a dumb-bell is given to the retina during an eye-movement, and in such a way that the handle of the image, while positively above the threshold of perception, is yet of brief enough duration to fade completely before the end of the movement, it then happens that both ends of the dumb-bell are seen but the handle not at all. The fact of its having been properly [41]given to the retina is made certain by the presence of the now disconnected ends.

The second proof is that, similarly, if during an eye-movement two stimulations of different colors are given to the retina, superposed and at such intensity and rate of succession as would show to the resting eye two successive phases of color (in the case taken, reddish-orange and straw-yellow), it then happens that the first phase, which runs its course and is supplanted by the second before the movement is over, is not perceived at all. The first phase was certainly given, because the conditions of the experiment require the orange to be given if the straw-yellow is, since the straw-yellow which is seen can be produced only by the addition of green to the orange which is not seen.

These two phenomena seem inevitably to demonstrate a moment during which a process on the retina, of sufficient duration and intensity ordinarily to determine a corresponding conscious state, is nevertheless prevented from doing so. One inclines to imagine a retraction of dendrites, which breaks the connection between the central end of the optic nerve and the occipital centers of vision.

The fact of anæsthesia demonstrated, other phenomena are now available with further information. From the phenomena of the 'falsely localized' images it follows that at least in voluntary eye-movements of considerable arc (30° or more), the anæsthesia commences appreciably later than the movement. The falsely localized streak is not generated before the eye moves, but is yet seen before the correctly localized streak, as is shown by the relative intensities of the two. The anæsthesia must intervene between the two appearances. The conjecture of Schwarz, that the fainter streak is but a second appearance of the stronger, is undoubtedly right.

We know too that the anæsthesia depends on a mechanism central of the retina, for stimulations are received during movement but not transmitted to consciousness till afterward. This would be further shown if it should be found that movements of the head, no less than those of the eyes, condition the anæsthesia. As before said, it is not certain that the eyes do not move slightly in the head while the head moves. The movement of [42]the eyes must then be very slight, and the anæsthesia correspondingly either brief or discontinuous. Whereas, the phenomena are the same when the head moves 90° as when the eyes move that amount. It seems probable, then, that voluntary movements of the head do equally condition the anæsthesia.

We have seen, too, that in reflex eye-or head-movements no anæsthesia is so far to be demonstrated. The closeness with which the eye follows the unexpected gyrations of a slowly waving rush-light, proves that the reflex movement is produced by a succession of brief impulses (probably from the cerebellum), each one of which carries the eye through only a very short distance. It is an interesting question, whether there is an instant of anæsthesia for each one of these involuntary innervations—an instant too brief to be revealed by the experimental conditions employed above. The seeming continuity of the sensation during reflex movement would of course not argue against such successive instants of anæsthesia, since no discontinuity of vision during voluntary movement is noticeable, although a relatively long moment of anæsthesia actually intervenes.

But decidedly the most interesting detail about the anæsthesia is that shown by the extreme liability of the eye to stop reflexly on the red or the green light, in the second experiment with the pendulum. Suppose the eye to be moving from P to P' (Fig. 5); the anæsthesia, although beginning later than the movement, is present when the eye reaches O, while it is between O and N, that is, during the anæsthetic moment, that the eye is reflexly caught and held by the light. This proves again that the anæsthesia is not retinal, but it proves very much more; namely, that the retinal stimulation is transmitted to those lower centers which mediate reflex movements, at the very instant during which it is cut off from the higher, conscious centers. The great frequency with which the eye would stop midway in its movements, both in the second pendulum-experiment and in the repetition of Dodge's perimeter-test, was very annoying at the time, and the observation cannot be questioned. The fact of the habitual reflex regulation of voluntary movements is [43]otherwise undisputed. Exner24 mentions a variety of similar instances. Also, with the moving dumb-bell, as has been mentioned, the eye having begun a voluntary sweep would often be caught by the moving image and carried on thereafter reflexly with the pendulum. These observations hang together, and prove a connection between the retina and the reflex centers even while that between the retina and the conscious centers is cut off.

But shall we suppose that the 'connection' between the retina and the conscious centers is cut off during the central anæsthesia? All that the facts prove is that the centers are at that time not conscious. It would be at present an unwarrantable assumption to make, that these centers are therefore disconnected from the retina, at the optic thalami, the superior quadrigeminal bodies, or wheresoever. On broad psychological grounds the action-theory of Münsterberg25 has proposed the hypothesis that cerebral centers fail to mediate consciousness not merely when no stimulations are transmitted to them, but rather when the stimulations transmitted are not able to pass through and out. The stimulation arouses consciousness when it finds a ready discharge. And indeed, in this particular case, while we have no other grounds for supposing stimulations to the visual centers to be cut off, we do have other grounds for supposing that egress from these cells would be impeded.

The occipital centers which mediate sensations of color are of course most closely associated with those other centers (probably the parietal) which receive sensations from the eye-muscles and which, therefore, mediate sensations which furnish space and position to the sensations of mere color. Now it is these occipital centers, mediators of light-sensations merely, which the experiments have shown most specially to be anæsthetic. The discharge of such centers means particularly the passage of excitations on to the parietal localization-centers. There are doubtless other outlets, but these are the chief group. The [44]movements, for instance, which activity of these cells produces, are first of all eye-movements, which have to be directly produced (according to our present psychophysical conceptions) by discharges from the centers of eye-muscle sensation. The principal direction of discharge, then, from the color-centers is toward the localization-centers.

Now the experiment with falsely and correctly localized after-images proves that before the anæsthesia all localization is with reference to the point of departure, while afterwards it is with reference to the final fixation-point. The transition is abrupt. During the anæsthesia, then, the mechanism of localization is suffering a readjustment. It is proved that during this interval of readjustment in the centers of eye-muscle sensation the way is closed to oncoming discharges from the color-centers; but it is certain that any such discharge, during this complicated process of readjustment, would take the localization-centres by surprise, as it were, and might conceivably result in untoward eye-movements highly prejudicial to the safety of the individual as a whole. The much more probable event is the following:

Although Schwarz suggests that the moment between seeing the false and seeing the correct after-image is the moment that consciousness is taken up with 'innervation-feelings' of the eye-movement, this is impossible, since the innervation-feelings (using the word in the only permissible sense of remembered muscle-sensations) must precede the movement, whereas even the first-seen, falsely localized streak is not generated till the movement commences. But we do have to suppose that during the visual anæsthesia, muscle-sensations of present movement are streaming to consciousness, to form the basis of the new post-motum localization. And these would have to go to those very centers mentioned above, the localization-centers or eye-muscle sensation centers. One may well suppose that these incoming currents so raise the tension of these centers that for the moment no discharge can take place thither from other parts of the brain, among which are the centers for color-sensations. The word 'tension' is of course a figure, but it expresses the familiar idea that centers which are in process of receiving [45]peripheral stimulations, radiate that energy to other parts of the brain (according to the neural dispositions), and probably do not for the time being receive communications therefrom, since those other parts are now less strongly excited. It is, therefore, most probable that during the incoming of the eye-muscle sensations the centers for color are in fact not able to discharge through their usual channels toward the localization-centers, since the tension in that direction is too high. If, now, their other channels of discharge are too few or too little used to come into question, the action-theory would find in this a simple explanation of the visual anæsthesia.

The fact that the anæsthesia commences appreciably later than the movement so far favors this interpretation. For if the anæsthesia is conditioned by high tension in the localization-centers, due to incoming sensations from the eye-muscles, it could not possibly commence synchronously with the movement. For, first the sensory end-organs in the eye-muscles (or perhaps in the ligaments, surfaces of the eye-sockets, etc.) have their latent period; then the stimulation has to travel to the brain; and lastly it probably has to initiate there a summation-process equivalent to another latent period. These three processes would account very readily for what we may call the latent period of the anæsthesia, as observed in the experiments. It is true that this latent period was observed only in long eye-and head-movements, but the experiments were not delicate enough in this particular to bring out the finer points.

Finally, the conditioning of anæsthesia by movements of the head, if really proved, would rather corroborate this interpretation. For of course the position of the head on the shoulders is as important for localization of the retinal picture as the position of the eyes in the head, so that sensations of head-movements must be equally represented in the localization centers; and head movements would equally raise the tension on those centers against discharge-currents from the color-centers.

The conclusion from the foregoing experiments is that voluntary movements of the eyes condition a momentary, visual, central anæsthesia.

TACTUAL ILLUSIONS.

BY CHARLES H. RIEBER.

I.

Many profound researches have been published upon the subject of optical illusions, but in the field of tactual illusions no equally extensive and serious work has been accomplished. The reason for this apparent neglect of the illusions of touch is obviously the fact that the studies in the optical illusions are generally thought to yield more important results for psychology than corresponding studies in the field of touch. Then, too, the optical studies are more attractive by reason of the comparative ease and certainty with which the statistics are gathered there. An optical illusion is discovered in a single instance of the phenomenon. We are aware of the illusion almost immediately. But in the case of most of the illusions of touch, a large number of experiments is often necessary in order to reveal any approximately constant error in the judgments. Nevertheless, it seems to me that the factors that influence our judgments of visual space, though their effects are nearly always immediately apparent, are of no more vital significance for the final explanation of the origin of our notion of space than the disturbing factors in our estimations of tactual space whose effects are not so open to direct observation.

The present investigation has for its main object a critical examination of the tactual illusions that correspond to some of the well-known optical illusions, in the hope of segregating some of the various disturbing factors that enter into our very complex judgments of tactual space. The investigation has unavoidably extended into a number of near-lying problems in the psychology of touch, but the final object of my paper will be to offer a more decisive answer than has hitherto been given to the question, Are the optical illusions also tactual illusions, or are they reversed for touch?

[48]Those who have given their attention to illusions of sight and touch are rather unequally divided in their views as to whether the geometrical optical illusions undergo a reversal in the field of touch, the majority inclining to the belief that they are reversed. And yet there are not wanting warm adherents of the opposite view. A comparison of the two classes of illusions, with this question in view, appears therefore in the present state of divergent opinion to be a needed contribution to experimental psychology. Such an experimental study, if it succeeds in finding the solution to this debate, ought to throw some further light upon the question of the origin of our idea of space, as well as upon the subject of illusions of sense in general. For, on the one hand, if touch and sight function alike in our judgment of space, we should expect that like peripheral disturbances in the two senses would cause like central errors in judgment, and every tactual analogue of an optical illusion should be found to correspond both in the direction of the error and, to a certain extent, quantitatively with the optical illusion. But if, on the other hand, they are in their origin and in their developed state really disparate senses, each guided by a different psychological principle, the illusion in the one sense might well be the reverse of the corresponding illusion in the other sense. Therefore, if the results of an empirical study should furnish evidence that the illusions are reversed in passing from one field to the other, we should be obliged to conclude that we are here in the presence of what psychologists have been content to call the 'unanalyzable fact' that the two senses function differently under the same objective conditions. But if, on the contrary, it should turn out that the illusions are not reversed for the two senses, then the theory of the ultimate uniformity of the psychical laws will have received an important defence.

These experiments were carried on in the Harvard Psychological Laboratory during the greater part of the years 1898-1901. In all, fifteen subjects coöperated in the work at different times.

The experimental work in the direction of a comparison of the optical illusions with the tactual illusions, to the time of the present investigation, has been carried on chiefly with the familiar [49]optical illusion of the overestimation of filled space. If the distance between two points be divided into two equal parts by a point midway between them, and the one of the halves be filled with intermediate points, the filled half will, to the eye, appear longer than the open half. James1 says that one may easily prove that with the skin we underestimate a filled space, 'by taking a visiting card, and cutting one edge of it into a saw-toothed pattern, and from the opposite edge cutting out all but two corners, and then comparing the feelings aroused by the two edges when held against the skin.' He then remarks, 'the skin seems to obey a different law here from the eye.' This experiment has often been repeated and verified. The most extensive work on the problem, however, is that by Parrish.2 It is doubtless principally on the results of Parrish's experiments that several authors of text-books in psychology have based their assertions that a filled space is underestimated by the skin. The opposite conclusion, namely, that the illusion is not reversed for the skin, has been maintained by Thiéry,3 and Dresslar.4 Thiéry does not, so far as I know, state the statistics on which he bases his view. Dresslar's experiments, as Parrish has correctly observed, do no