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MEDTCAL LIBRARY COLUMBIA UMVERSIS| 
 
 COLLEGE OP PHYSICIANS AND SUROEOSa 
 
 67.0 WET 163th STREET 
 
- 
 
 ON AFTER-IMAGES 
 
 SHEPHERD IVORY FRANZ, A.B. 
 
 SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR 
 
 THE DEGREE OF DOCTOR OF PHILOSOPHY 
 
 IN THE 
 
 Faculty of Philosophy, Columbia University 
 
 COLUMBIA UNIVERSITY 
 
 DEPARTMENT OF PHYSIOLOGY 
 
 College of Physicians and Surgeons 
 fst w£8t fifty ninth. street 
 
 NEW YORK 
 
 NEW YORK 
 
 JUNE, 1899 
 
ON AFTER-IMAGES 
 
 SHEPHERD IVORY FRANZ, A.B. 
 
 SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR 
 
 THE DEGREE OF DOCTOR OF PHILOSOPHY 
 
 IN THE 
 
 Faculty of Philosophy, Columbia University 
 
 NEW YORK 
 
 JUNE, 1899 
 

 
 PRESS OF 
 
 The New Era Printing Company, 
 Lancaster, Pa. 
 
CONTENTS. 
 
 Page. 
 
 Introduction 2 
 
 Part I. Experimental 3 
 
 Sec. 1. Apparatus and Methods 3 
 
 Sec. 2. Threshold 4 
 
 Sec. 3 . Latent Period 11 
 
 Sec. 4. Duration 18 
 
 Sec. 5. Fluctuations 37 
 
 Sec. 6. Qualitative Changes 38 
 
 Sec. 7. Space-relations 41 
 
 Sec. 8. Retinal Transfer 44 
 
 Part II. Historical and Descriptive 51 
 
 Sec. 9. Relation to Sensation, to Memory, and to Imagination 51 
 
 Sec. 10. History 54 
 
 Bibliography 60 
 
Digitized by the Internet Archive 
 
 in 2010 with funding from 
 
 Open Knowledge Commons (for the Medical Heritage Library project) 
 
 http://www.archive.org/details/onafterimagesOOfran 
 
ON AFTER-IMAGES. 
 Introduction. 
 
 After-images were first described in the De Somniis by 
 Aristotle, who regarded them as closely allied to the centrally 
 excited images of the dream-state ; they were rediscovered and 
 described anew by St. Augustine, and again by the Arab Al- 
 hazan (a student of Aristotle's works) in the eleventh century. 
 Peiresc, in describing these appearances in the seventeenth 
 century, thought that he had discovered a new phenomenon. 
 Among other prominent investigators to note and experiment 
 with the images were Boyle, Newton, Buffon, Goethe, the elder 
 Darwin and Fechner. 
 
 These vestiges of sensation owe their present interest in 
 large measure to their seemingly twofold character, being al- 
 lied both to sensation and to memory- and imagination-images. 
 To the earlier observers after-images were more nearly like 
 the images of the imagination ; later, they were considered 
 almost a sensation ; whilst most recently the original position of 
 Aristotle is again prominent. In the history of after-images we 
 seem to have an epitome of the interrelations of physics, physi- 
 ology and psychology ; and probably no other single phenome- 
 non is so good an example of the growth of experiment and 
 measurement in psychology. 
 
 The succeeding portions of the present monograph, apart 
 from the bibliography, deal respectively with (i) an experi- 
 mental analysis of the conditions affecting the production, the 
 duration, the latent period, the space-relations, etc., of the after- 
 image, and with (2) a history of the phenomena and their rela- 
 tion to sensation, to imagination and to memory. 
 
 In the bibliography I have given references to what seem 
 the most important contributions on the general subject of after- 
 images. In Professor Koenig's excellent bibliography 1 of 
 
 1 Helmholtz, Handbuch der Physiologischen Optik, 2te Aufl. 
 
2 5. /. FRANZ. 
 
 vision will be found references to almost all the work on this 
 subject done previous to 1894. In the numbers of the Psy- 
 chological Index, and in the bibliographies in the Zeitschrift 
 filr Psychologie and in the ZSAnnee Psychologique will be 
 found references to current literature since 1893. 
 
 The experiments to be subsequently described were all 
 made in the psychological laboratory of Columbia University ; 
 and as subjects eleven advanced students in psychology took 
 part, viz., B, C, D, F, G, H, Ho, K, M, S and W. Other 
 subjects, too numerous to mention individually, chosen for their 
 naivete, were used for check-experiments. To all I am greatly 
 indebted for their help and suggestions throughout the progress 
 of the research. 
 
 The terminology used in the succeeding portions is as fol- 
 lows : (1) Positive after-image is an after-image in which the 
 image and its background bear the same intensity-relation as in 
 the stimulus. (2) Negative image is one in which the relation 
 of intensity is reversed. Thus, if the stimulus is a red cross on 
 a black background, and the resultant after-image is projected 
 on a white wall, the image will be darker than the background, 
 and accordingly negative ; if the image is projected on gray, it 
 may be darker or lighter than the field and either negative or 
 positive. In either case the image maybe of the same color as 
 the original or of a different color. Four different kinds of 
 images must then be distinguished, viz. : 
 
 same-colored. , T . C same-colored. 
 
 _ . . C same-colored. , T ,. C 
 
 Positive < , , Negative < 
 
 C other-colored. C 
 
 other-colored. 
 
 Wundt's definitions, (1) that the image is positive when it is 
 of the same or greater intensity than the stimulus, and negative 
 when it is of lesser intensity ; J and (2) that positive images are 
 same-colored and negative, are other-colored, 2 have not been 
 accepted by the most recent and best writers. 
 
 1 Grundziige der Physiologische Psychologie, 4 Ed., I., 513. 
 
 2 Human and Animal Psychology, Eng. trans., p. 109. 
 
ON AFTER-IMAGES. 3 
 
 Part I. Experimental. 
 
 Section i. Apparatus and Methods. — The apparatus 
 used throughout the series on threshold, latent period and dura- 
 tion was that used by Fullerton and Cattell, 1 adapted by the 
 writer for the present purposes. The accompanying illustra- 
 tion shows the instrument from the standpoint of the experi- 
 menter. 
 
 The apparatus consists of a vertical screen (6") with an 
 adjustable opening (Zf), of a long arm (A) and of a screen 
 pendulum (P). At the opening (H) arrangements were made 
 for inserting ground-glass plates for equalizing the illumination 
 of the stimulus and for rapidly changing the areas. The long 
 arm (A ) carried a kerosene-lamp (Z) (in the experiments upon 
 duration and latent period an electric arc lamp), the changes in 
 intensity being made by moving the source of light toward and 
 from the observer. On the pendulum (.P) was a screen, which 
 
 1 Perception of Small Differences, p. 135; University of Pennsylvania Press, 
 1892. 
 
4 5. I. FRANZ. 
 
 in its usual position, as the pendulum is held up by the electro- 
 magnet (M), shuts off the light from the area to be illumined 
 at (If). When the pendulum is released, however, the light 
 shines on the area for one second, and, returning, it is caught 
 up by the electro-magnet. The key (K) was used to make 
 and break a circuit to the electro-magnet (M) for releasing and 
 holding the pendulum. Before the upright screen (S) is a 
 tachistoscope (not shown in the figure), with drop screens to 
 give a light-stimulation of T oV<P ihj> or iV secon d- The whole 
 instrument was placed in a dark-room before a cabinet with an 
 opening opposite (H). All experiments, unless otherwise noted 
 hereafter, were made in a dark-room. The subject remained 
 inside the dark-cabinet for from ten to fifteen minutes for adap- 
 tation before any experiments were made. Then, with the eyes 
 on a level with the stimulating light, the area was uncovered, 
 covered again, and the subject reacted in the manner appropri- 
 ate to the series of experiments. 
 
 Section 2. The Threshold. 1 — Not every stimulus is fol- 
 lowed by a sensation ; a sound may not have the requisite in- 
 tensity ; the weight be not sufficiently heavy : the light not large 
 enough or long enough continued. In like manner minimal 
 amounts of energy, time and space stimulation will fail to pro- 
 duce an after-image. Some questions immediately suggested 
 are: What intensity of light will produce the after-sensation? 
 How large an area is necessary to get an after-image? How 
 long must the stimulus last to leave its effect? Then, we 
 may ask whether or not there is a relation between the varying 
 changes in the light, whereby a small area may be counterbal- 
 anced by a longer stimulation or by a greater intensity, andvice 
 versa. 
 
 Time. — As noted above, three variables were used, viz., 
 time, area and intensity. With a fixed area and intensity, 
 what time must the light stimulate the eye to produce an after- 
 image? The lamp placed at 25 cm. from the screen S, giving 
 an intensity of -£$ c.p., and the area 64 sq. mm., were used 
 
 1 Sec. 2 is largely reprinted from an article on The After-image Thresh- 
 old; Psychol. Rev., II., 130-136 (1896). 
 
ON AFTER-IMAGES. 5 
 
 as constants throughout this series. The time was varied from 
 ToV'tf to x secon d. Two intermediate steps were used, y-J-g- and 
 -j'j- second. 
 
 Adaptation completed, the subject sat in position, a stimulus 
 was given, and the subject announced the appearance or non- 
 appearance of an after-image. Usually thirty to forty minutes 
 were taken for each day's experiments, the series being stopped 
 before fatigue became apparent. For each subject the number 
 of experiments upon each variation in time was one hundred. 
 Where a greater or less number were made the small numbers 
 in parentheses in the appropriate columns of the tables show 
 how many tests of this kind were made. The experiments 
 were all made with the eyes open, so as not to disturb the after- 
 image. The subject's eyes were at the normal distance (about 
 30 cm.) from the stimulus ; his head was steadied by a support. 
 Rests were taken between the separate experiments to allow any 
 trace of the previous image to disappear, a signal was given, five 
 seconds were allowed for preparation and the light was shown. 
 
 The intensity of the light was very constant. The lamp was 
 trimmed before the whole series of experiments ; and the photo- 
 metric determinations made before, during and after a sitting 
 showed only the variations likely to occur in any series with the 
 photometer used (Bunsen's). A fixation-point could not be se- 
 cured throughout the experiments. At first this was a some- 
 what disturbing factor ; but as the light used as a stimulus was 
 so small and the after-image so indistinct, it was judged best 
 not to have a fixation-point of light, owing to the confusion 
 likely to result from mistaking it for an after-image. With 
 practice, however, the observer learned to look for the stimulus 
 in the proper direction ; and in the case of the writer in not over 
 5 per cent, of the times was it necessary to focus the eyes con- 
 sciously after any part of the light was seen. 
 
 The following table shows the results of the experiments for 
 time obtained from two observers, C, an advanced student of 
 psychology, and F, the writer, upon whom the experiments 
 were made by another worker familiar with the apparatus. 
 The table shows on the first lines the percentages of times that 
 after-images appeared. The results were also grouped in 
 
5. /. FRANZ. 
 
 series of tens, and the variations of these groups were calcu- 
 lated. These variations are shown on the second lines, marked 
 A. var. 
 
 Table I. — Time. 
 
 Time in sec. 
 Subjects. 
 
 I 
 
 l 
 TTT 
 
 1 
 
 1 
 1000 
 
 r j Per cent. 
 *■" I A. var. 
 
 ■p, j Per cent. 
 r " 1 A. var. 
 
 97 
 3-5(7°) 
 
 IOO 
 
 95 
 7- 
 
 97 
 4.2 
 
 75 
 17 
 
 82.5 
 8.3(120) 
 
 12 
 10.8 
 
 19 
 7-4 
 
 Average. 
 
 99 
 
 96 
 
 79 
 
 r 5-5 
 
 As noted above, 100 experiments of each kind were made, ex- 
 cept where another figure is shown in parentheses. Thus, for 
 F, with the stimulus -j^- second, 120 experiments were made; 
 while for C, using one second stimulation, only 70 trials were 
 made. 
 
 The accompanying curve shows the results graphically — the 
 abscissa denoting divisions of time, the ordinate the percentage 
 
 of times after-images ap- 
 peared. Only the points for 
 
 T0V0' Tlhp and iV seconds 
 are shown. 
 
 The shape of the curve 
 indicates what might have 
 been expected in accord- 
 ance with the results of ex- 
 periments upon other time- 
 phenomena of vision. With 
 the shortest time after-im- 
 ages seldom appeared ; then 
 with a slight increase in the 
 duration of the stimulus 
 there was a rapid rise in the 
 number of appearances, fol- 
 lowed by only a slight in- 
 crease for longer stimulation. The same character of curve is 
 found in investigating the time it takes to see a light or a color. 
 
 00- 
 
 % 
 
 jm^^j^^^^ 
 
 80- 
 
 r^^g^ 
 
 60- 
 
 1 TKscm®, 
 
 <r0- 
 
 1 
 
 20- 
 
 1 re« 2. 
 
 sec. 
 
ON AFTER-IMAGES. 7 
 
 When shown for a very short time the color can never be seen ; 
 but as the time is slowly increased, it becomes very plain, but 
 is seen no oftener and becomes no plainer if the time be further 
 increased. In both cases (recognizing a color and seeing after- 
 images) the stimulus at first is insufficient to make the sensation 
 appear above the threshold of conscious experience. This 
 threshold is then passed suddenly, and the stimulus soon reaches 
 a point at which an after-image or the color (as the case may 
 be) is always seen, unless numerous distractions of great in- 
 tensity interfere. 
 
 This effect of increase in the time of stimulation is quite in 
 accord with the commonly accepted theory regarding the after- 
 image : that the phenomenon, at least in its positive form, is 
 due to inertia of the retina. From this point of view, the longer 
 the light works upon the retina — the greater the amount of 
 energy used — the greater will be the effect on consciousness. 
 
 Area. — A corresponding, though not a proportionate, in- 
 crease in the number of appearances was noted when the area 
 was varied from Jg to 64 sq. mm. Six areas were used — 
 squares of J^, y 2 , 1, 2, 4 and 8 mm. on each side. These were 
 made on ground glass, opaque paper being glued over all but 
 the small area wanted. The error in making the larger sizes 
 was less than ^ mm., which amount is inappreciable in com- 
 parison with the whole area. Difficulty was experienced in 
 making the areas under 1 mm. ; but from numerous ones made 
 the best were chosen. In those chosen the error was not more 
 than J ? mm. The smallest areas were measured with a mag- 
 nifying-glass, and those were chosen in which the errors com- 
 pensated each other. 
 
 The intensity of the light was kept at -^ c. p., and the 
 time of stimulation was one second. With these constant, an 
 after-image was very seldom seen when the smallest area was 
 used, while the largest square produced an image at all times. 
 The same precautions that were observed in the experiments 
 upon the effect of time-changes were taken in this series and in 
 all the experiments that are described later. 
 
 The following table and accompanying curve give the re- 
 sults obtained. The figures in the table are as in Table I. : 
 
5. /. FRANZ. 
 
 first are given the changes in stimulation, then the percentage 
 of times the image was seen under these conditions and then the 
 average variations of the experiments when grouped in sets of 
 ten. The abscissa of- the curve denotes the relative amount of 
 the stimulus ; the ordinate shows the percentage of times after- 
 images were seen. 
 
 Table II. — Area. 
 
 Area in sq. mm. 
 Subjects. 
 
 64 
 
 16 
 
 4 
 
 1 
 
 1 
 
 1 
 TB - 
 
 M /Percent. 
 m - I A. var. 
 
 „ f Per cent, 
 t A. var. 
 
 100 
 
 -(5o) 
 
 100 
 
 90 
 7-5(8o) 
 
 96 
 4.8 
 
 72 
 14.8 
 
 S8 
 8.8 
 
 52 
 12.4 
 
 57 
 13 
 
 27 
 9.2 
 
 31 
 15-4 
 
 20 
 6 
 
 8 
 8 
 
 Average. 
 
 100 
 
 93 
 
 80 
 
 54-5 
 
 29 
 
 *14 
 
 CA. 
 
 WRUSMOIL®. 
 
 re« D 
 
 ±± I + sq.cm. 16 
 
 Some preliminary work in which a different standard of in- 
 tensity was used — 2V can dle power — shows the same effect of 
 area-increase. These experiments were made before the gen- 
 eral line of work was fully planned. Seventy experiments 
 were made with each area, and the writer was the only subject. 
 The results, because of their confirmatory character, are ap- 
 pended : 
 
 Table III. — Area. 
 
 Area in sq. mm. 
 
 64 
 
 16 
 
 4 
 
 1 
 
 1 
 
 _. f Per cent. 
 r ■ \ A. var. 
 
 96 
 
 5 
 
 89 
 10 
 
 67 
 14.7 
 
 41 
 10 
 
 19 
 9-9 
 
ON AFTER-IMAGES. 9 
 
 A frtori, one might not expect such an increase in the num- 
 ber of appearances to follow an increase in the size of the 
 stimulus. For, with intensity and a time of stimulation constant, 
 one might well say that the structure of the eye is such that 
 only the number of stimulated elements, not the stimulation of 
 the individual elements, changes with any increase in size of 
 the physical stimulus. This is undoubtedly true, and the result 
 is paralleled by the results of experiments upon the extensive 
 threshold of vision. A color or a light cannot be seen when 
 only a small area is present ; but on increasing the size it is 
 readily perceived. This may be analogous to the so-called 
 ' summation process,' or it may and probably can be resolved 
 into a matter of the attention. As a matter of fact, the largest 
 area may not produce any more after-images than the smallest. 
 The apparent difference may be the result of inability of the 
 smallest sensations to fix the attention. 
 
 Intensity. — A similar series was made changing the inten- 
 sity of illumination. The lamp was moved along the arm (A) 
 of the apparatus (see Fig. i) so as to give intensities varying 
 from 23- to ^jj-jj c.p. Three intermediate steps were chosen 
 for convenience — £$, ^-J^ and -gj^ c.p. The amounts of 
 energy in these experiments had an effect corresponding to 
 what was found for the changes in the time and in the area of 
 stimulation. The results obtained and a graphic representation 
 follow : 
 
 Table IV. — Intensity. 
 
 Intens. in c.p. 
 Subjects. 
 
 TS 
 
 1 
 
 50 
 
 1 
 
 8~0"0~ 
 
 1 
 
 3200 
 
 „ f Per cent. 
 \A. var. 
 
 ,-, ("Percent. 
 F> \A.var. 
 
 IOO 
 
 — (80) 
 
 IOO 
 
 94 
 
 7 
 96 
 
 5-4 
 
 43 
 
 25 (no) 
 
 44 
 19-5(13°) 
 
 17 
 20.8(110) 
 
 15-5 
 13-5 (13°) 
 
 3-6 
 
 1 
 
 1.8 
 
 Average. 
 
 IOO 
 
 95 
 
 46 
 
 46 
 
 i-5 
 
 Two results in Tables I., II., III. and IV. should be noted : 
 (1) the close correspondence of the observers, and (2) the 
 comparatively large variation for the different individuals. The 
 first of these observations is, I believe, of little significance. 
 
io S. I. FRANZ. 
 
 The great variation, however, seems to point to a considerable 
 mental influence affecting the appearances. This will probably 
 be more clearly brought out in the discussion of the experiments 
 upon the influences determining the duration of the after-image. 
 
 I00-] 
 
 /o 
 
 F!@MI4. 
 
 -i- _!_ _i- c.p. _n 
 
 3200 800 200 r so 
 
 Summary. — Experiments were made upon four trained sub- 
 jects to find the amount of time-, area- and intensity-stimula- 
 tion that would produce an after-image (conscious). It was 
 found that to see an after-image 75 per cent, of the times the 
 eye is stimulated, it is necessary to have as stimulation (1) for 
 yJ-q second and 64 sq. mm. surface of light, an intensity of 
 fa c. p. ; or (2) for one second exposure, an area of 4 sq. mm. 
 and an intensity of fa c.p. ; or (3) for 1 second exposure, an 
 area of 64 sq. mm. and a light of ^-^ c. p. 
 
 The average results for the three series are indicated in the 
 following curve, which gives a general view of the relative 
 effects of the physical units. It will be seen that the effects of 
 time, intensity and extensity-changes are varied. This shows 
 that for physiological or mental processes the units of physical 
 energy do not counterbalance. The compensating relation 
 seems to be a rather complex one. In its simplest form this re- 
 lation may be stated as follows : squaring the time equals doub- 
 ling the intensity or quadrupling the area ; and vice versa, re- 
 ducing the area to one quarter equals halving the intensity or 
 taking the square root of the time. 
 
ON AFTER-IMAGES. II 
 
 Based upon the well-known phenomena of color-mixing by 
 means of Maxwell's discs, the objection may be raised to these 
 conclusions that lights of much less intensity produce after- 
 images, inasmuch as colors fuse in any illumination. While this 
 is true, it should not be forgotten, on the other hand, that the 
 after-images producing the fusion are not consciously perceived 
 as after-images. The usually low intensity of light in conjunc- 
 tion with a short time of stimulation does not produce an after- 
 image that can be consciously noted. Moreover, the overlap- 
 pings of images and succeeding stimuli produce a total effect 
 which lacks many of the distinctive features of the simple after- 
 image. 
 
 100 
 
 T0Z&? 
 
 TTOI 
 
 1F0(§MKI 
 
 During the three series of over 3000 experiments only five 
 times were negative after-images seen. These were noticed 
 only with the largest area, the longest time and the greatest in- 
 tensity, and always toward the close of an hour's session after 
 the eyes had been stimulated forty or fifty times. This fact 
 would seem to support the hypothesis that the negative after- 
 image is due to eye-exhaustion. 
 
 Section 3. The Latent Period. — That a light does not 
 immediately produce an after-image is now well known. The 
 observation seems to have been made first by Young, 1 in 1872, 
 
 iC. A. Young, Note on Recurrent Vision; Taylor's PJiilos. Mag., 4 Ser. 
 XLI1L, 343-345> 1872. 
 
12 6'. /. FRANZ. 
 
 who, in using a Holtz machine which gave a large spark, noted 
 that if a single spark was made any conspicuous object in the 
 darkened room was seen at least twice — with an interval of a 
 trifle less than a quarter of a second — the first time vividly, 
 the second time faintly ; often the object was seen a third and 
 sometimes (but only with great difficulty) even a fourth time. 
 This interval between the sensation and the after-image has 
 been called the latent period. 
 
 Young's observation seems to have been quite forgotten for 
 several years till the appearance of Bidwell's note on the image 
 following an illumined vacuum tube. 1 Bidwell found that, if a 
 Geissler tube be revolved at the rate of once in three seconds, 
 the tube is followed at a distance of about forty degrees by a 
 ghost-like image of the original. If the rotation stopped, the 
 ghost moved on, merging finally with the tube. The latent 
 period in this case was about one-third of a second. The ob- 
 servation was repeated upon about ten people, with approxi- 
 mately the same results. 
 
 Independent observers 2 at the same time announced varia- 
 tions in the experiment, and the suggestion was made that the 
 intermittent appearance of lightning might be due to a similar 
 cause. The effect may be produced in a simple manner with 
 a match or a piece of glowing coal revolved by hand. The 
 ghost-image is seen to follow close after the stimulus. After 
 some experience even a pencil or a bright-colored book when 
 moved shows the phenomenon. 
 
 Later and more exact measurements of the appearances have 
 been made by Bidwell, 3 Charpentier 4 and Hess. 5 Charpentier 
 repeated Bidwell's observations and noted a second appearance 
 after the sensation, as Young and Bidwell had. This, he adds, 
 is easier to note in indirect than in direct vision. A similar os- 
 
 1 S. Bidwell, On Certain Spectral Images Produced by a Rotating Vacuum 
 Tube; Nature, XXXII., 30-31, 1885. 
 
 2 Davis, Laurin, Newall and others. 
 
 3 On the Recurrent Images following Visual Impressions; Proc. Roy. Soc, 
 LVI., 132-145, 1894. 
 
 4 Reaction oscillatoire de la retine sous Tinnuence des excitations lumineuses ; 
 Arch, de Physiol. (Ser. 5), IV., 54 I -5S3. l8 92- 
 
 6 Untersuchungen liber die nach kurzdauernder Reizung des Sehorgans; 
 Archivf. die gesammte Physiol., XLIX., 190-208, 1891. 
 
ON AFTER-IMAGES. 13 
 
 dilatory effect was" found by Charpentier in using a black disc 
 with a comparatively small white sector. Revolving the disc 
 once every second or every two seconds, and fixating the center, 
 the white sector was seen to be crossed in places by dark bands. 
 These seem closely related to the disappearances and the reap- 
 pearances of the after-image, but need more careful study in the 
 light of Exner's work upon the progress of a visual sensation. 
 
 Bidwell's later work is largely a repetition of his earlier ob- 
 servations, but using spectrum colors instead of the illuminated 
 Geissler tube. A spectrum, revolving every 1 1/2 seconds, was 
 thrown on a screen and the ghost-image observed. The ghost 
 followed the spectrum at a distance of about 50 degrees. The 
 latent time was 0.2 second. It is interesting to note that Bidwell 
 observed no image following red or violet light. The recurrent 
 image of an illumined slit in an opaque disc was found to follow 
 after 0.15 second. Another disc, 15 cm. diameter, having two 
 opposite radial slits 1/2 mm. wide, was then rotated one turn a 
 second before illumined ground glass. As observed from a 
 distance of 1 1/2 meters, each slit seemed to give four and pos- 
 sibly five luminous images arranged like the ribs of a fan. The 
 usual blue recurrent image could also be seen following the 
 images of the radial slits at an angle of 80 degrees. Through- 
 out, the eye was fixed upon the center of the disc. This obser- 
 vation is readily confirmed, but seems not to be understood. The 
 fact that Bidwell speaks of the oscillations and the ghost sepa- 
 rately seems to indicate that he did not believe they were similar 
 in character. Even if we admit Charpentier's contention of an 
 oscillatory process in the retina, which makes a white disc at 
 any phase seem alternately dark and light, we could not admit 
 it for the dark opaque disc of Bidwell nor for the after-image 
 accompanying it. 
 
 One fact seems to have escaped notice — at least it has not 
 been recorded. In the simple experiment with a burning coal 
 or with a Geissler tube the ghost follows the stimulus ; but, in 
 addition, there is an after-image of the path taken by the light. 
 In this image the oscillations described by Bidwell do not occur. 
 None of the fluctuations usually apparent in the after-image are 
 noted in this case, the image gradually fading away. 
 
5. /. FRANZ. 
 
 Experiments of a different character have been made by 
 Hess. In his work moving objects were not used, momentary 
 illumination of a stationary stimulus taking the place of the re- 
 volving discs. A photographic shutter gave exposures of y^-g- 
 to 2T5- second. Hess discovered a negative after-image of 
 very short duration following immediately upon the termination 
 of the stimulus. This preceded what is commonly known as 
 the positive after-image. 
 
 The nature of the experiments in the present work is more 
 nearly related to the work of Hess than to that of Charpentier 
 and Bidwell. A stationary light was used and the reaction-time 
 taken. As in the work on threshold, I endeavored to find the 
 relative effects of varying stimuli in the three respects of in- 
 tensity, time and area. The apparatus used was essentially 
 the same as that described in Section 1. The kerosene lamp, 
 however, was replaced by an arc light of about 1000 c. p. 
 The intensities, as given to the subjects, were 1, Jg- and j-^-g- c. 
 p. ; these were obtained as in the threshold-experiments. \, 1 
 
 and 4 sq. cm. in squares were 
 (LATIEMT IPH^O©® used as areas, and the varying 
 times were yjj-g-, yg- and 1 sec- 
 ond. As constants were used 
 y T c. p., 1 sq. cm. and 1 sec. 
 Only one of the factors was 
 varied at any one time. A 
 chronoscope was placed in 
 series with a reaction key and 
 the pendulum (P) and the drop 
 tachistoscope. The observer 
 was instructed to react the in- 
 stant the after-image appeared, 
 and the time between the end 
 of the stimulus and the begin- 
 ning of the after-image could 
 thus be read direct from the 
 chronoscope. 
 
 The individual variation is quite large, owing to the mental 
 attitude of the subject, one reacting to the first approximation 
 
 100 10 
 
 sec. 
 
ON AFTER-IMAGES. 
 
 15 
 
 of an after-image, another waiting till he was certain that the 
 appearance was what he sought. Although his attitude was 
 constant with each individual, we shall not be able to compare 
 the subjects, and to group them for an average representing 
 a typical result. So also the individual attitude toward the 
 stimuli made other variations in the results. To one the stimuli 
 of greatest intensity were too bright ; at times another could 
 scarcely get an after-image with the smallest area. 
 
 Accompanying will be found the tables and curves for the av- 
 erage results for time-, area- and intensity-experiments upon four 
 subjects. Twenty experiments were made upon each variable. 
 In the tables the average time between the stimulus and the 
 first appearance of the after-image is first given, and this is fol- 
 lowed by the average variations of the twenty experiments. 
 
 Table V. 
 
 Time : constants, T lj candle power, 1 sq. cm.- 
 
 Stimulus in Seconds. 
 Subjects. 
 
 .01 
 Aver. A. Var. 
 
 .1 
 Aver. A. Var. 
 
 1.0 
 
 Aver. A. Var. 
 
 B. 
 D. 
 G. 
 W. 
 
 I.62 
 •49 
 •76 
 ■7i 
 
 .42 
 .19 
 .22 
 •13 
 
 .99 .12 
 
 •83 -35 
 .61 .23 
 
 .46 .14 
 
 I.76 
 •65 
 •71 
 .92 
 
 •74 
 .11 
 .18 
 .20 
 
 Aver. 
 
 90 
 
 72 
 
 1. 01 
 
 ?00 
 
 \. HATIOT PUBOO®. 
 
 
 sec. 
 
 \ ^^^ "** 
 
 
 1.75- 
 
 \ 
 
 
 1.50- 
 
 \ " — 
 
 |_ 
 
 125- 
 
 \ mm. 
 
 
 L00 
 
 \ 
 
 M*> 
 
 •75- 
 
 \ <s 
 
 •50- 
 
 
 © 
 
 -25- 
 
 F0669R1 7, 
 
 
 sei.cm. 
 
1 6 S. I. FRANZ. 
 
 Table VI. 
 
 Area: constants, I sec, and j^ c . p. 
 
 Area in sq. cm. 
 
 X 
 
 
 
 : 
 
 
 \ 
 
 Subjects. 
 
 Aver. 
 
 A 
 
 Var. 
 
 Aver. 
 
 A. Var. 
 
 Aver. 
 
 A. Var. 
 
 B. 
 
 2. II 
 
 
 ■65 
 
 1.76 
 
 •74 
 
 1.50 
 
 .46 
 
 D. 
 
 I.91 
 
 ) 
 
 .2,5 
 
 ■65 
 
 .11 
 
 •58 
 
 .07 
 
 G. 
 
 •63 
 
 
 ■13 
 
 •71 
 
 .18 
 
 ■65 
 
 .11 
 
 W. 
 
 .90 
 
 
 ■15 
 
 .92 
 
 .20 
 
 .86 
 
 ■13 
 
 Aver. 
 
 i-39 
 
 1. 01 
 
 .90 
 
 Table VII. 
 
 Intensity : constants, 1 sec, and 1 sq. cm. 
 
 Intensity in c. p. 
 
 .01 
 
 
 [ 
 
 I 
 
 
 Subjects. 
 
 Aver. 
 
 A. Var. 
 
 Aver. 
 
 A. Var. 
 
 Aver. 
 
 A. Var. 
 
 B. 
 
 i-39 
 
 •34 
 
 I.76 
 
 •74 
 
 3-36 
 
 1.58 
 
 D. 
 
 •57 
 
 .08 
 
 •65 
 
 .11 
 
 ■Si 
 
 .14 
 
 G. 
 
 .67 
 
 .12 
 
 •71 
 
 .18 
 
 1.09 
 
 •33 
 
 W. 
 
 •77 
 
 •13 
 
 .92 
 
 .20 
 
 .72 
 
 ■" 
 
 Aver. 
 
 ■85 
 
 I. OI 
 
 1.42 
 
 sec 
 
 2.50' 
 
 Summary . — While the grouping and averaging of these 
 diverse figures cannot be well justified, I have done so because, 
 
OJST A FTER-IMA GES. 1 7 
 
 in a general way, the figures of the averages seem to show 
 what ordinarily would be expected. With the lesser times the 
 eyes have not time to move and to get a sensation on many con- 
 tiguous portions of the retina. Accordingly when the time of 
 stimulation is short the after-images are clearer and more easily 
 recognized. The larger areas have the advantage of giving 
 after-images of considerable size, which attract the attention 
 sooner than the smaller ones ; while the great intensities seem 
 to be so blinding in their effect that the image is very hard to 
 recognize. I append a curve showing the comparative effects 
 of the variables, but with full knowledge that its meaning 
 should not be taken as absolute, but only as an indication of the 
 results that a much larger number of individuals may approxi- 
 mate. 
 
 ISO 
 
 sec 
 
 1.25 
 1.0& 
 .75 ■ 
 
 .50 
 .25 
 
 
 FDStilRE 9. 
 
 In the existing theories explaining the phenomena no reason 
 is noted for any variation in this latent period. It seems prob- 
 able that an explanation must be looked for in the mental at- 
 titude of the subject. Such an explanation would not only ac- 
 count for the great individual differences, but would also 
 account for the great variation of the same individual under 
 precisely the same physical conditions. The subject G, who is 
 distinctly visual, accustomed to note variations in color, etc., 
 apparently recognized the image sooner than any of the others 
 who were not so trained. This mental attitude will also ex- 
 plain why the after-image of the largest areas was seen sooner 
 than that of the smaller one. The attention in this case, was 
 probably attracted sooner to the large object. The conditions 
 attending the changes in latent time for intensity- and time-ex- 
 
1 8 S. /. FRANZ. 
 
 periments are not so evident. In the intensity-experiments the 
 subjects B and G, for whom the i c. p. stimulus was too strong, 
 dominated thereby the totals. Considered in connection with 
 the results of the experiments upon duration (Section 4), it 
 seems likely that the attention is a predominant factor in all 
 measurements of the phenomena. 
 
 This view is further strengthened if the average variations 
 under the different conditions are considered. Usually those 
 conditions which the subject considered favorable show the 
 least variation. Thus, B felt that the T l_-second exposure was 
 too short a time for him to see the after-image, and the great 
 variation shows that there must have been some great disturb- 
 ing factors. The same occurred with both G and B under 
 stimulation of I. c. p., which was considered by both as too 
 strong. 
 
 Section 4. Duration. — The earliest accounts of the du- 
 ration of after-images seem to be those given by Boyle and by 
 Newton. In his work on color, Boyle relates that a man who 
 had looked through a telescope at the sun without a protecting 
 medium had his eyesight so injured that thereafter whenever he 
 looked toward windows or other bright objects he fancied he 
 saw a globe of light about the size of the sun. This appearance 
 was a source of trouble to the man for nine or ten years. 
 
 To Locke, who had called his attention to this interesting 
 observation, Newton wrote an account of a similar personal ex- 
 perience. The letter, which contains further observations of 
 interest, is quoted fully i 1 
 
 " The observations you mention in Mr. Boyle's book of colours, I once made 
 upon myself with the hazard of my eyes. The manner was this : I looked a 
 very little while upon the sun in a looking-glass with my right eye, and then 
 turned my eyes into a dark corner of my chamber to observe the impression 
 made, and the circles of colours which encompassed it, and how they decayed by 
 degrees, and at last vanished. This I repeated a second and a third time. At 
 the third time, when the phantasm of light and colours about it were almost 
 vanished, intending my fancy upon them to see their last appearances, I found, 
 to my amazement, that they began to return, and by little and little to become 
 as lively and vivid as when I had newly looked upon the sun. But when I 
 
 l D. Brewster, Memoirs of the Life of Sir I. Newton; Edinburgh, 1855, 
 Vol. I., p. 236 : 
 
ON AFTER-IMAGES. 19 
 
 ceased to intent my fancy upon them they began to vanish again. After this 
 I found that as often as I went into the dark and intended my fancy upon them, 
 as when a man looks earnestly to see anything which is difficult to be seen, I 
 could make the phantasm return without looking any more upon the sun ; and 
 the oftener I made it return the more easily I could make it return again. And 
 at length, by repeating this without looking any more at the sun, I made such 
 an impression upon my eyes that, if I looked upon the clouds, or a book, or any 
 light object, I saw upon it a round bright spot of light like the sun, and, which 
 is still stranger, though I looked tipon the sun -with my right eye only, and not 
 -ivilh my left, yet my fancy began to make an impression upon my left eye as well 
 as upon my rig/it. For if I shut my right eye, or looked upon a book or the 
 clouds with my left eye, I could see the spectrum of the sun almost as plain as 
 ■with my right eye, if I did but intend my fancy a little while upon it ; for at first 
 if I shut my right eye and looked with my left, the spectrum of the sun did 
 not appear till I intended my fancy upon it ; but by repeating this appeared every 
 time more easy. And now in a few hours' time I had brought my eyes to such 
 a pass that I could look upon no bright object with either eye, but I saw the sun 
 before me, so that I durst neither write nor read ; but to recover the use of my 
 eyes shut myself up in my chamber made dark for three days together, and used 
 all means to divert my imagination from the sun. For if I thought upon him 
 I presently saw his picture, though I was in the dark. But by keeping in the 
 dark, and employing my mind about other things, I began in three or four days 
 to have some use of my eyes again ; and by forbearing to look upon bright objects 
 recovered them pretty well, though not so well, but that, for some months after, 
 the spectrum of the sun began to return as often as I began to meditate upon 
 the phenomena, even though I lay in bed at midnight with my curtains drawn." 
 
 Several interesting observations are here noted. The one of 
 most importance seems to be that regarding the effect of imagi- 
 nation and attention upon the after-image. This will be dis- 
 cussed later ; as will also the one regarding the transfer of the 
 image from the right to the left eye. 
 
 The philosopher Tetens, who seems in some way to have 
 anticipated the modern idea of mental measurement, mentions 
 that he had investigated the duration of the after-image. Only 
 a few words of description are given, and details were left for a 
 future paper, no record of which I have been able to find. 
 
 Some few records of the duration are to be found in Fech- 
 ner's observation of the color-changes, but the material is so 
 slight as to be of little value. 
 
 Helmholtz notes that the duration and intensity of the stimu- 
 lating light affect the duration and character of the resultant 
 after-image: "The greater the intensity of the primary light 
 the brighter is the positive after-image and the longer it con- 
 
20 S. I. FRANZ. 
 
 tinues." 1 " Greater intensity of the primary light gives the 
 negative after-image greater clearness and duration." 2 
 
 The most extended observation of the changes occurring in 
 connection with the duration of the phenomena seem to be those 
 by Titchener. 3 Used primarily for another purpose (see Sec- 
 tion 8), the observations are of considerable value in this con- 
 nection, because they were made upon subjects na'ive as to the 
 present problem. Dr. Titchener asked his subjects to record 
 the duration of various after-images in a stimulated eye. 
 These experiments, though few in number for the single ob- 
 servers, show in general the same effect as the more extended 
 series by nryself . The conditions were as follows : The sub- 
 ject was placed before a screen with a variable sized opening, 
 which was to be illumined by a lamp. The photometric determi- 
 nation of the light is not given. The area- and time-stimulations 
 were changed and the records taken. The light used was 
 made vari-colored by the use of gelatin plates ; white light was 
 not used. The following table of results with red lights will 
 illustrate the general tendency, and will act as a basis for com- 
 parison with the present work. In this table, which I have con- 
 structed from the individual records as given by Titchener, will 
 be found the averages of at least three observers for each series. 
 The maximum number of experiments made under any con- 
 
 Table VIII. 
 
 
 Area 
 
 of Stimulation in mm. 
 
 
 
 
 
 3° 
 
 40 
 
 5° 
 
 o 
 
 o 
 w 
 
 S 
 
 
 9.9 (14) 
 
 27.2 (9) 
 
 g 
 
 10 
 
 12.5 (14) 
 
 17.4(16) 
 
 26.7 (8) 
 
 
 
 H 
 
 15 
 
 16.3 (15) 
 
 22. (14) 
 
 29.7 (7) 
 
 P 
 
 § 
 
 20 
 
 19.1 (10) 
 
 23-9 (15) 
 
 39- (8) 
 
 H 
 
 25 
 
 21.9 (11) 
 
 30.1 (10) 
 
 30.S (7) 
 
 1 Handbu 
 
 2 Ibid., p 
 3 E.B. Ti 
 
 :h der Physiol. C 
 
 5°5- 
 chener, Ueber b 
 
 )ptik, p. 503. 
 noculare Wirkui 
 
 lgen monoculare 
 
 r Reize ; Philos. 
 
 Stud., VIII., 231-310 (1892). 
 
ON AFTER-IMAGES. 31 
 
 dition by a subject was six, the minimum two. In paren- 
 theses will be found the number of experiments used in making 
 the averages. The other figures give, in seconds, the average 
 total duration of the after-image. 
 
 The individual variations from these averages are com- 
 paratively large, although each individual's results are quite 
 constant. The variability is about the same as was found in 
 my experiments upon the effect of time-, area- and intensity- 
 changes. 
 
 The present series took its rise, however, largely from two 
 incomplete minor studies made at the Columbia Psychological 
 Laboratory, 1891-93. These investigations were begun by Mr. 
 L. V. Southack and Miss E. G. Seebring, respectively ; but the 
 results and methods were unknown to the writer till after the 
 present series was begun. The results, which have remained 
 unpublished, I give in the appropriate place, with the corre- 
 sponding present series. The method employed by both ex- 
 perimenters was to use an incandescent lamp, enclosed in a 
 long box, to illumine a ground-glass Greek cross in the front of 
 the box. Each arm of the cross was one square centimeter. 
 Variations in intensity were obtained by changing the position 
 of the lamp, and variations in time of stimulation were meas- 
 ured by means of a metronome. The results were written 
 down by the observers after each experiment, the various fluc- 
 tuations being noted as well as possible. 
 
 The apparatus for the present series was about the same as 
 that used in the work on latent period ; but for the chronoscope 
 a kymograph was substituted. Upon this a continuous record 
 of the after-image could be recorded. The time of stimulation 
 was noted upon the drum, and the subject was instructed to close 
 the key-circuit as soon as an after-image appeared, to keep it 
 closed while the image lasted, and to break and make the circuit 
 as quickly as possible when any changes in the character of the 
 after-image took place. When any change was noted by the 
 breaking or the closing of the circuit the subject told the experi- 
 menter its character, and this was indicated upon the kymo- 
 graph-record. In this way not only the total, but the actual 
 time of duration, the fluctuations and, to some extent, the latent 
 
2 2 S. I. FRANZ. 
 
 period could be determined. In this series the observations 
 of the latent period were open to criticism owing to a slight 
 degree of inaccuracy in recording the end of the stimulating 
 light. Such an error was relatively large for the short times of 
 the latent period, but was inappreciable for the comparatively 
 long durations. As in the preceding work on threshold and 
 latent period, the three physical units of energy were varied. 
 A standard stimulus of i cm., J^ c.p. and 5 seconds was chosen ; 
 and the variables were \, 1 and 4 sq. cm., y-g-^-, yjr and 1 c.p., 
 and 1, 5 and 10 seconds. It was found impossible to record the 
 exact time of each fluctuation. The changes in the character 
 of the image are not clear-cut, but there is a fading of one color 
 into another. This could not be recorded as a distinctive color- 
 change. Even the usually marked changes from the positive 
 to the negative phases were not always clear-cut. 
 
 The following tables and curves give two results : the total 
 time of the after-image — i. e., the time from the end of the 
 stimulus to the final disappearance of the after-image ; and the 
 actual time — i. e., the total time minus the period when unseen. 
 In the curves the total durations are denoted by the continuous 
 lines, and the actual times by the broken lines. 
 
 Time. — The constants in these experiments were an intensity 
 of -jJ-jj- c.p. and an area of 1 sq. cm. 
 
 Table IX. 
 Total Duration of After-image. 
 
 Time of Stimu- 
 lation iu seconds. 
 Subjects. 
 
 1 
 Aver. A. Var. No. Exp 
 
 5 
 Aver. A. Var. No. Exp. 
 
 10 
 
 Aver. A. Var. No. Exp. 
 
 K. 
 H. 
 F. 
 M. 
 
 32 
 
 48 
 29 
 So 
 
 6 
 
 10 
 4 
 13 
 
 20 
 10 
 10 
 10 
 
 56 
 
 59 
 32 
 
 77 
 
 24 
 IS 
 
 s 
 29 
 
 20 
 10 
 
 10 
 
 10 
 
 56 
 §4 
 
 34 
 96 
 
 14 
 
 27 
 
 4 
 30 
 
 20 
 10 
 10 
 10 
 
 Average. 
 
 39-8 
 
 
 50 
 
 56 
 
 
 So 
 
 67-5 
 
 
 SO 
 
ON AFTER-IMAGES. 
 
 2 3 
 
 Table X. 
 Actual Duration of After-image. 
 
 Time of Stimu- 
 
 
 
 
 
 5 
 
 
 
 
 
 lation in seconds. 
 
 
 
 
 
 
 
 
 
 Subjects. 
 
 Aver. A. Var. No. Exp. 
 
 Aver. A. Var. No. Exp. 
 
 Aver. A. Var. No. Exp. 
 
 K. 
 
 26 
 
 4 
 
 20 
 
 40 
 
 12 
 
 20 
 
 40 
 
 9 
 
 20 
 
 H. 
 
 32 
 
 9 
 
 10 
 
 38 
 
 17 
 
 10 
 
 64 
 
 23 
 
 10 
 
 F. 
 
 25 
 
 2 
 
 10 
 
 30 
 
 5 
 
 10 
 
 29 
 
 3 
 
 10 
 
 M. 
 
 32 
 
 10 
 
 10 
 
 54 
 
 20 
 
 10 
 
 77 
 
 20 
 
 10 
 
 Average. 
 
 28.7 
 
 
 50 
 
 4°-5 
 
 
 5° 
 
 52-5 
 
 
 5° 
 
 DURATION 
 lOOi 
 
 sec 
 
 80 
 60 
 40 
 
 20 
 
 DURATION 
 100 
 
 sec 
 
 80 
 
 AK1A., 
 
 $mmE 00. 
 
 "£ 1 sq.cm. ? 
 
 1 s sec. 10 
 
 The corresponding series in Miss Seebring's experiments 
 show the following results for the total time of duration : 
 
 Table XI. 
 
 Time of stimulation. 
 
 3 
 
 Aver. A Var. No. Exp. 
 
 15 
 Aver. A. Var. No. Exp. 
 
 60 
 Aver. A. Var. No. Exp. 
 
 First series. 
 Second series. 
 
 21 
 23 
 
 9 
 
 4 
 
 5 
 5 
 
 47 1 6 I 5 j i°7 1 5 I S 
 43 1 6 1 5 1 101 | 7 | 5 
 
 Averages. 
 
 22 
 
 
 10 
 
 45 10 
 
 104 10 
 
 Full records of the fluctuations were taken only for the 
 second series, and the actual time that the image was seen can 
 
24 
 
 5. /. FRANZ. 
 
 be given only for these experiments. The following table 
 shows the relation between the total and the actual duration : 
 
 Table XII. 
 
 Time of stimulation. 
 
 Total time. 
 Aver. A. Var. 
 
 Actual time. 
 Aver. A. Var. 
 
 No. Exper. 
 
 3 sec. 
 15 sec. 
 60 sec. 
 
 22.6 
 42.6 
 IOI.2 
 
 A- 
 6. 
 7- 
 
 10.9 3.9 
 36. 4-4 
 98.6 8.4 
 
 s 
 s 
 
 5 
 
 In Titchener's results the same effect is noted. The varied 
 intensities and areas do not permit, however, of a direct com- 
 parison of the three series. But they have enough in common 
 to show the rapid, but not proportionate, increase in duration 
 due to the longer times of stimulation. 
 
 Intensity. — With 1 sq. cm. and 5 seconds exposure as con- 
 stants. 
 
 Table XIII. 
 Total Duration of After-image. 
 
 Intensity in c.p. 
 Subjects. 
 
 Aver. A 
 
 .01 
 
 Var. No. Exp. 
 
 Aver. 
 
 .1 
 A. Var. 
 
 No. Exp. 
 
 Aver. 
 
 1.0 
 A. Var. No. Exp. 
 
 K. 
 H. 
 
 F. 
 
 40 
 29 
 26 1 
 
 12 
 
 t 
 
 20 
 10 
 IO 
 
 56 
 59 
 32 
 
 | 24 
 
 5 
 
 20 
 IO 
 10 
 
 70 
 84 
 50 
 
 1 * 
 19 
 
 3 
 
 20 
 10 
 10 
 
 Average. 
 
 3i-8 | 
 
 40 
 
 49 J 
 
 40 
 
 68 1 
 
 40 
 
 Table XIV. 
 
 Actual Duration of After-image. 
 
 Intensity in c. p. 
 Subjects. 
 
 Aver. 
 
 .01 
 A. Var 
 
 No. Exp. 
 
 Aver. 
 
 .1 
 A. Var. 
 
 No. Exp. 
 
 Aver. 
 
 1.0 
 A. Var. No. Exp. 
 
 K. 
 H. 
 
 F. 
 
 28 
 22 
 25 
 
 6 
 
 7 
 3 
 
 20 
 
 1 IO 
 
 1 IO 
 
 40 
 38 
 3° 
 
 12 
 17 
 
 s 
 
 20 
 IO 
 10 
 
 52 
 63 
 50 
 
 9 
 18 
 
 3 
 
 20 
 10 
 IO 
 
 Average. 
 
 25 
 
 | 4° 
 
 36 
 
 
 40 
 
 55 
 
 
 40 
 
 Using a cross with 1 sq. cm. arms and 15 seconds exposure, 
 Mr. Southack found for himself and a second observer the 
 following results. The standard intensity, 1, was a 100 c. p. 
 
ON AFTER-IMAGES. 
 
 25 
 
 incandescent lamp at a distance of 30 cm. from the ground- 
 glass cross. The varying intensities were obtained in the 
 usual manner. 
 
 Table XV. 
 
 Total Duration of After-image. 
 
 Relative Intensi- 
 ties. 
 
 Subjects. 
 
 Av. A- Var. 
 
 No. 
 Exp. 
 
 1 
 Av. A. Var. 
 
 No. 
 Exp. 
 
 1 
 T 
 
 Av. A. Var. 1 
 
 No. 
 
 Exp. 
 
 I 
 
 No. 
 Av. A. Var. 1 Exp. 
 
 A. 
 B. 
 
 3 1 5 
 11.8I 1.4 
 
 20 
 IO 
 
 10.6 1.5 
 14.4 J 3.2 
 
 20 
 10 
 
 II. 2 
 
 18. I 
 
 i-7 
 1.9 
 
 20 
 10 
 
 60.7 1 10.7 
 26.4 1 I 
 
 20 
 IO 
 
 Area: constants, 5 sec. and i c. p. 
 
 Table XVI. 
 
 Total Duration of After-image. 
 
 Area of Stimulus 
 in sq. cm. 
 
 
 1 
 ¥ 
 
 
 
 1 
 
 
 
 4 
 
 
 Subjects. 
 
 Aver. A. Var. No. Exp. 
 
 Aver. A. Var. No. Exp. 
 
 Aver. A 
 
 . Var. No. Exp. 
 
 K. 
 
 34 
 
 15 
 
 20 
 
 56 
 
 2 4 
 
 20 
 
 74 
 
 24 
 
 20 
 
 H. 
 
 3i 
 
 12 
 
 10 
 
 59 
 
 15 
 
 10 
 
 68 
 
 17 
 
 10 
 
 Ho. 
 
 24 
 
 3 
 
 5 
 
 24 
 
 4 
 
 5 
 
 45 
 
 15 
 
 5 
 
 F. 
 
 24 
 
 S 
 
 10 
 
 32 
 
 5 
 
 10 
 
 44 
 
 6 
 
 10 
 
 Average. 
 
 28 
 
 
 45 
 
 42.8 
 
 
 45 
 
 57-8 
 
 
 45 
 
 Table XVII. 
 
 Actual Duration of After-image. 
 
 Area of stimu- 
 
 
 1 
 
 
 
 
 
 
 4 
 
 
 lus in sq cm. 
 
 
 T 
 
 
 
 
 
 
 
 Subjects. 
 
 Aver. A. Var. No. Exp. 
 
 Aver. 
 
 A. Var. 
 
 Vo. Exp. 
 
 Aver. A. Var. N o. Exp. 
 
 K. 
 
 23 
 
 7 
 
 20 
 
 4 2 
 
 12 
 
 20 
 
 58 
 
 10 
 
 20 
 
 H. 
 
 21 
 
 10 
 
 10 
 
 38 
 
 17 
 
 10 
 
 45 
 
 9 
 
 10 
 
 Ho. 
 
 21 
 
 3 
 
 5 
 
 20 
 
 5 
 
 5 
 
 37 
 
 11 
 
 5 
 
 F. 
 
 23 
 
 5 
 
 10 
 
 3° 
 
 5 
 
 10 
 
 37 
 
 11 
 
 10 
 
 Average. 
 
 22 
 
 
 45 
 
 32 
 
 
 45 
 
 45 
 
 
 45 
 
 The increase in duration for the greater intensities and the 
 longer times of stimulation is easily explained on purely physio- 
 logical grounds. In both cases more of the photo-chemical matter 
 
 1 The average variation of the averages when the experiments were grouped 
 in sets of five. This was the only calculation left by Mr. Southack. The 
 records of the individual experiments could not be found. 
 
26 
 
 5. /. FRANZ. 
 
 of the retina may be decomposed, and it takes the retina longer to 
 come to a state of equilibrium. The effect of the larger areas, 
 however, cannot be explained by such a simple process. In the 
 latter case there is only an increase in the number of retinal 
 elements stimulated, and not a greater stimulation of any one 
 element. An explanation of the greater durations with the 
 large areas must probably be looked for in another direction. 
 In the threshold experiments the same effect was noticed, and 
 it was concluded that the cause was mental. In this case, 
 when the after-image is fading, a small light is hard to notice, 
 and the large one, with its numerous points for the shifting of 
 the attention, is easy to keep in view. The different durations 
 for the large and the small areas is, accordingly, only an ap- 
 parent one. 
 
 The relative effects of the physical units are shown in the 
 accompanying curves, which are constructed from the averages 
 of subjects H, K and F. The results from these three ob- 
 servers only were used because a slight variation due to the 
 fourth subject would disturb the common point of coincidence 
 in the three series. Fig. 13 represents the results of actual time 
 of the after-image, while Fig. 14 shows the results for the total 
 time. 
 
 Such a simple relation as was found in the threshold ex- 
 periments between the units of physical energy seems not to 
 
ON AFTER-IMAGES. 27 
 
 hold here. This is not surprising when it is considered that the 
 longer time that the image is in view gives greater opportunity 
 for outside influences to affect the process. 
 
 sec. 
 
 60. 
 
 DwawSE. 
 
 m 
 
 The constancy of the relation of the actual to the total 
 duration should be noted. The average proportion is 76 : 
 100, while the variation for this is about 3. 
 
 Quality of Light. — If a spectrum projected on a wall is 
 looked at for some time and an after-image obtained therefrom, 
 it will be noticed, besides the qualitative differences, that the 
 image of the central portion, the yellow and green parts, lasts 
 relatively longer than that of the red or violet ends. This has 
 sometimes been considered as evidence that the central portion 
 of the spectrum is of greater intensity than the ends. This 
 hypothesis seems at first sight to be the correct one ; but at times 
 it has not been sufficiently considered by investigators. 
 
 Bidwell x notes that while the after-image obtained from the 
 
 1 Op. cit., p. 140. 
 
5. /. FRANZ. 
 
 whole spectrum was violet, he could get no after-image from 
 the red and the violet of the spectrum. This fact he attempts 
 to explain without considering the intensity differences, and he 
 uses the observation to prove that the image is due to an ex- 
 citation of the violet nerve-fibers only. However, the accuracy 
 of Bidwell's observation has been questioned by Hess, who- 
 found that an image was produced by red lights whose wave- 
 length was less than 623 //. 
 
 Charpentier ' found that the persistance des impressions 
 retinniennes was affected by the quality of the light ; but 
 though he believed the difference due to the differences in the 
 intensity of the stimuli rather than to the quality of the light, his 
 results were not reported in full. 
 
 In his work with transmitted light, Titchener 2 used various 
 colored gelatins, which resulted generally in showing a longer 
 duration for yellow than for red light. In these experiments, 
 however, the relative intensities were not the same, and in this 
 connection little value attaches to the results. The results 
 obtained by Titchener for effects of other colors — violet, green, 
 etc. — are incomparable for the same reason. 
 
 I have attempted to exclude the factor of illumination- 
 differences by using gelatin films whose absorbent powers were 
 known. The light was so arranged for each gelatin that the 
 intensity was equal for all. The following table shows the 
 results for stimulation of 5 seconds and an area of 1 sq. cm. : 
 
 Table XVIII. 
 
 
 V. 
 
 B. 
 
 G. 
 
 Y. 
 
 O. 
 
 R. 
 
 Duration in 
 seconds. 
 
 24 
 
 26 
 
 30 
 
 24 
 
 27 
 
 22 
 
 A. Var. 
 
 6 
 
 5 
 
 3 
 
 5 
 
 7 
 
 6 
 
 Ten determinations were made with each kind of light. On 
 the first line will be found the average duration under the 
 various conditions ; and below, the average variations for each 
 series of experiments. It will be noticed that practically no 
 
 1 C. Rend, de la Soc. de Biol., Ser. 8, Vol. IV., p. 92, 1887. 
 
 2 Op. cit. 
 
ON AFTER-IMAGES. 
 
 2 9 
 
 difference is found to exist between the after-image-producing 
 power of any colors. The variations make an overlapping of 
 the times in such a way as to show that the slight differences 
 noticed at first in the table are only apparent. 
 
 In this connection it would be interesting to test individuals 
 who are partially or totally color-blind. Tests on such a class 
 would probably be. the only valid ones as indicating the part 
 played by the quality and by the intensity of the light. 
 
 Parts of the Retina. — -Purkinje, Aubert and Exner have 
 noted independently that light-sensations continued longer in the 
 fovea than on the periphery of the retina. These observations 
 have been disputed by more recent investigators, however. 
 The experiments, dealing only with the phenomenon of 
 visual persistence, do not indicate what might be expected for 
 other similar phenomena, particularly the longer continua- 
 tions, such as after-images. In view of the fact that we can 
 see less clearly with the peripheral portions of the retina, 1 and 
 that these parts are more easily fatigued, 2 it seemed probable 
 that the after-image would also be less distinct and would con- 
 tinue a shorter time. The supposition was proved correct by 
 the results of the experiments. 
 
 Each of seven points along the inner side of the right eye 
 were stimulated with a stationary light, and the total and the 
 actual times of duration of the after-image were noted. As 
 stimulus, I used a small lamp with an intensity of about y 2 c. p. 
 
 Table XIX. 
 
 1 See Aubert, Grundziige d. Physiol. Optik, 1876, p. 539 ft'. 
 2 Ibid., p. 545. 
 
 Angular distance 
 from fovea. 
 
 0° 
 
 15° 
 
 30 
 
 45° 
 
 60° 
 
 75° 
 
 90 
 
 Total Aver. 
 
 40 
 
 32 
 
 10 
 
 0? 
 
 
 
 
 
 
 
 Duration A. Var. 
 
 S 
 
 9 
 
 4 
 
 - 
 
 - 
 
 - 
 
 - 
 
 No. Exp. 
 
 10 
 
 10 
 
 10 
 
 5 
 
 5 
 
 2 
 
 2 
 
 Actual Aver. 
 
 35 
 
 24 
 
 8 
 
 0? 
 
 
 
 
 
 
 
 Duration A. Var. 
 
 7-5 
 
 9 
 
 5 
 
 — 
 
 - 
 
 — 
 
 - 
 
 No. Exp. 
 
 10 
 
 10 
 
 10 
 
 5 
 
 5 
 
 2 
 
 2 
 
30 5. /. FRANZ. 
 
 This was looked at for about 5 seconds, and the angular dis- 
 tance subtended was about two degrees. The light was at all 
 times at a constant distance from the eye (retina), viz., 30 cm. 
 The table on page 29 shows the results of the experiment upon 
 each of the points, o° being the fovea. 
 
 Not only were there differences in the duration of the after- 
 image, but the fainter appearance of a sensation toward the 
 periphery was duplicated in the after-images. No variations in 
 color of the after-image corresponding to the different points 
 tested was apparent. 
 
 This gradual, but finally absolute, lack of ability to distin- 
 guish an after-image is probably due to several factors. Aubert 
 mentions that the periphery is more easily fatigued than the 
 fovea ; but he does not seem to consider that this may be partly 
 mental and not entirely physiological. From observations made 
 during the progress of the experiments it seems likely that the in- 
 ability to attend to these things not in its immediate vicinity is the 
 primary reason for the lack of images toward the periphery, 
 and for the long durations at the fovea. At 45 it was felt at 
 times that an image might be present, but so indistinct that it 
 was impossible to give any definite answer. In the table these 
 results are accordingly given as o (?). 
 
 A similar but more extended set of experiments upon the 
 quality as well as the duration would be important for testing 
 color theories, but this point of view was not considered till the 
 present series had been finished. 
 
 Individual Differences. — Throughout the foregoing pages 
 one or more individuals have been considered as giving typical 
 results. That this is not strictly true, however, has been fully 
 recognized ; and from the material at hand I have endeavored 
 to discover some of the individual conditions upon which the 
 duration depends. The numerous difficulties in such a study 
 are evident : each individual is not the same at any two mo- 
 ments, and various distracting thoughts cannot be excluded. 
 
 During the past few years there has been made at the Co- 
 lumbia laboratory a series of mental and physical tests. Among 
 these was one on after-images, in which the duration, the color, 
 etc., were noted. One great defect of these measurements is to 
 
ON AFTER-IMAGES. 31 
 
 be found in the fact that there were many experimenters instead 
 of one, making methods of experimenting and of recording to 
 some extent dissimilar. In general, however, the results are 
 fairly comparable, and are as accurate as the method of correla- 
 tion used in this case. The individual differences of recording 
 had little influence upon the measurement of the total time ; but 
 by some the actual time with the various fluctuations were not 
 given at all. For this reason only the total time can be used as 
 a basis for comparison. 
 
 The stimulus was a cross, with arms 1 sq. cm., illumined by 
 a 100 c. p. incandescent lamp in a box. The lamp was 30 cm. 
 from a plate of ground glass, which was used to make the cross 
 of uniform brightness. The subject was seated in a semi-dark 
 room, and no adaptation time was needed. The cross was 
 shown for ten seconds and then shut off by a screen. The sub- 
 jects, having closed and covered the eyes without pressing upon 
 them, informed the experimenter of the various phases as they 
 occurred, and these were recorded as well as possible. Gener- 
 ally two trials were given. 
 
 The tests chosen for comparison with the results of the after- 
 image test were sharpness of vision, color-vision and imagina- 
 tion-type. These were so chosen because they seemed closely 
 allied to, or were thought to have an influence upon, the after- 
 image. It would have been interesting to compare, in addition, 
 visual and auditory memory, time of reaction to visual and to 
 auditory stimuli, etc. ; but at that time such tests were not in- 
 cluded in the anthropometric series. 
 
 Sharpness of vision was tested with the Galton eyesight- 
 tester. This is an instrument having at certain distances cards 
 of numerals in diamond type which are to be read by the sub- 
 ject, who looks through a hole in a screen. The type was 
 illumined by a 100 c. p. incandescent lamp at a distance of one 
 meter from the card, which was 37 cm. from the eyes. The 
 normal distance for reading correctly eight or more out of ten 
 numerals was for each eye 42-52 cm. 
 
 Color-vision was tested with spools of colored wools ; but 
 this, being only a rough-and-ready method of detecting color- 
 blindness, did not show defects which did not amount to blind- 
 
32 S. /. FRANZ. 
 
 ness. Only six color-defectives were found, and their after- 
 images may be paralleled by numerous other observers who 
 were considered nearer normal. The results, accordingly, 
 show no unusual variations. 
 
 The imagination-type was discovered by means of the fol- 
 lowing series of questions, part of which are the classic ones of 
 Galton : Think of your breakfast-table as you sat down to it 
 this morning ; call up the appearance of the table, the dishes 
 and food upon it, the persons present, etc. Then write answers 
 to the following questions : Are the outlines of the objects dis- 
 tinct and sharp? Are the colors bright and natural? Where does 
 the image seem to be situated? In the head? Before the eyes? At 
 a distance? How does the size of the image compare with the 
 actual size of the scene ? Can you call to mind better the face 
 or the voice of a friend? When violin is suggested, do you 
 first think of the appearance of the instrument or the sounds 
 made when it is played? Can you call to mind natural scenery 
 so that it gives you pleasure? Music? The taste of fruit? Have 
 you ever mistaken a hallucination for a perception ; e. g\, appar- 
 ently heard a voice or seen a figure when none was present? 
 
 Of the subjects who answered the questionnaire, 59^ would 
 be considered as of a distinct visual type, ^j4fo were distinctly 
 auditory and 36^/0 would be considered of mixed type. 
 
 For comparing these results with the duration of the after- 
 image, only the simple method of correlation was used. The 
 results of the subjects that were considered as having hyper- 
 normal, normal or subnormal vision were grouped and the aver- 
 age duration of the after-image was calculated ; a similar method 
 was used for the imagination-type correlation. This method is 
 not above criticism ; but it is sufficiently accurate to show cer- 
 tain tendencies and influences. The results for the accuracy 
 of vision in the right and left eyes, and for the type of imagina- 
 tion, are summarized in the following tables. 
 
 Objection may be made to this method of correlation, to the 
 grouping the subjects and the use of one or, at most, two ex- 
 periments, as giving a typical value for the individual. The 
 objections would apply to all anthropometric series, and it has 
 weight only when one attempts to apply results to individuals. 
 
ON AFTER-IMAGES. 
 
 33 
 
 It is only in considering an individual as of a class that the 
 figures have a value. 
 
 Table XX. 
 
 Acuteness of vision, 
 right eye. 
 
 No. of 
 cases. 
 
 No. of 
 exper. 
 
 Av. durat. of after- 
 image in sees. 
 
 Av. 
 
 Percentage of 
 times no image 
 
 0-37 cm. 
 Subnormal. 
 44-52 cm. 
 normal. 
 61-85 cm. 
 Hyper normal. 
 
 37 
 63 
 35 
 
 57 
 92 
 
 59 
 
 26 
 37 
 37 
 
 
 49 
 
 27 
 25 
 
 Table XXI. 
 
 Acuteness of vision, 
 left eye. 
 
 No. of 
 cases. 
 
 No. of 
 exper. 
 
 Av. durat. of after- 
 image in sees. 
 
 Av. 
 
 Percentage of 
 
 times no image 
 
 was seen. 
 
 0-37 
 
 subnormal. 
 
 44-52 
 
 normal. 
 
 61-72 
 
 hyper normal. 
 
 37 
 76 
 26 
 
 57 
 106 
 
 42 
 
 32 
 34 
 36 
 
 
 46 
 26 
 26 
 
 Table XXII. 
 
 Mental Type. 
 
 No. of 
 cases. 
 
 No. of 
 exper. 
 
 Av. durat. of after- 
 image in sees. 
 
 Av. 
 
 Percentage of 
 times no image 
 
 Visual. 
 Mixed. 
 Audile. 
 
 77 I T2 i 
 
 47 65 
 
 6 8 
 
 36 
 37 
 16 
 
 
 26 
 32 
 63 
 
 Probably the most interesting fact to be noted in the tables 
 is not that the duration is longer for the visual type, or for the 
 man with strong eyes, but that in these classes the total number 
 of times when no image was noticed is small compared with the 
 percentage for the audible type, or for those having weak eyes. 
 
 Granting that these variations are true criteria of the differ- 
 ent classes, an explanation must be sought. It is easy to see why 
 an individual of a visual type might have an after-image of 
 longer duration than one of an audile type. Accustomed, as he 
 is, to note principally those phenomena which appeal to his 
 eyes, practice enables him to select the visual stimuli and to at- 
 tend to these even if of little intensity. Accordingly, an after- 
 image in its final stages, when very dim, would be noted by 
 
34 5. I. FRANZ. 
 
 him ; whereas an auditory type of individual might neglect 
 this minimal amount of sensation. A similar hypothesis would 
 explain why audiles should see less images than visuals. Many 
 of the images are undoubtedly of little intensity, and they would 
 be unnoticed by those individuals unaccustomed to note small 
 visual differences. This would reduce the differences to a 
 question of the attention or of habit, rather than to a physio- 
 logical process ; and this explanation is in accord with what was 
 noted above in the experiments upon the increase in the area of 
 stimulation. 
 
 In a similar series made upon a class of seven boys by Dr. 
 F. B. Brandt this explanation is further confirmed. Using a 
 stimulation of i sq. cm. of the northern sky fixated for 15 
 seconds at a distance of 30 cm., Dr. Brandt found that those 
 boys who held the highest rank in school-work, those who had 
 learned to fix their attention, could see the after-image for a 
 longer time than those who were dull and inattentive. The 
 attention in this case may have been largely influenced by 
 ' interest,' but the general result and conclusions are the same. 
 The ' bright' boy is he who is interested in each novel experi- 
 ence ; is he who attends with all his might to what is immediately 
 present. During the course of the experiments this view was 
 clearly indicated by a study of the actions of the subjects. A 
 and B had been intellectual rivals throughout their school- 
 work, and when B found that A was having after-images of 
 longer duration than he, his interest flagged and it was difficult 
 to persuade him to continue. While there are some variations 
 in the relation between scholarship and the duration of the after- 
 image, the results show that the three boys of excellent ability 
 very easily outrank the other four. The following table gives 
 the results of ten experiments upon each subject. 
 
 In conjunction with the attention, imagination undoubtedly 
 plays an important role in these phenomena. Miss Washburn 
 has noted that she was able to control voluntarily the colors of the 
 after-image ; J and in addition she mentions that when she visual- 
 ized a color and tried to force the after-image to take this quality, 
 
 J M. F. Washburn, Subjective Colors and the After-image ; Mind, N. S., 
 VII., 25-34, l8 99- 
 
ON AFTER-IMAGES. 
 
 Table XXIII. 
 
 35 
 
 Subject. 
 
 Scholarship grade. 
 
 Duration'of after- 
 image in seconds. 
 
 Average variations. 
 
 A. 
 
 9 6% 
 
 320 
 
 36 
 
 B. 
 
 95 
 
 169 
 
 64 
 
 C. 
 
 92 
 
 267 
 
 65 
 
 D. 
 
 82 
 
 60 
 
 26 
 
 E. 
 
 81 
 
 69 
 
 51 
 
 F. 
 
 80 
 
 13s 
 
 40 
 
 G. 
 
 79 
 
 42 
 
 31 
 
 ' the image of the visualized color was brought on sooner,' 
 and ' held longer than usual.' Miss Washburn concludes, as 
 I do above, that the image is largely influenced by the attention, 
 and that perhaps the imagination is to be explained as the call- 
 ing the attention to any part of the image ; the color is there, 
 but it remains unnoticed till we are prepared for it. In this way 
 the after-image would also appear to be in view a longer time. 
 
 Miscellaneous. — Besides these factors, other influences affect 
 the duration of the phenomena. Helmholtz l noted that if an 
 electric current be sent through the eye and the optic nerve, 
 the image is changed in character and the time of duration is 
 (probably) shortened. This change is analogous to that oc- 
 curring when the eye is stimulated with light while an image is 
 in view. Exner 2 found that the retinal circulation and pressure 
 on the ball of the eye influenced the duration. Brewster 3 in 
 1834 n °ted that a smart blow on the head would stop an image 
 in its course. Similar results were obtained from my subjects, 
 but sometimes the image would also seem of less intensity. 
 In the morning, when the eyes were fresh, the image lasted 
 about 30^ longer than in the evening or when fatigue was ap- 
 parent. 
 
 All these factors may be resolved into an influence of the 
 attention on the one hand, or of the physiological process in the 
 sense-organ on the other. Either explanation would suffice ; 
 but it seems that both should be considered. In conjunction 
 
 1 Physiol. Optik, 509. 
 
 2 Ueber die Funktionsweise d. Netzhautperipherie und den Sitz der Nach- 
 bilder; Archiv f. Ophthal., XXXII. (2), 233-252, 1886. 
 
 3 Accounts of Two Experiments on Accidental Colors ; Philos. Mag., 3 Ser., 
 IV., May, 1834. 
 
36 S. f. FRANZ. 
 
 with the material of the foregoing paragraphs it seems likely 
 that the mental element is very great — that is, the blow on 
 the head or the general fatigue, the pressure on the eyeball, 
 etc., are influences inhibiting the focussing of the attention 
 upon the after-image. Particularly is this the case when these 
 factors are brought in in an artificial manner that their effect may 
 be observed. Voluntary winking, where the attention must be 
 turned away from the sensation to the movement, shows the 
 same effect. 
 
 When the stimulus was not attended to, when the subject 
 was distracted with a continued conversation, while all distract- 
 ing influences ceased with the end of the stimulus, the duration 
 and the intensity of the after-image seemed not affected. When 
 however, distracting sensations, sounds and conversation were 
 introduced during the progress of the after-image the attention 
 was distracted sufficiently to cause an apparent shortening in 
 the duration of the after-image by about 40%. 
 
 Summary. — The after-image, particularly in its duration, if 
 affected by many mental and physical conditions. The most 
 influential of these seems to be the mental attitude of the sub- 
 ject. If he looks intently for the after-image, if he is accus- 
 tomed to note visual changes, the image will apparently remain 
 longer than if his attitude is more motor or auditory. When 
 the attention is directed in its greatest intensity to the after- 
 image, either from habit, because one is visual-minded, or 
 voluntarily, the duration is about one-third longer than when 
 the attention is not so directed. Various physical conditions — 
 e. g., having the image on the periphery of the retina, physical 
 fatigue, etc., which give similar results — may also be con- 
 sidered in this class. Of the physical light-changes, the most 
 influential seems to be the time of stimulation. This corre- 
 sponds to what was found in the experiments on the threshold 
 and latent period. The area- and intensity-changes also have 
 effects similar to what was found in the experiments upon thresh- 
 old and latent period ; but no general relation of these effects 
 seems to be discovered. Figures 13 and 14 show composites of 
 the average results from Tables IX.— XVII., and from them the 
 relative effects of the physical units may be noted. Contrary to 
 
ON AFTER-IMAGES. 37 
 
 the generally accepted view, variations in quality (color) seem 
 to have little effect unless there is, as usual, a change in 
 intensity. 
 
 Section 5. Fluctuations. — In all discussions of the after- 
 image one of the questions that has been largely neglected is 
 that regarding the fluctuations from positive to negative, its 
 disappearances and reappearances. Immediately upon the close 
 of the stimulus an after-image due to the continuation of the 
 light is seen. A vacant period (the latent time, see Sec. 3) fol- 
 lows, and then an image which in turn fades away into another 
 color or disappears and reappears several times. This fluctu- 
 ation was noted by R. W. Darwin, 1 and has been variously as- 
 cribed to many physiological causes connected with the sense- 
 organ — to eye-movements, to winking, to pressure on retina, 
 etc. During the progress of the present work I endeavored to 
 note the various effects of these conditions. Most of the experi- 
 ments were made in a dark-room, and the image was looked for 
 with eyes open. Pressure on the eyeball was thus prevented, 
 but the fluctuations continued. Winking, if unconsciously done, 
 disturbed the image momentarily, but had no lasting effect. 
 However, if consciously done (i. e., when the attention was di- 
 rected away from the after-image to the movements of the eye- 
 lids), the image seemed lost for an appreciable time. The 
 numerous involuntary eye-movements could not be controlled. 
 The eyes tended to follow the image, which would remain in 
 view while these movements were not inhibited. If consciously 
 prevented, if the eyes were brought back to what appeared to 
 be their normal position, if the eyes were consciously rotated 
 while the phenomena lasted, the image would disappear, only to 
 reappear if consciously brought back — i. e., if the attention 
 was again directed to the organ of sensation rather than to the 
 movements of this organ. From these observations of my sub- 
 jects and myself I am inclined to believe many of the fluctua- 
 tions are mental in character, just as we have seen the duration 
 to be largely a mental matter. In other words, the various 
 
 1 R. W. Darwin, New Experiments upon the Ocular Spectra of Light and 
 Colours; Phil. Trans., LXXVL, 313-348, 1786. 
 
3S S. f. FRANZ. 
 
 phases are influenced more by the mental attitude of the subject 
 than by the physiological condition of the retina. 
 
 Helmholtz noted that violent movements of the body 
 changed the character of the after-image from positive to nega- 
 tive, 1 and that an electric current sent through the optic 
 nerve changed the intensity of the after-image present. 2 This 
 may also be seen to agree with the attention hypothesis. 
 
 In some respects the fluctuations of the image arising from 
 stationary stimuli are analogous to those noted by Bidwell 3 
 and by Charpentier. 4 However, the images noted by these in- 
 vestigators were so complex that it seems uncertain whether 
 we are dealing with the same phenomena. The Bidwell recur- 
 rent images are explained satisfactorily upon physiological 
 grounds ; but in repeating the experiments I was led to believe 
 that at least the brightness of the various bands was due in large 
 measure to the fixation of the attention. 
 
 The changes in the physical stimuli seemed to have no 
 effect in the present case. The after-images when they lasted 
 longer did not have more fluctuations, but the various phases 
 remained for longer times. Such a great variation in the num- 
 ber and in the character of the fluctuations was noted that it 
 seemed impossible that any general relation held true. Op- 
 posed to these results, however, are observations by Helmholtz 5 
 and by Miss Washburn. 6 Helmholtz noted that the duration 
 of the stimulus had an influence upon the negative as well as 
 upon the positive phases ; but he only makes this general state- 
 ment without further detail. Miss Washburn also observed vari- 
 ations in the fluctuations due to intensity-changes of the stimulus. 
 
 Section 6. Qualitative Changes. — As one of the most 
 difficult and most striking of the phenomena to be explained, 
 the color-changes of the after-image have attracted a great 
 amount of attention. But throughout the various accounts of 
 the color-changes it seems to have been assumed that the ex- 
 perimenter was normal, and what occurred in his eyes would or 
 
 1 Physiol. Optik, p. 504. *Op. cit. 
 
 2 Ibid. 5 Op. cit., p. 505. 
 
 3 Op. cit. 6 Op. cit., p. 27. 
 
ON AFTER-IMAGES. 39 
 
 should occur to another, if he saw the same light. This 
 belief has led in some measure, I fancy, to the disagreements 
 regarding the phenomena. The individual element seems not 
 to have been considered by any one. In the experiments noted 
 in Section 4 it was found that the various observers differed 
 greatly in their account of the color-changes ; and, moreover, 
 an individual did not always see the same colored after- 
 image when stimulated with the same light. At one time, all 
 the images would be seen as light alone, at another, they ap- 
 peared gray or reddish. This change appeared with all my sub- 
 jects. The difference was also noted by R. W. Darwin, 1 who 
 considered the changes as due to his making too many experi- 
 ments at one time, his eye being ' not quite free from the 
 spectra of the colours previously attended to.' The difference 
 of individuals is naively noted by Miss Washburn in her recent' 
 paper. In this paper she says her subjects had to be drilled to 
 see a normal image («'. e., like her own). 2 
 
 From the after-image experiments in the series of anthro- 
 pometric tests (see Sec. 4), it was found that " the after-image, 
 when first seen, was sometimes positive and sometimes nega- 
 tive : and the colors varied greatly, being distributed in the first 
 place as follows : negative or dark, 33^ ; light or white, 29.4 ; 
 blue, 13.7; purple, 9.8; green, 5.9; yellow, 3.9; red, 2.0; 
 miscellaneous, 2.0." 3 
 
 Various trials were made of physical conditions to discover 
 whether or not the change was due to these ; but for the condi- 
 tions tested (position of eyes in relation to light, accommodation, 
 adaptation, changes in intensity, duration and area of stimulus), 
 nothing definite was discovered. 
 
 Fechner found that various durations of the stimulus gave 
 different colored after-images, but I found no such definite 
 relation with the comparatively long times used by me. The 
 change in quality of the after-image under any one condition — 
 e.g., 5 seconds — seemed as great as that observed between 5 
 seconds and 1 second stimulation. The only uniform change 
 
 !Op. cit. 
 
 2 Op. ct., p. 27. 
 
 3 Cattell and Farrand, Physical and Mental Measurements of the Students of 
 Columbia University; Psycho,. Rev., III., 618-648, 1896. See especially p. 645. 
 
40 5. I. FRANZ. 
 
 noted was that with the greater intensities and the longer stim- 
 ulations the after-image appeared oftener as negative. With 
 medium intensities such as I used this result is almost never 
 noted ; but the change is clearly brought out in the results of 
 the experiments upon threshold (where only \ of ifo were nega- 
 tive) compared with the results upon duration (where fully 8of> 
 of the images had negative phases). 
 
 The quality (or intensity) of the image was changed by 
 Helmholtz by sending an electric current through the eye. " If 
 one has developed in the eye a negative after-image and then 
 sends an electric current through the eye and the optic nerve, 
 * * * the negative after-image becomes darker, and if an 
 image is just on the borderland between positive and negative, 
 it can be made negative by sending a current through the eye." 1 
 This change is somewhat analogous to what occurs when an 
 after-image is produced and external light is then admitted to 
 the eye. 
 
 I can attempt no explanation for these individual differences. 
 The suggestion has been made that the differences may be due 
 to a varied sensitiveness of the retina to light-waves of different 
 lengths. Such an explanation seems, however, too hypothetical 
 when one considers the great differences. One subject saw 
 the after-image always as green ; to another it was always red ; 
 to a third it varied in the white-black series. The explanation 
 could not be stretched in any case to cover the change in an in- 
 dividual under practically the same conditions. The importance 
 of these differences for theories of color-vision will readily be 
 recognized. If later experiments are made, a most careful 
 study of each subject's color-vision should also be made. 
 
 The control of the color of the* after-image should again be 
 noted. Miss Washburn attempted to control the color-changes, 
 but concludes that the control is not real, but apparent. In her 
 case the attention intensified certain colors that were present, 
 but which would be unnoticed under ordinary circumstances. 
 Such an explanation of the effects of attention probably could 
 not be made to include the variation of one individual from 
 another. The problem needs more detailed and careful study. 
 
 1 Op. cit., p. 509. 
 
ON AFTER-IMAGES. 41 
 
 An attempt to change voluntarily the character of the after- 
 image from positive to negative has been uniformly unsuccess- 
 ful in my own case ; Miss Washburn seems to indicate that this 
 was done by all her subjects, and it may be that being a poor 
 visualizer accounts for my inability. 
 
 Section 7. Space-relations. — In his characterization of 
 the difference between after-images and the images of the im- 
 agination, Fechner considers as one of the greatest differentia 
 the spatial character of the phenomena. The after-image is, 
 according to him, of two dimensions only, while the images of 
 the imagination are tridimensional. This statement has not 
 often been disputed, but there seems little truth in it as a state- 
 ment of universal fact. Professor Hyslop found that if a 
 " picture hanging obliquely on the wall, say thirty degrees, 
 more or less, and I look at it while lying on a bed or lounge 
 and then look at the wall vertically near me,* * * I notice that the 
 after-image does not lie in the plane of the wall, but in the same 
 ■position relatively to the plane of vision as in its real position.^ 
 In other words, we have here a case where the after-image 
 was distinctly opposed in character to what Fechner said we 
 have it. 
 
 In view of these contradictory statements, it seemed advis- 
 able to test the matter more thoroughly, using several subjects. 
 A few simple experiments were devised for the purpose. The 
 subjects, naive as to the purpose of the experiment, were asked to 
 gaze fixedly for fifteen seconds at one corner of a highly colored 
 rectangular block, and to project the resultant after-image 
 upon a white wall about four meters distant. Note was to be 
 made of size, color, etc., but no suggestion was given regard- 
 ing the tridimensional character. Throughout the experiments 
 the eyes were open. A brightly lacquered round resonator, 
 two brightly colored books placed like a V with the apex turned 
 away from the subject, and two incandescent lamps placed at dif- 
 ferent distances from the eyes, were also used as stimuli. Some 
 of the observations with the various stimuli are as follows : 
 
 l J. H. Hyslop, Experiments in Space Perception; Psychol. Rev., I., 588, 
 
42 5. /. FRANZ. 
 
 " Projected from the wall, but not distinctly." 
 
 " Seemed to stand back. Solid." 
 
 " Some suggestion of solidity, but no definite outline." 
 
 "Perhaps (?) solid." 
 
 "Vague feeling of projection from wall." 
 
 Most of the results, however, seem negative. With closed 
 eyes, on the other hand, the image became more nearly like 
 the imagination-images ; and while there was a feeling of in- 
 security, the images acquired the tridimensional character of the 
 mental products. This may be due (in some cases it was due) 
 to the superposition or to the strengthening of the after-image 
 by the imagination-image. Here we must also consider the 
 effects of the attention. We ordinarily see what we wish to 
 see, and the numerous questions regarding the space-relations 
 may have tended to suggest to the subjects the answer sought. 
 With some subjects, however, no amount of experiments or 
 suggestions could get them to say anything beyond that the 
 image seemed ' almost solid.' 
 
 If my own attitude is to be considered typical, I should say 
 that the image appears at first distinct only in outline. The dif- 
 ferences of light and shade which we note in the original solid 
 object do not appear in the after-image till we look for them, 
 and then it is difficult to say whether the differences are 
 really due to the imagination or whether they are in the after- 
 image. The time it took many of my subjects to note the 
 third dimensions would indicate that this space-character was 
 ' read into ' the sensation rather than given. 1 This much, how- 
 ever, seems clear, that if there is any appearance of solidity 
 connected with the after-image, it is not nearly so evident as it 
 is in sensation or in imagination. The fact that the image is 
 projected by us into space, and is there localized, leads one to 
 believe that the idea of depth is not wholly wanting in the ordi- 
 nary after-image. This would seem to class the phenomena 
 rather as perceptions than as sensations. 
 
 1 Professor Hyslop advises me that almost seventy-five per cent, of the stu- 
 dents in his introductory classes fail to perceive at once the third dimension in 
 stereoscopic pictures. Many succeed after the suggestion has heen given, but 
 some fail to obtain the usual result even after many experiments. 
 
ON AFTER-IMAGES. 43 
 
 Stereoscopic experiments, in which the sensation was one of 
 solidity, gave no indication of apparent solidity in the resultant 
 after-image. 
 
 The size of the image, as is well known, apparently in- 
 creases or decreases if we look respectively at a far or at a near 
 object. This apparent change in size is due to the fact that a 
 certain portion of the retina is concerned with the seeing of 
 any after-image, and the amount of space subtended by this 
 portion of the eye increases as the square of the distance of the 
 wall or screen. When projected on the same screen the size 
 of the after-image from any light is approximately constant 
 for all observers ; the few variations noted are probably due to 
 hasty judgments or poor observations. It may be that the mov- 
 ing of the eyes during the stimulus produced in these cases a 
 larger image, more of the retinal elements having been stimu- 
 lated. A curious fact in connection with the apparent change in 
 size is that if a long image is projected upon a receding wall 
 one end is seen wider than the other. A more wonderful and 
 more puzzling phenomenon is mentioned by James. If an 
 after-image is projected on a flat surface ' resembling a re- 
 ceding screen,' the image takes on the form it would have if 
 seen under similar actual conditions just noted. Such a result 
 points strongly to mental influences, which we have seen 
 are important factors in all conditions of the after-image, viz., 
 attention and imagination. The experiment makes it evident 
 that the phenomena are closely related to the imagination and 
 to perceptual processes. The experiment was repeated by 
 several of the subjects, but at first all united in affirming no 
 such change. The suggestion made, however, to one in- 
 dividual gave the effect. It is impossible to tell from James' 
 remark whether the observation is original with him or whether 
 it is quoted. 
 
 To discuss the apparent size of the after-image seen when 
 the eyes are closed leads one into all the difficulties that have 
 followed the discussion of the moon's size. It is interesting 
 to note, however, that when asked the size of the image result- 
 ing from a cross with arms one decimeter square as viewed at a 
 distance of three meters the average of the sizes noted by 
 
44 5. /. FRANZ. 
 
 observers was 7.5 cm. and the image was said to be about 
 1.87 m. from the eyes. The variation is large and the individ- 
 ual answers show characteristics of the several observers. The 
 subject might repay fuller and more extended observation. 
 
 Section 8. The Retinal Transfer of the After- 
 image. — One of the questions of considerable interest raised by 
 Newton in his letter to Locke was that regarding the transfer of 
 an image from the stimulated to the unstimulated eye. "Though 
 I looked with my right eye only" (at the sun), he says, " and 
 not with my left, yet my fancy began to make an impression 
 upon my left eye as well as upon my right. * * * With my left 
 eye I could see the spectrum or the sun almost as plain as with 
 my right." 
 
 Brewster 1 independently noticed the phenomenon, and it has 
 been discussed fully by many observers, notably by Helmholtz, 
 Fechner, Charpentier and Titchener. A complete discussion of 
 previous views will be found in Titchener's 2 article, and only a 
 brief resume of the general views need here be given. 
 
 It will readily be seen that the transfer may be explained in 
 any of the following ways : (1) The appearance is an extension 
 of the well-known phenomena of binocular contrast. (2) There 
 is a functional connection between the retinas, whereby one is 
 affected by what affects the other. (3) The after-image has its 
 seat in the brain, and not in the retina. (4) The transfer is 
 only apparent. 
 
 Fechner 3 and Helmholtz i are the sponsors of the first theory. 
 The former stimulated one eye with a bright-colored light, and 
 the other was darkened or stimulated with a very weak gray 
 light. According to him, the unstimulated eye saw the con- 
 trasted color during the continuation of the stimulus, and this 
 left its effect (a true after-image, apparently) in this eye. In 
 other words, the after-image was not transferred, but the op- 
 
 1 Article on Accidental Colours, in Edinburgh Encyclopedia. 
 2 E. B. Titchener, Ueber binoculare Wirkungen monocular Reize; Philos. 
 Stud., VIII., 231-310, 1S93. 
 
 3 Fechner, G. Th., Ueber einige Verhaltnisse des binocularen Sehens ; Abh. 
 d. k. Sack. Ges. d. Wi'ss., VII., 481, i860. 
 
 4 Op. cit. 
 
ON AFTER-IMAGES. 45 
 
 posite nerve-fibers of the unstimulated eye were excited during 
 the progress of the sensation, whence an after-image was pro- 
 duced. This explanation, accordingly, is only a different form 
 of the next theory. 
 
 In support of the co-excitation hypothesis many other 
 physiological phenomena may be cited. The most evident of 
 these are the facts that both eyes are used as a single instru- 
 ment ; that they are moved together ; and that if one eye is in 
 the dark while the other is in light, the iris of the former will 
 expand as the iris of the latter does. Similar other points indi- 
 cate that the two eyes are controlled as one. Charpentier 1 is the 
 main exponent of this theory ; but his experiments upon the 
 varied sensitiveness of the two eyes do not seem to have been 
 well chosen or conclusive. 
 
 Parinaud, 2 Ebbinghaus 3 and Binet 4 are supporters of the third 
 explanation. Almost the same experiments that convinced 
 Fechner and Charpentier were used by them in making the 
 new hypothesis. It is taken for granted that the image is seen 
 by the eye that is open, and to them the only satisfactory ex- 
 planation is that the image lies in the cerebral center of vision, 
 and not in the retina. 
 
 While accepting the same general point of view, Titchener 
 introduces a new series of experiments to prove that the transfer 
 is real, and is not an apparent one. The experiments included 
 observations upon( i )the relative duration in the stimulated and in 
 the non-stimulated eye ; (2) the variations in the fluctuations ; (3) 
 the color-changes, and (4) the differences in brightness. The 
 results of these experiments by Titchener are summed up as fol- 
 lows : (1) The image in the non-stimulated eye lasts a shorter 
 time than in the stimulated eye. (2) The relative time which 
 the two phases of the secondary (i. e., in the non-stimulated 
 eye) image take is in no way similar in the primary eye. 
 (3) The phenomenon occurs also under conditions when a mix- 
 
 1 Charpentier, A., La lumiere et les couleurs, Paris, 188S. Sur les connex- 
 ions functionnelles des deux retines. C. Rend, de la Soc. de Biol., 8 Ser., II., 
 364, 1885. 
 
 2 C. Rend, de la Soc. de Biol., 13 May, 1S82. 
 
 3 Archiv f. die gesammle Physiol., XLVI., 498, 1S91. 
 
 4 La Psychologie du raisonnement. Paris, 1886, p. 45 ft. 
 
4° 5. /. FRANZ. 
 
 ture of both fields of vision is made impossible. (4) There is 
 a constant difference in brightness between the negative comple- 
 mentary images in the two eyes. (5) Experiments upon a 
 monocularly color-blind person gave evidence of the second 
 image. 
 
 On the other side of the question we find Delabarre. To 
 him the secondary after-image is an illusion ; the transfer is only 
 apparent. Considering the experiments made by Parinaud and 
 Binet, he says: "A serious difficulty in settling the question 
 lies in the well-known impossibility of separating the visual 
 fields of the two eyes. Whether one eye or both are open ; 
 whether they are focussed on the same point or are held par- 
 allel, or squinted, or even jammed into all sorts of relative posi- 
 tions by fingers inserted into their sockets, the field of each will 
 appear to coincide with the field of the corresponding portion of 
 the retina of the other. If an after-image be found in both to- 
 gether, one image only will be seen whatever their relative 
 positions ; and if the image be found on one alone, it will yet be 
 seen in the corresponding portion of the field of the other, pro- 
 vided that the brilliancy of the second field be not so great as to 
 obscure the much weaker sensation of the image. In reality, 
 in this experiment the after-image never does appear on the left 
 field until the last eye has so greatly darkened as to allow it to 
 be seen ; and in the periodical increases in brilliancy of the left 
 field the image disappears. It will thus be seen that a retinal 
 seat of the after-image explains all the facts as easily as does a 
 central seat. Hence the assumption of the latter by M. Binet 
 and others is entirely superfluous * * * no i merely super- 
 fluous, but impossible." 1 
 
 The results of five experiments tend to strengthen this posi- 
 tion : (1) There is a difference in brightness of the image 
 when the stimulated eye is open or shut. (2) With an image in 
 the right eye, open it and the image is clearly seen ; open the left 
 eye with the right closed, and the image is blotted out. Such 
 an effect is probably due to binocular mixture. (3) With both 
 eyes open, the image being in the right, place a pencil or 
 
 1 Delabarre, E. B., On the Seat of Optical After-images ; Amer. Jour. 
 Psych., II., 326-328. 1889. 
 
ON AFTER-IMAGES. 47 
 
 similar object before the left ; no change in the image occurs. 
 Place an obstruction before the right, and the image disappears 
 temporarily or permanently. (4) Obtain a strong image with 
 the right eye ; if projected on a white surface, it appears light 
 green. If both eyes are covered tightly, so that no light gets in, 
 the image is dark green. If the eyes are closed, but not 
 covered, and light gets in, the image is rose-colored. Now 
 "obtain an image with the right eye, then close and cover it 
 and open the left eye. Now the left field darkens and the dark- 
 green after-image appears ; this gives place to the rose-colored 
 image if the covering is removed from the right eye, and the 
 eye kept closed ; and this in turn to the light green if the right 
 eye is opened. These three colors can be made to succeed 
 one another indefinitely without in any way interfering with the 
 open left eye, which, alone, according to M. Binet, is the 
 source of all the visual impressions present." (5) In retinal 
 rivalry, on obtaining an image with the right eye, look with 
 both eyes through colored glasses at a background. When the 
 color seen with the right eye is predominant the image is seen ; 
 when the color seen with the left eye is predominant no image is 
 apparent. 
 
 Professor Titchener's experiments, like Binet's, : may also be 
 explained by considering the image a peripheral as well as a 
 central event. The variation in brightness is explained from 
 the peripheral point of view, to my satisfaction, by Delabarre's 
 considerations. The apparent difference in the duration of the 
 image is readily understood when one learns that pressure of any 
 sort on the eye will disturb the image. The mere closing of 
 the eyelid would suffice for this. When Titchener speaks of 
 the occurrence of the phenomena even under ' conditions when 
 a mixture of both fields is impossible,' he seems to forget that 
 the two eyes always act as one, and that it is impossible to tell 
 (by ordinary inspection) in which eye a sensation is. One of 
 the subjects used in the present research, a man who had 
 worked during two years upon binocular rivalry, advises me that 
 he is now unable, even after such great practice, to tell in 
 which eye the sensation is. It seems probable, in view of other 
 experiments noted above, that Dr. Titchener's subjects were in- 
 
4*5 5. /. FRANZ. 
 
 fluenced to a great extent by suggestion. It has been seen what 
 a great part this plays in the duration and in the spatial charac- 
 ter of the after-image ; and it is not unlikely that a similar in- 
 fluence is felt when looking for an image with an unstimulated 
 eye. 
 
 To my mind, the only experimental results that might indi- 
 cate the central seat of the after-image are the well-known 
 experiments with suggested after-images. Binet gives the 
 following account of the experiments •} " Wundt has shown 
 that the simple image of a color, imagined for a long time, 
 gives rise to an after-sensation of the complementary color. * 
 If one mentally looks fixedly for some time at a red image, he 
 perceives a green tint upon opening the eyes and looking to- 
 ward a white surface. This experiment is difficult to repeat, 
 for it is necessary to have a power of visualization that not 
 every one has. To take me as an example, I am not even able 
 to imagine a color clearly. I am a very mediocre visualizer, 
 so it is not surprising that I cannot obtain the colored after- 
 image. But my friend Dr. F£re does this readily. He can 
 represent to himself a red cross sufficiently intense to see on a 
 piece of paper a green cross following it ; moreover, he sees not 
 only the color but the shape." A similar experiment has been 
 made with hypnotized subjects. A red cross is suggested and 
 then suggested away, and the green after-image noted. In the 
 case mentioned by Binet, however, it seems not to have occurred 
 to him that possibly the resultant after-image was suggested 
 just as was the original red cross. The experiment has not 
 been made in a rigidly scientific manner, and is inadequate. I 
 have been unable to get a sufficiently good visualizer who was 
 at the same time na'ive regarding after-images, and I have not 
 been able to repeat the experiment with due precautions. 
 
 The case with a full hypnotized subject seems not well au- 
 thenticated. However if the question of its authenticity be 
 waived for a moment, the question immediately arises, " Is the 
 after-image the result of seeing the imaginary red cross, or is it 
 not in this case also a new suggestion?" The precaution not to 
 give such a suggestion seems not to have been taken, and this 
 
 1 La Psychologie du Raisonnement, Paris, 1886, p. 41. 
 
ON AFTER-IMAGES. 49 
 
 would vitiate the experiment. Even if this precaution were 
 taken and the same result found, the central hypothesis would 
 not be proven. Other experiments with hypnotized subjects 
 would seem to indicate that from suggestion certain tissues 
 may be modified so as to leave thereafter a noticeable ef- 
 fect. This is true of the experiments in which a blister is 
 produced by suggesting the application of some known medium 
 which ordinarily will produce such an effect. With such a 
 case explained upon purely peripheral grounds, would it not 
 seem probable that the eye was also so stimulated when the red 
 cross was imagined? 
 
 Delabarre suggests the following experiment, a trial of which 
 might aid in giving some negative testimony. Hypnotize a sub- 
 ject and suggest a sensation that he may get a good after- 
 image. Then by suggestion paralyze the sight of this eye. If 
 no after-image is seen in the other eye, it will indicate that there is 
 no transfer ; and if an image is presumably seen, " it will merely 
 indicate that the paralysis of the right optic nerve has not been 
 complete." 1 Delabarre's radical statement should have its will 
 toned to may. The result in either case would not decide 
 whether the image was central or peripherally transferred. 
 
 An experiment which may indicate a line for investigation 
 has been tried by me. I attempted to stimulate only the optic 
 nerve and the cerebral visual centers to discover what effect, if 
 any, was left by such stimulation. Electrodes were placed upon 
 the head, and when the circuit was closed and broken a vivid 
 flash of light was produced. All the experiments gave nega- 
 tive results. No after-image was produced. It would be sup- 
 posed that, if the image was of central origin, the stimulation 
 of the central centers would give such an after-image. In this 
 experiment it is not certain that the retina was not stimulated, but 
 that the brain center was (either directly by the electricity or by 
 the retina, which was in turn stimulated by the electric current). 
 It would seem as if an after-image should have been produced 
 in either case if the seat was cerebral. 
 
 Another experiment, which gives only a partial and nega- 
 tive answer to the question of transfer, has been made by me. 
 
 ^p. cit. 
 
50 5. /. FRANZ. 
 
 That portion of the right eye which corresponds to the blind spot 
 of the left eye was stimulated to see whether there would be an 
 apparent transfer. If a transfer (apparent) occurred, it could not 
 be accepted as real, since with the corresponding portion of the 
 left eye we can see nothing. If no image is apparent on open- 
 ing the left eye, we learn nothing new, and neither theory is 
 benefited. The fact is, an image appeared upon opening the 
 left eye, and we are left to conclude that the transfer in this case 
 is apparent and in other cases it is likely to be apparent. 
 
 Such an experiment is of value in that it is a link in the 
 evidence tending to exclude one hypothesis. The image seems 
 not to be transferred, but is either central or peripheral. The 
 central situation seems to me improbable in view of the results 
 of Delabarre's expei - iments and of the experiment on brain 
 stimulation. The changes in the image under objective changes 
 in the organ originally stimulated indicate a peripheral seat only 
 in the stimulated eye ; and the interference with the stimulated 
 eye interfering with the progress of the image indicates that 
 the after-image is not transferred. 
 
ON AFTER-IMAGES. 51 
 
 Part II. Historical and Descriptive. 
 
 Section 9. Relation of After-images to Sensation, 
 to Imagination and to Memory. — The intimate relation of 
 after-images to sensation, to memory and to imagination makes 
 the phenomena of great psychological interest. Seeming to be 
 the connecting link between sensation and the idea, the study 
 of this relation is of considerable epistemological importance. 
 
 The differences between visual after-images and the images 
 of the imagination have been thoroughly discussed by Fechner. 1 
 His conclusions are summarized by James as follows : 2 
 
 "After-images first coercive ; seem unsubstantial, vaporous ; are sharp in 
 outline ; are bright ; are almost colorless ; are continuously enduring ; cannot be 
 voluntarily changed ; are exact copies of the originals ; are more easily got with 
 shut than with open eyes ; seem to move when the head or eyes move ; the field 
 within which they appear (with eyes covered) is dark, contracted, flat, close to 
 the eyes and the images have no perspective ; the attention seems directed 
 forward toward the sense organ in observing after-images. " On the other hand, 
 " imagination-imagesizA subject to our spontaneity; have, as it were, more 
 body ; are blurred ; are darker than the darkest black of the after-images ; have 
 lively coloration ; incessantly disappear and have to be renewed by an effort 
 of the will (at last even this fails to revive them); can be exchanged at will 
 for others ; cannot violate the necessary laws of appearance of their originals ; 
 e. g., a man cannot be imagined from in front and behind at once ; the imagi- 
 nation must walk around him, so to speak ; are more easily had with open than 
 with shut eyes ; need not follow the movements of head or eyes ; the field is ex- 
 tensive in three dimensions, and objects can be imagined in it above or behind 
 almost as easily as in front ; in imagining, the attention feels as if drawn back- 
 wards towards the brain. Finally Fechner speaks of the impossibility of 
 attending to both after-images and imagination-images at once, even when they 
 are of the same object and might be expected to combine." 
 
 The above account is true only of Fechner himself ; he re- 
 marks that results from other individuals show certain character- 
 istic differences. Dr. Lay finds that he agrees with Fechner 
 only in the following particulars : the after-images are coercive, 
 cannot be voluntarily changed, seem to move with the eyes, and 
 the attention is directed forward toward the sense organ, 
 while the images of the imagination are directly opposite 
 
 1 Elemente der Psychophysik, II., 46S ff. 
 
 2 Principles of Psychology, Vol. II., 50. 
 
52 S. L FRANZ. 
 
 in these particulars. 1 Regarding the other differences, it is not 
 clear whether Dr. Lay disagrees with Fechner regarding the 
 characterization of the after-image or of the images of imagi- 
 nation. 
 
 From the numerous observations of my subjects and myself 
 any one of these qualifications of the after-image may be contra- 
 dicted. The fact that we do not see an after-image after every 
 visual sensation, the fact that it requires an amount of attention 
 to perceive it, would indicate that the phenomenon is not so 
 coercive as Fechner believed. The after-images are not color- 
 less ; sometimes they are colored more lively than ordinary * 
 sensations, and often their color is more intense than that of the 
 imagination-images. The images are not continuously endur- 
 ing, but have many fluctuations. We have already seen (Part 
 i, Sec. 7) that the after-image often has the appearance of 
 solidity. The probable reason for the usual two-dimensional 
 character lies in the fact that nearly all objects bright enough 
 to produce after-images are of two dimensions only. The 
 colored paper, the window panes, the gas flame, etc., are 
 plane surfaces and in the sensation the effect of solidity is not 
 gotten directly from these, but rather from their surroundings, 
 which usually have not the requisite intensity for the production 
 of after-images. To Fechner's differences it may be added that 
 the after-image is sharp and clear only if very near the point of 
 fixation, while the imagination-image may be a clear represen- 
 tation of a scene all of which could not be noted by the eye at 
 one time. James adds as a universal proposition that the after- 
 images seem larger if we project them on a distant screen, and 
 smaller if we project them on a near one, while no such change 
 takes place in mental pictures. 2 
 
 The name of memory-after-image is given by Fechner to 
 the instantaneous positive effects of sensation. 3 These images 
 are distinguished from ordinary after-images by the following 
 characteristics : (i) Their originals must have been attended to 
 
 1 W. Lay, Mental Imagery, p. 2. Monograph Supplement No. 7 to The Psy- 
 cJiological Jieview, 1898. 
 
 2 Principles of Psychology, p. 51, note. 
 3 Elemente der Psychophysik, II, p. 491 ff. 
 
ON AFTER-IMAGES. 53 
 
 only such parts of the compared originals as have been attended 
 to appearing : this is not the case in common visual after- 
 images. (2) The strain of attention toward them is inward, as 
 in ordinary remembering ; not outward, as in observing an or- 
 dinary after-image. (3) A short fixation of the original is better 
 for the memory-after-image ; a long one for the ordinary after- 
 image. (4) The colors of the memory-after-image are never 
 complementary of those of the original. It is difficult to state 
 the relation between the memory-after-image and the memory- 
 image, but the two seem almost equally different from the true 
 after-image. Many memory-after-images have an overpower- 
 ingly coercive quality. Examples of this are numerous. A 
 revolting scene will often leave a lasting memory-image. The 
 sight of some one drowning or a railway accident will remain 
 in the mind for weeks ; the details will appear in consciousness 
 unexpectedly, and will overpower us as did the originals. But 
 the effect is not produced so often with the visual as with the 
 auditory memory-after-images. The snatches of melody that 
 one hears continually in the mind's ear after a concert, or the 
 popular tunes that often rise in consciousness and compel us to 
 hum them over and over again, have a coercive quality un- 
 paralleled in the phenomena of after-images. 
 
 In many ways the after-images can be considered a true sen- 
 sation. One of the chief sensational conditions is present, viz., 
 a change in the sense organ ; and many of the more mental 
 conditions of the sensation are equalled. At times it is impossi- 
 ble to say whether it is an after-image or a sensation that is in 
 consciousness. A friend relates that his child, who had looked 
 at the sun and then turned around, tried to point out to a sister 
 the second sun (7. e., the after-image, which was real to him). 
 Older people sometimes make the same mistake, and correct the 
 error only from later observations. Such an uncertainty is due 
 primarily to a wonderful clearness of outline, brightness of color 
 and great intensity of the after-image. To these differences 
 Sully 1 adds " definiteness of localization (either in the field of 
 objects if the eyes are open, or in the dark field if they are 
 shut)." Such conditions are not often produced in one's daily 
 
 iThe Human Mind, I., p. 17S. 
 
54 S. /. FRANZ. 
 
 life, but may be obtained by suitable experiment. On the other 
 hand, the moving of the after-image with the eye, its fluctua- 
 tions, its negative quality, its (usually) plane character, and the 
 fact that it is not doubled by lateral pressure upon the eyeball, 
 differentiate the after-image from a sensation. 
 
 Section io. History. 1 — The phenomena variously known 
 as after-images, recurrent-images and ocular spectra (Germ. 
 nachbilder ; Fr. couleurs accidentelles, persistence des im- 
 pressions') seem to have been noted first by Aristotle. He 
 compared the images of the dream state to them, and speaks of 
 them as if they were familar to his audience. The account is 
 as follows : 
 
 " It is evident that when we look at anything for a long time, and then 
 turn away our eyes, the sensation continues ; just, for example, as when we 
 look at something dark after having looked at the sun, it happens that on ac- 
 count of the force from the light still remaining in our eyes we see nothing 
 (*'. e., of the shaded object). And if, after having looked at a color for a long 
 time we turn away our eyes, this same thing happens ; and if we should turn 
 away our eyes after having looked at the sun or some other bright object, it 
 happens that the eye sees first the same color, this color then changes to red, 
 then to purple, and after becoming black disappears." 2 
 
 Later the after-image is mentioned by St. Augustine, and 
 by the Arab Alhazin, who was probably attracted to their study 
 from reading Aristotle's works. 
 
 In the seventeenth century (according to Helmholtz, in 
 1634) Peiresc "observed 1000 times that when he had looked 
 upon the window distinguished with wooden bars and squares 
 of paper, he carried the form thereof some time after in his 
 eyes ; but with this difference, that if he kept his eyes shut, he 
 seemed to behold the bars dark, and the paper squares white, 
 as he had at first seen them ; but if he looked with his eyes 
 upon a dark wall, then the paper squares seemed dark, and the 
 bars of the same whiteness with the wall." 3 
 
 Kircher, Mariotte, Boyle, Fabri, in the succeeding century 
 
 1 A short historical resume will be found in Helmholtz, Physiol. Optik, 
 2 Aufl., pp. 836-837. 
 s De Somniis. 
 3 Life of Peiresc, London, 1657, Book IV., p. 101. 
 
ON AFTER-IMAGES. 55 
 
 made slight additions to our knowledge regarding the appear- 
 ances. Newton's account of his memorable after-image (see 
 Part I., Sec. 4, p. 37) was sent to Locke about this time. In it 
 he seems to be the first after Aristotle to call attention to their 
 intimate relation to the more (so-called) mental images. 
 
 Buff on 1 in 1743 related his experience with after-images 
 projected on differently colored backgrounds. He found that 
 the image fused with the background, and formed a color which 
 was a combination of the true color of the after-image and 
 that of the background. On account of the variety of the 
 phenomena and because he was unable to account for all the 
 appearances, Buff on called the after-images ' couleurs acci- 
 dentelles.' These experiences of Buffon were later confirmed 
 by Gergonne, who made a number of new experiments similar 
 in character. 
 
 The first theory after Aristotle's (^continuation of the 
 stimulus) was that of Jurin (1758?). Apparently considering 
 only the negative phases, he regarded the after-image as due 
 to a process in the retina the reverse of what went on in normal 
 sensation. In expression this view is strikingly like some most 
 recent ones, although Jurin had not the same ideas as the more 
 modern writers. 
 
 Scherffer 3 (i76i), noting the negative images on a light back- 
 ground, supposed the phenomena to be caused by a temporary 
 loss of sensibility of the retina for one color. He thought that 
 the eye, having undergone a prolonged action from rays of a 
 certain color, lost momentarily its sensibility for a weaker 
 stimulus of rays of that color. Thus, on looking at white the 
 eye is stimulated by rays of light of different color — red, green, 
 blue, etc., and the retina recombines these into white. Now, 
 after having looked for a time at red, the retina becomes fa- 
 tigued for rays of that color, and when a white object is then 
 fixated the various rays composing white are seen with the 
 exception of the red, thus producing a bluish-green image. 
 
 1 Memoires de l'Acad. des Sciences de Paris, 1743, p. 213. 
 "Essay on Distinct and Indistinct Vision, p. 170 ff, of Smith's Optics, Cam- 
 bridge, 1738. 
 
 3 Dissertation sur les couleurs accidentelles ; Jour, de Physique, XXVI., 1785. 
 
56 5. /. FRANZ. 
 
 Plateau 1 conclusively shows that this theory is inadequate in that 
 it does not account for the negative after-images which are seen 
 perfectly in most complete darkness. A later theory proposed 
 by Scherffer was that the after-image is due to a prolongation 
 of a feebler stimulation produced by rays different from the 
 dominant color. For example, in looking at a red square we 
 see not only red light, but also some blue and green light, and 
 when the red is taken away the blue and green, which have 
 not been too intense to overpower the eye, continue to be seen, 
 thus producing the after-image. Were this true, it would be an 
 example of a lesser light having a greater effect than one of 
 great intensity. 
 
 De Godart 2 is responsible for two (so-called) theories. The 
 first of these is a very fanciful one. Arranging the colors 
 like the tones in a musical scale (black, blue, green, red, yellow, 
 white), he believed that the direct continuation of a sensation was 
 as much lower than white as the sensation was higher than 
 black. A sympathetic action was set up in the retina just as 
 sympathetic tones are noted on a musical instrument. " Such a 
 theory," Plateau remarks, "scarcely needs refutation." The 
 other theory of De Godart is as follows : 
 
 Voici une autre thdorie de ces phdnomenes : c'est de dire tout uniment qu'une 
 fibre dbranlee par un objet reste incapable de dormer la sensation d'un autre, 
 aussi longtemps qu'elle conserver l'impression du premier, et que les diffdrentes 
 couleurs dtant experimdes par des portion d'une meme fibre d'autant plus, 
 courtes que le ton de la couleur est plus vif, c'est la partie quin, a pas joud qui 
 excitde par le blanc a le faire, donne la couleur accidentelle. 3 
 
 This seems to be only a badly conceived, a poorly expressed 
 theory of insensibility like that proposed by Schaeff er ; and 
 coming, as it does, without elaboration toward the close of his 
 paper, it seems to indicate that the hypothesis is not well con- 
 sidered by the author. 
 
 Darwin* (R. W.), having considered all the known facts, 
 
 1 Essai d'une Theorie generate comprenant l'ensemble des apparences visuel- 
 les qui succedent a la contemplation des objets coleres. * * * la persistance 
 des impressions de la Retine, les couleurs accidentelles, etc. ; Ann. de Chimie et 
 de Physique, LVIII., 337-406, 1835. 
 
 ''■Jour- de Physique, VIII., 1776. 
 
 s S5id. 
 
 4 R. W. Darwin, New Experiments on OcularjjSpectra of Light and Colours; 
 Philos. Trans., LXXVI., 313-348, 17S6. Also found in E. Darwin's Zoonomia, 
 4 Am. Edit., 1818, Vol. I., p. 443-466. See also Vol. I., p. 10 ff. 
 
ON AFTER-IMAGES. 57 
 
 attempted to group them into four classes: (i) Images owing 
 to a less sensibility of a defined part of the retina. (2) Images 
 owing to a greater sensibility of a defined part of the retina. 
 (3) Images that resemble their object in color as well as 
 form. (4) Images that are of a color contrary to that 
 of their object. From the consideration of these different 
 facts, he was led to believe that a part of the retina became fa- 
 tigued by a color and became insensible to rays of that color, 
 and that this part of the retina then took up a mode of action 
 opposite to that which produced the sensation. The details of 
 this theory are interesting in view of what is at present known of 
 the retina's action. He says "the effect of the activity of the 
 retina may be to alter its thickness or thinness, so as better to 
 reflect or transmit the colours which stimulate it into action." 1 
 Possibly "the muscular actions of the retina constitute the 
 sensation of lights and colours ; and the voluntary repetitions of 
 them, when the object is withdrawn, constitute our memory of 
 them." 2 
 
 Contrast was used as an explanation by C. A. Prieur (or 
 Prieur de la Cote d'Or. [Fechner] ) . Numerous observations 
 were made in the succeeding years, but they were only slight 
 variations from the previous work. They are unimportant. 
 
 In 1835, Brewster 3 discussed the various color-changes, and 
 concluded that the primary color and the color of the after-image 
 existed in the retina simultaneously, in the same manner as a 
 fundamental tone and its harmonic. After the primary light 
 has ceased the color of the after-image continues. This is al- 
 most identical with the first theory of de Godard. In construct- 
 ing his theory Brewster seems to rely greatly upon the various 
 phenomena of simultaneous contrasts, considering these as repre- 
 sentatives of the after-image process. 
 
 Having considered the inadequacy of each of the foregoing 
 theories from Jurin to Brewster, Plateau 4 made a careful ex- 
 
 l P/tilos. Trans., LXXVL, p. 348. 
 *Ibid. 
 
 8 In Edinburgh Encyclopedia, Vol. I., article Accidental Colours. Also 
 Philos. Mag., IV., 354, 1S34. 
 
 *Ann. de Chimie et de Physique, LVIII., 337-406. 
 
58 5. /. FRANZ. 
 
 amination of all the known facts preparatory to constructing a 
 new theory. His theory, which follows, is partly a combination 
 of the ideas of his predecessors, particularly those of Jurin and 
 Darwin. He says, " we must conclude that the accidental 
 image results from a particular modification of the organ, which 
 spontaneously gives us a new sensation." "When the retina 
 has undergone the action of rays of a certain color, it resists 
 the action of that color and tends to regain its normal state with 
 a force more and more intense. Then if the excitation is sud- 
 denly removed, it returns to its normal state by an oscillatory 
 movement as much more intense as the action has been pro- 
 longed." The first primary image, which he considered a pro- 
 longation of the stimulus, was noted by him, and he concludes 
 " that when the retina, after having been excited for some time 
 by the presence of a colored object, is suddenly removed from 
 this excitation, the sensation produced by the object continues 
 to exist for a very short time, after which the retina spon- 
 taneously takes on a state opposite to the first, whence there 
 results the sensation of the accidental color. 
 
 Dove, Scoresby, Grove, Seguin, Brucke and Aubert noted 
 the after-images of moving objects, and the appearance and 
 reappearance of the images under increased and decreased eye 
 illumination. Brucke and Aubert noticed the after-images re- 
 sultant from instantaneous illumination by the electric spark. 
 
 Fechner, who lost his eyesight mainly because of his long- 
 continued study of after-images from very bright lights, pro- 
 posed the theory which has usually been associated with the 
 name of Helmholtz, viz., that the positive phase of the image 
 is a continuation of the stimulus, and the negative and comple- 
 mentary phases are due to retinal exhaustion. This hypothesis 
 is the one accepted by Wundt. The theory is inadequate 
 because it does not account for an after-image whose course 
 (fluctuation) is as follows: Pos., neg., neg., pos., pos., neg., 
 neg., neg., etc. 
 
 Hering regards the positive after-image as a continuation of 
 the stimulus, and the negative phases as reactions of the visual 
 elements to a state of equilibrium, the ' assimilation or dissimi- 
 lation in some of the visual substances.' The theory of light 
 
ON AFTER-IMA GES. 59 
 
 sensation proposed by Mrs. Franklin would account for the phe- 
 nomena in approximately the same manner. It will be noticed 
 that both these theories are in terms, though not wholly in sense, 
 the same as Darwin's. 
 
 Acceptingthe general theoretical position of the Young-Helm- 
 holtz hypothesis of color vision, Bidwell would account for the 
 phenomena as due to a ' reaction of the violet nerve fibers 
 only.' 1 Four reasons are given for this view: " (i) With 
 white light the recurrent colour is violet. (2) In the recurrent 
 spectrum of the complete spectrum no colour but violet can be 
 detected. (3) A pure red light, however intense, gives no re- 
 current image. * * * (4) The apparently blue colour of the 
 ghost of simple spectrum yellow is just as well produced by a 
 compound yellow consisting of green and red, the latter of 
 which is inert when tested separately." It should be remarked, 
 however, that some of these observations have been disputed, 
 and there is always danger of accepting a theory which is 
 proved by experiments devised after the acceptance of the 
 theory. 
 
 It is to be regretted that all these theories have made little 
 addition to our knowledge regarding the phenomena. Except 
 the work mentioned in Part I., not very much has been learned 
 regarding the conscious after-images since Fechner's time. 
 The mixing of colors by means of discs and the summation 
 effects of intermittent retinal excitation, have largely been con- 
 sidered during this time, but in a historical account of the ap- 
 pearances, these investigations have little place. 
 
 'On the Recurrent Image following Visual Impressions ; Proc. Roy. Soc, 
 LVL, 140, 1894. 
 
5. /. FRANZ. 
 
 Bibliography. 
 
 Aubert, H., Ueber die durch den elektrischen Funken erzeugten Nachbilder. 
 
 Molescbott. Untersuchungen, V., 279-314, 1859. 
 Aubert, H., Physiologie der Netzhaut. Breslau, 1SS4, pp. 347-386. 
 Aubert, H., Grundziige der pbysiologischen Optik. Leipzig, 1S76, p. 508 fF. 
 Bidwell, S., Ou the recurrent images following visual impressions. Proc. Roy. 
 
 Soc, LVL, 132-145, 1894. 
 Bosscha, H. P., Primare, secundare und tertiare Netzhautbilder nach momen- 
 
 tanen Lichteindriicken. Arcb. f. Opbthal., XL., L, 22-42, 1S94. 
 Buffon, — ., Dissertation sur les couleurs accidentelles. Memoires de 1' Acad 
 
 des Sciences de Paris, 1743, p. 147. (See also the first article in the same 
 
 volume, which seems not to be by Buffon. ) 
 Cbarpentier, A., Oscillations retiniennes. Compt. Rend, de 1' Acad, des 
 
 Sciences, CXIIL, 147-150, 1891. 
 Charpentier, A., Reaction oscillatoire de la retine sous 1' influence des excita- 
 tions lumineuses. Archives de Physiol., 5 Ser., IV., 541-553, 1892. 
 Darwin, R. W., New experiments on the ocular spectra of light and colours. 
 
 Philos. Trans., LXXVI., 313-348, 1786. Same article is reprinted verbatim 
 
 in E. Darwin's Zoonomia, Vol. I., pp. 444-466. 
 Delabarre, E- B., On the Seat of Optical After-images. Amer. Jour, of Psych., 
 
 II., 326-328, 1889. 
 Ebbinghaus, H., Ueber Nachbilder im binocularen Sehen und die binocularen 
 
 Farbenerscbeinungen uberhaupt. Arch. f. d. gesammte Physiol., XLVL, 
 
 498-509, 1890. 
 Exner, S., Ueber den Sitz der Nachbilder im Centralnervenorgan. Rep. d. 
 
 Phys., XX., 374, 1884. 
 Exner, S., Ueber die Funktionsweise der Netzhautperipherie und den Sitz der 
 
 Nachbilder. Arch. f. Ophtbal., XXXII., I., 233-252, 1886. 
 Fechner, G. T., Ueber die Subjectiven Complementarfarben. Pogg. Ann., 
 
 xliv., 221-245 ; 513-53°. 1838. 
 
 Fechner, G. T., Ueber die subjectiven Nachbilder und Nebenbilder. Pogg. 
 
 Ann., L., 193-221 ; 427-465, 1840. 
 Fechner, G. T., Elemente der Psychophysik. 2teAufl. Leipzig, 1889. Espec. 
 
 pp. 46S-525. 
 Fick, A. E., Ueber Ermiidung und Erholungder Netzhaut. Arch. f. Ophthal., 
 
 XXXVIII., 1, 118-126, 1892. 
 Fick, A. E., und Gerber, A., Ueber Erholung der Netzhaut. Arch. f. Ophthal., 
 
 XXXVL, 2, 245-301, 1890. 
 Franklin, C. L., The Bearing of the After-image. Psychol. Review, I., 396- 
 
 399, 1894. 
 Franz, S. I., The After-image Threshold. Psychol. Review, II., 130-136, 
 
 1895- 
 Helmholtz, H. von, Handbuch der Physiologischen Optik. 2te. Aufi. Ham- 
 burg und Leipzig, 1896. Espec. pp. 501-537. 
 Hering, E., Ueber Ermiidung und Erholung des Sehorgans. Arch. f. Ophthal., 
 
 XXXVIL, 3, 1-36, 1891. 
 
ON AFTER-IMAGES. 6 1 
 
 Hess, C, Untersuchungen liber die nach kurzdauernder Reizungdes Sehorgans 
 
 auftretenden Nachbilder. Arch. f. d. gesammte Physiol., XLIX., 190- 
 
 208, 1891. 
 Hess, C, Studien iiber Nachbilder. Arch. f. Ophthal., XL,., 1, 259-279, 1S94. 
 Plateau, J., Essai d'une theorie generale comprenant l'ensemble des apparences 
 
 visuelles qui succedent a la contemplation des objects colores * * * la 
 
 persistence des impressions de ratine, les couleurs accidentelles, etc. 
 
 Mem. de l'Acad. royale de Belgique, VIII. Also in Ann. de Chimie etde 
 
 Physique, Ser. 2, LVIII., 337-4 06 , lS 35- 
 Plateau, J., Sur les couleurs accidentelles ou subjectives. Bull, de l'Acad. 
 
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 Seguin, J. M., Recherches sur les couleurs accidentelles. Ann. de Chimie et 
 
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 Titchener, E- B., Ueber binoculare Wirkungen monocularen Reize. Philos. 
 
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 for the Theory of the Attention. Mind, N. S., VIII, 25-34, 1899. 
 
VITA. 
 
 The author was born May 27, 1874. He attended elemen- 
 tary and secondary schools in Jersey City, N. J., Columbia Col- 
 lege, 1890-4, Columbia University, 1894-7, and the University 
 of Leipzig, 1896. His graduate work consisted of courses and 
 of seminars in Psychology ( Professors Cattell, Wundt and 
 Meumann, Dr. Farrand and Mr. Strong ) in Education (Pro- 
 fessors Butler, Meleney, Hervey and Reigart), in Anthropology 
 (Professor Boas, Drs. Farrand and Ripley) and in Philosophy 
 (Professor Butler). He received the A.B. degree from Co- 
 lumbia College in 1894. In 1895-6, and 1896-7 he was Uni- 
 versity Fellow in Psychology in Columbia University, and was 
 assistant in Psychology in 1897-9. 
 
 In addition to the foregoing monograph the author has pub- 
 lished the following articles : 
 
 The After-image Threshold, Psychol. Rev., II., 130-136 
 
 On the Conditions of Fatigue in Reading, Psychol. Rev., III., 
 513-530(1896). (With H. Griffing.) 
 
 The Accuracy of Observation and of Recollection in School 
 Children, Psychol. Rev., III., 531-535 (1896). (With H. E. 
 Houston. ) 
 
 The International Psychological Congress, Science, N. S. 
 IV., 640-647(1896). 
 
 Education at the Psychological Congress, Educational Re- 
 view, Sept., 1896. 
 

 Date Due 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 f> 
 
 
 
 
Franz 
 On after-. 
 
 QP481 
 P85 
 
 1899