THE COLOR SENSITIVITY OF THE PERIPHERAL RETINA The Color Sensitivity of the Peripheral Retina by JOHN WALLACE BAIRD Of the Department of Psychology, fohns Hopkins University Research Assistant, Carnegie Institution of Washington, 1903—1904 WASHINGTON, D. C.: Published by the Carnegie Institution of Washington May, 1905 CONTENTS. Historical. 7 Experimental. Changes of Color Tone in Indirect Vision. 42 The Relative Extension of the different Color Zones upon the Retina. 59 Discussion of Results. 5 3 Interpretation of Results. 67 Summary. ?2 Bibliography. , The Color Sensitivity of the Peripheral Retina. By John Wallace Baird, This paper is the result of an experimental investigation conducted during the year 1903-4 in the Psychological Laboratory of Cornell Uni¬ versity. It aims to make a contribution to our knowledge of the phe¬ nomena of indirect vision. It will present data obtained by the writer in a series of experiments upon the peripheral retina, and will attempt to correlate these data with the phenomena previously established, and with the theoretical principles advanced from time to time by other investigators. An unequivocal and undisputed statement of the facts of the case is, of course, a necessary prerequisite to satisfactory progress in the domain of theory. So long as uncertainty prevails as to what sensa¬ tions arise under given conditions of color stimulation, there can be no definite and certainly no unanimous envisagement of the process of color vision. That the investigation of the present problem has yielded wholly different results in the hands of different investigators is only too patent from the literature; but in late years a distinct tendency has set in toward harmony in the findings of the various experimenters who have attacked the problem, and it appeared for a time that we were at last upon the verge of a unanimity of agreement. The hope that this goal was ultimately to be reached, at least along lines already laid down, was, however, dashed by the results recently published by Hellpach. It seems eminently desirable, therefore, to work over the ground covered by Hellpach with a view to verifying his data and to discovering, if pos¬ sible, the ground for their variation from those of his predecessors. It might appear that one is doomed to disappointment who attempts to solve a problem which has already been attacked without satisfactory result by so many illustrious investigators. But a perusal of the litera¬ ture of the topic reveals the fact that although these investigators have for the most part failed in their ultimate object, they still made a valu- 5 6 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. able contribution to the solution of the problem; and one is encouraged to grapple with it anew by a recognition of the value of this heritage from the past—a heritage which consists not only in an intimate knowl¬ edge of many of the facts of vision, but also in an extraordinary refine¬ ment of apparatus and of method. In entering into possession of this inheritance it seems distinctly worth while to go over the ground and discover its extent and value. 'We shall therefore preface the report of our own experiments with a detailed history of the problem; and, in the light of the accrued advantages of the present generation, we shall attempt to evaluate the results of previous investigations. Our paper represents the work of the past year. The leisure re¬ quired for a year’s concentration upon this topic was rendered possible by a grant from the Carnegie Institution of Washington. The writer takes this opportunity to express his gratitude to the members of the Institution for their assistance in his undertaking. The experiments upon which this paper is based will be described in detail in a later section.* The experimenter was in consultation throughout with Professor E. B. Titchener and Assistant Professor I. M. Bentley, to both of whom he is indebted for advice and criticism. He is also under obligation to Professor Eightner Witmer, of the Uni¬ versity of Pennsylvania; to Mr. Louis Wilson, of Clark University; to Mr. Willard Austen, of the library of Cornell University; and to Major Walter D. McCaw, of the Surgeon-General’s library at Washington, D. C., for aid in obtaining access to the literature of his subject. *See pp. 42ff. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. J HISTORICAL. Mariotte’s discovery of the blind-spot seems to have played an important part in instigating the examination of the peripheral retina. It is, however, to Thomas Young that we owe the first published account of an attempt to explore the retina with a view to determining the form and extent of its sensitive surface. Troxler attacked the problem inde¬ pendently a few years later and discovered that the periphery possesses a characteristic which is absent from the central regions. After the lapse of twenty years Purkinje confirmed and extended the work of Troxler in an exceedingly important series of experiments; but it was not until the days of Szokalsky and Aubert that the problem of peri¬ pheral as compared with central vision assumed definite shape, and that the characteristic features of each were investigated in detail. In a paper read before the Royal Society of England, in 1800, Thomas Young* communicated the following statement: The visual axis being fixed in any direction, I can ait the same time see a luminous object placed laterally at a considerable distance from it; but in various directions the angle is very different. Upwards it extends to 50 degrees, inwards to 60, downwards to 70, and outwards to 90 'degrees. These internal limits of the field of view nearly correspond with the external limit formed by the different parts of the face, when the eye is directed forwards and slightly downwards, which is its most natural position. * * * The whole extent of perfect vision is (little more than 10 degrees; or more strictly speaking, the imperfection begins within a degree or two of the visual axis, and at the distance of 5 or 6 degrees becomes nearly stationary, until, at a still greater distance, vision is wholly extinguished. The imperfection as partly owing to the unavoidable aberration of oblique rays, but principally to the insensibility of the retina. * * * The motion of the eye has a range of about 55 degrees in every direction; so that the field of perfect vision, in succession, is by this motion extended to no degrees. Troxler-j- found that images which fall upon eccentric parts of the retina fade out rapidly and soon disappear, while those upon the central area of the retina persist. This peculiarity of indirect vision he dis¬ covered accidentally while engaged upon an attempt to prove that the blind-spot is not wholly insensitive to light, as its discoverer had stated. Troxler had pasted a series of white paper figures upon a light-blue background, and was surprised to find that when each was fixated monocularly in turn, all of the others gradually paled out and finally * Thomas Young. On the Mechanism of the Eye, Philosophical Transactions, XCII, 1801, pp. 23ff; Miscellaneous Works of Thomas Young, London, 1855, pp. I2ff. fD. Troxler. Ueber das Versohwinden gegebener Gegenstande innerhalb unseres Gesichtkreises. Himlyund Schmidt’s Ophthalmologische Bibliothek, Jena, 1804, II, 2, S. I-S 3 - 8 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. disappeared, leaving nothing but the uniform background and the fixation-object. Similar experiments with papers of various colors showed that certain tones disappear more rapidly in indirect vision than do others. The degree of persistence of the retinal image is, in his opinion, conditioned by two factors—the color of its object and the angular distance of the object from the fixation-point. The latter factor he explains, in part at least, in terms of luminosity of image; the former he makes no attempt to explain, nor even to describe in satisfactory detail. It is to be noticed in this connection that the manner of disap¬ pearing, as described by Troxler, is peculiar and interesting. The colors gradually paled out and vanished, frequently reappearing—“ floating into view as out of still water ”—before ultimately disappearing.* In no case do they seem to have been followed by an after-image, f PurkinjeJ confirmed Troxler’s observations as to the rapid fading of color from peripheral images and endeavored to bring them into relation with certain subjective phenomena—the disappearance and reappearance of objects which occurs when one is drowsy, and the float¬ ing phantoms which may be observed when one fixates a point in a dark room—both of which classes of phenomena he describes at length in his earlier paper. In his later paper he gives a more detailed discussion of central and peripheral vision, for which he proposes the names, now commonly employed, of direct and indirect vision. There is, he says, but a single retinal point—the fovea—at which the brightness, the color, and the form of objects are seen with maximal clearness. This three-fold sensitivity of the retina diminishes with in- *T'his reappearance of objects which had previously disappeared from view is doubtless due to eye-movements. Failure to maintain a constant fixation would result in the exposure, to the action of the stimulus, of a non-fatigued area of the retina; and there would ensue a series of appearances and disappearances correlate with the successive shiftings of regard and the subsequent fatigue of the new area stimulated. The absence of after-images upon the peripheral retina has been pointed out by several investigators; it was also characteristic of our own experiments. (See pp. 56 and 58.) tit is also to be noted that Aubert refers to Troxler as having been the first to discover that objects appear colorless in indirect vision (Grundziige der Physiologischen Optik, S. 539). We can find no mention of this phenomenon in Troxler’s paper. It is true that he speaks of the colored papers merging into the uniform (blue) background; but that this is a final stage of the process, Troxler himself is careful to emphasize. What Troxler is interested in demonstrating is the relatively rapid chromatic adaptation of the peripheral retina. That under certain conditions it is wholly insensitive to color, clearly escaped his notice. fjohannes Evangelista Purkinje. Beobachtungen und Versuche zur Physi¬ ologic der Sinne, I, Beitrage zur Kenntnis des Sehens in subjectiver Hinsicht, Prag, 1823, Sections V and IX, particularly S. 77; II, Neue Beitrage, u. s. w.,’ Berlin, 1825, S. 3 - 25 - Various spellings of the name of this author have crept into the literature. We have adopted the form given by Purkinje himself in the two papers oited. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 9 crease of distance from the fovea, until at the extreme periphery there is found a zone “not of objective blackness and darkness, but of insen¬ sibility and unconsciousness.” Purkinje devised an apparatus for the purpose of determining the spatial extension of retinal sensitivity, and of investigating the phenomena of indirect vision in general. His apparatus consisted of a sheet of heavy cardboard cut in the form of a quadrant. The subject was seated in a dark room and the apparatus was brought into such a position that the pointed apex of the quadrant rested upon the bridge of his nose, while the side of the apparatus lay against his cheek. A lighted taper was then moved along the graduated arc of the quadrant and the point was noted at which the taper first appeared or disappeared from view. Purkinje’s measurements of the absolute extension of the normal field of vision gave somewhat larger values than had been obtained by Young (see p. 7), namely, 6o° up¬ wards, 6o° inwards, 8o° downwards, and ioo° outwards. In a series of experiments with color-stimuli,* he discovered that colored objects appear gray when their images fall upon peripheral parts of the retina and that they pass through regular transitions of color tone as their images approach the fovea. Neue Beitrage, S. 15-16.) Thus, purple appeared successively black at 90°, blue at 8o°, violet at 70°, and pur¬ plish at 50°; red appeared pale grayish-yellow at 90° to 70°, then orange, and finally its true color; “bright blue” appeared white at 90°, bluish at 80°; “saturated blue” appeared white at 90°, bluish at 8o°; violet appeared black at 90°, blue at 8o° to 70°, violet in different tones from 6o° onward; “ saturated green ” appeared black at 90° and 8o°, greenish at 70°; rose-red appeared white at 90° and 8o°, reddish at 70°; bright yellow and orange appeared in their own tones at 90°. Purkinje confesses that he is unable to find a satisfactory explana¬ tion of these phenomena. He points out that peripheral images must, in the nature of the case, receive less light than central images, and discusses the influence of the yellow spot and of different conditions of optic refraction, upon color vision, but he regards these factors as in¬ adequate to explain the transitions of color tone. He also discusses the significance for practical life of the indistinctness of indirect vision, and finally concludes (Neue Beitrage, S. 19) : “ It is difficult to determine whether the clearer vision of the central part of the retina is due to the presence, at the fovea, of a greater light intensity, or to a peculiar struc¬ ture and a closer relation to the Seelenkraft, at that point.” *Just how these experiments were arranged is not clear from Purkinje’s description. It is probable, however, that he employed the same apparatus in diffuse daylight, and that colored papers served as stimuli. 10 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. Although Purkinje admits that the problem baffles him, he makes an important contribution toward its final solution. He was the first to observe that sensitivity to light has a wider retinal extension than sen¬ sitivity to color, that the different colors have retinal zones of different extension, and that colored objects appear in different color tones at different parts of the retina. He also discovered the significance of retinal adaptation in color vision; for in the discussion of his color experiments he recommends that the eye be closed for a time after each exposure, “ um seine Empfindlichkeit zu sammeln ” (1. c., S. 6). Szokalsky* also found that sensitivity to color is not uniformly dis¬ tributed over the whole retina, and that colored objects appear in differ¬ ent tones when their images move across the retina. His apparatus consisted of a black background, containing at its center a white fixation- point. Pigment colors were moved in from the edges of the back¬ ground towards the center. His experiments showed that purplish-red appears first black, then blue, then violet, and finally assumes a tone of purple-red, only when it is near the fixation-point; violet passes through black and blue before becoming violet; bright blue and rose-red appear white at the periphery. “The other colors,” he adds, “gave similar results.” In consequence of these experiments, Szokalsky conceives the retina to be made up of three concentric zones; the outermost zone is assumed to be sensitive only to black and white, the intermediate zone only to blue, yellow, black, and white, while the innermost zone is capable of sensing these four, and red besides. This happy conjecture was unfortunately not elaborated in detail by its author; its chief inter¬ est lies in the fact that it foreshadows the brilliant discovery made half a century later by Bull and Hess. Hueckf attacked the problem by means of a new method. He endeavored to determine the minimal extent of retinal image which is capable of producing a sensation of color, at the different parts of the retina, i. e., he set out to determine the minimum visibile of color for various degrees of eccentricity. He, too, found that color sensitivity decreases with increase of distance from the fovea; he confirmed the existence of Purkinje’s black-white zone, but does not mention having observed any transitions of color tone. His essential contribution to the *V. Szokalsky. Essai sur les sensations des couleurs, etc., Annales d’Oculist., II, 1839, .and III, 1840. These papers were reprinted in book form, under the’ same tide, Paris, 1841; they were subsequently elaborated and published in Ger¬ man (Ueber die Empfindungen der Farben in physiologischer und pathologischer Hinsicht, Giessen, 1842). fA. Hueck. Von den Grenzen des Sehenvermogens, 'Muller’s Archiv. f. Anat., Phsyiol. und wiss. Med., 1840, S. 83-98. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. II literature was his discovery that the color zones are not fixed, but flexible areas, and that their extension increases with increased area of stimulus-object. By far the most fruitful investigator of this period was Aubert.* During the progress of an investigation of the “ space sense,” carried on by Aubert and Foerster in 1857, it had been observed that colored objects may appear colorless in indirect vision. Aubert was able to find in the literature but two references to this phenomenon—the papers of Purkinje and Hueck; and since neither discussed the matter in satisfac¬ tory detail, Aubert determined to undertake a thorough investigation of the phenomena of indirect vision. He employed an apparatus which had but recently been devised, and which has since been named the perimeter, and set himself the task of solving the following problems: (1) To what extent does the character of the background influence the degree of eccentricity at which a color stimulus can be recognized ? (2) What is the relation between decrease of color sensitivity and decrease of area of stimulus ? (3) Does color sensitivity decrease at a uniform rate along the various retinal meridians ? (4) Through what transitions of tone does each color pass during the movement of its image from the fovea to the periphery of the retina ? (5) What is the relative extension of the retinal areas within which stimuli appear in their true colors ? (6) In what degree are the phenomena of indirect vision analogous with those of direct vision ? Aubert’s method of experimentation was as follows: From papers of different colors he cut duplicate sets of squares, varying in size from 1 sq. mm. to 1,024 sq. mm. One set of these squares was then mounted upon white cards, the other upon black. The perimeter was set up in a room with a northern exposure and the experiments were performed in a diffuse daylight illumination. Each sheet of cardboard was inserted in the perimeter, in turn, and was gradually moved out along a retinal ^Hermann Auibert. Ueber die Grenzen der Farbenempfmdung auf den seitslichen Theilen der Netzhaut, Graefe’s Arohiv. fur Ophthalmologie, III, 2, 1857, S. 38-68; Ueber das Verhalten der Naohbilder auf den peripherisehen Theilen der Netzhaut., Molesohott’s Untersuchungen, IV, 1858, S. 215-240; Ueber die duroh den electrisohen Funken erzeugten Naohbilder, Ibid., V, 1859, S. 279ft.; Untersuchungen fiber die Sinnesthatigkeit der Netzhaut., Poggendorff’s Annalen, CXV, 1862, S. 87-116, CXVI, 1862, S. 249-278; Ueber subjective Lichterschein- ungen, Ibid., CXVII, 1862, S. 638ft.; Fhysiologie der Netzhaut, Breslau, 1865, S. 89-105 and 116-124; Grundzuge der physiologisdhen Optik, published in Graefe u. Saemisch’s Handbuoh der gesammten Augenheilkunde, II, 2, 1876, S. 393-670, republished in separate form under the same title, Leipzig, 1876. 12 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. meridian until its color had wholly disappeared. Eight meridians were explored by means of each stimulus. His results may be summarized as follows: (1) The brightness of the background has a most pronounced influence upon the extension both of the color sensitivity and of the brightness sensitivity. Red upon the white ground becomes colorless at 16°; upon the black ground at 30°. Blue upon white becomes colorless at 13 0 ; blue upon black at 37 0 . Beyond the limits of the color zones, all colors appear white when the ground is black, black when the ground is white. (2) The extension of the color zone increases with increase of area of stimulus. The former increase does not keep pace with the latter, but progresses much more slowly, as appears from the following table. The area of the stimulus-object is expressed in square milli¬ meters ; the width of the color zone is given in degrees. The squares were exposed at a distance of 20 cm. from the eye. Color stimulus. Area of stimulus-object. 1 sq. mm. 4 sq. mm. 16 sq. mm. 64 sq. mm. 256 sq. mm. Red. Degrees. Degrees. Degrees. Degrees. Degrees. 16 18 26 38 43 Y el low. 21 3 1 44 Green. 20 36 45 50 Blue. 13 23 37 49 57 (3) The color sensitivity decreases at very different rates upon different retinal meridians. It diminishes much more slowly, or extends much farther out, on the nasal side of the retina. (4) The transitions of color tone are as follows: Red passes through reddish-yellow and yellowish-gray to gray; green becomes yel¬ lowish, while yellow and blue undergo no change of tone, but decrease in saturation and finally appear gray. (5) The relative extension of the different color zones can not be determined with any degree of accuracy. Since the width of the color zone is a function of the luminosity of the stimulus, the color-stimuli employed in the determination of comparative retinal limits must all be equated in brightness. In the opinion of Aubert, the comparison of the relative brightness of stimuli of different colors is attended by such difficulties as to render its accurate accomplishment impossible. In his own experiments, with non-equated stimuli upon a black background, blue had the widest extension, red and yellow less, and green least of COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 13 all; upon a white ground, green had the widest extension, then followed yellow, blue, and red in order. In the face of this conflicting evidence Aubert averaged the values obtained with the different backgrounds, and arrived at the following results: Blue 34°, yellow 33 , green 31 °, and red 27 0 . From these data he concluded that there is but little difference between the extensions of the various color zones.* (6) There is a close analogy between the functioning of the central and peripheral parts of the retina. Colored objects appear colorless in both direct and indirect vision when the visual angle is sufficiently small, when the illumination is sufficiently weak, when the stimulation is sufficiently long-continued, and when certain conditions of contrast between stimulus and background are not fulfilled.t Aubert argues that the only difference between the color sensitivity of different parts of the retina is a difference of degree, the “color sense” of the periphery being less acute.$ And this lesser sensitivity is to be explained, in part from a different distribution of retinal elements, in part, from their more rapid fatigue.§ Helmholtz** pointed out that the changes of color tone which appear in indirect vision are similar to those which occur in direct vision when the luminosity of the stimulus is progressively increased. Thus red and green become distinctly yellow, blue passes over into gray with¬ out change of tone, and purple becomes bluish. Helmholtz found that a mixture of red and green-blue which appears colorless in direct vision becomes green-blue at but a slight distance from the visual axis. From this experiment it would seem that the peripheral retina is more sensi¬ tive to blue and to green than to red. Accordingly Helmholtz assumed that the red-sensing substance is lacking at the periphery, and referred the color phenomena of indirect vision to the red-blindness of the eccen¬ tric region. The view of Helmholtz was confirmed by Schelske,ff who was the first to employ spectral colors in the investigation of the problem. He determined the inner limits of the red-blind zone in his own eye to be 68° on the nasal side, 37 0 on the lower side, 53 0 in the temporal, and 38° in the upper direction. *Graefe’s Archiv., 1 . c., S. 55 - fGraefe’s Archiv., Ill, 2, S. 61. j Grade's Archiv., Ill, 2, S. 61; Grundziige, S. 545 - § Aubert devoted a paper to the phenomena of fatigue and after-images; this will ‘be referred to in connection with our own results. **Hermann von Helmholtz. Handbuch der physiologisohen Optik., Leipzig, i860, :S. 300-301, and S. 845. The publication of this volume covered a period extending from 1856 to 1866. The second Lieferung, which contains his first dis¬ cussion of indirect vision, appeared in i860. ffR. Schelske. Ueber Fanbenblindheit des normalen Auges, Graefe’s Archiv., IX, 3, 1863, S. 39-62. 14 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. A comparison of the sensations aroused by the same stimulus OOtif rn 1 1 1,1* 1 . i « Color tone seen in direct vision. Sensation aroused upon the “ red-blind ” zone. Spectral red (near end otf spectrum) .. Spectral yellow. Dark gray. Pale green.* Light bluish gray. Light greenish blue.* Dark blue. Spectral green (F-line) .. Spectral blue. Spectral violet. ------ than at the fovea Thisresulthas nofb^n 8 e^fi™^ pea ^ d ”*?!* ^enish at the Periphery to be no doubt that the opposite relation obtains. d by hef observers : lndeed there seems Experiments with mixed colors (pigments) gave the following: In direct vision. In indirect vision. Red -j- blue. Appeared light bluish. Appeared light grayish yellow. Appeared light bluish. Appeared bluish green. Appeared marine blue. Red -f- green. Green -(- 'blue. Blue -)- yellow. Violet -f- yellow. Schelske’s investigation convinced him of the validity of the Helm- holtzian theory of color vision, and of the existence of a zone of red- blindness at the periphery of the retina. In common with Helmholtz he assumed that every color stimulus in direct vision excites all three sorts of color-sensing substance; in indirect vision only green-sensing and violet-sensing substances are excited. This difference in the co¬ operation of the visual substances in the two cases would, in the opinion of Schelske, lead one to expect just such differences in color sensitivity as are observed. The Helmholtzian principle was accepted and extended by Woinow.* Employing pigment colors, this investigator observed the following transitions of tone when the stimulus passed out into indirect vision: Red becomes yellowish at but a slight distance from the fovea, gradually passes over into yellowish-brown, which becomes darker and darker, and finally black; if the background is much darker than the stimulus, the latter appears light gray instead of black at the periphery. Orange becomes yellowish, gradually grows murkier, and finally ap¬ pears gray, light or dark according as the background is dark or light. Yellow simply decreased in saturation until it appeared light gray at the periphery. Green passes out through yellow or yellowish to gray; this stimulus never appears black at the periphery. Blue passes over *M. Woinow. Zur Farbenempfindung. Graefe’s Archiv., XVI, i, 1870, S. 212-224. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 15 into gray with progressive decrease of saturation. Violet went out through blue to black or gray. Purple-red appears successively violet- ish, blue, and black or gray. Woinow is convinced that the areas of the color zones are fixed and invariable; he finds that their limits change neither with change of area of stimulus nor with change of brightness of background. He does not state what is the area of each zone, nor does he give data from which their relations may be calculated. Woinow concludes that the distribution of all three visual substances becomes more and more scant towards the periphery, until only green-sensing substance is present in the outermost zone. The extreme periphery is therefore not only red- blind but violet-blind as well. Bow* discusses the problem without any reference to the previous literature, and evidently under the impression that he is the pioneer investigator in the field. He reports that a blue-green glass held before the eye and regarded indirectly appears blue, while a yellow-green glass appears yellow under similar conditions. Scarlet becomes orange at 30° from the visual axis and yellow at 40°. Green passes over into yellow, and purple into blue, in indirect vision. On the basis of the different refrangibility of light of different colors, he advances the following remarkable explanation of his results: The retina contains a three-ply layer of sensitive elements. Each stratum is composed of fibers which are sensitive to but a single color, and each is separated from its fellows by intervening strata of non-sensitive tissue. The presence of the intermediate strata is a sine qua non of normal color vision. They are, however, absent from the peripheral retina, and since the sensitive layers are here in contact, they are incapable of function¬ ing normally, f Maxwell! found that when a white surface is observed through a bluish-green filter, the fixation-point appears to be a pinkish spot upon a bluish-green ground. He also reported that a certain mixture of red, *R. H. Bow. On the Changes of Apparent Color by Obliquity of Vision. Proceedings of the Royall Society of Edinburgh, VII, 1871, pp. 155-160. tSince we shall not have occasion to refer to this theory again, u/q yellow 574.5 ytf/q and blue 471 /qu. None of the spectral reds proved to be a stable tone; but from an appropriate *Carl Hess. Ueber den Farbensinn bei indirectem Seihen, Graefe’s Archiv., XXXV, 4, 1889, S. 1-62. f We shall refer to these as “ stable ” colors, a term which here signifies those color-stimuli which experience no change of tone in passing across the retina. 28 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. mixture of red and violet the desired tone was obtained. This latter tone was found to be complementary to the stable green; and the stable yellow turned out to be complementary to the stable blue. In his more accurate investigation of the behavior of the stable colors in indirect vision, Hess employed the Hering color-mixer.* In this apparatus the color-stimulus is observed through an aperture in a gray screen. By moving the fixation-point in a lateral direction, the image of the stimulus is made to fall upon the peripheral retina. The brightness of the background may be brought to equality with that of the stimulus by an appropriate inclination of the screen. By this means it is possible to secure accurate judgments as to the exact point of ap¬ pearance or disappearance of the color, since in indirect vision the stimulus may be made to fuse into the gray background. With stable stimuli whose white-values and color-values had pre¬ viously been equalized, Hess proceeded to determine the limits of retinal sensitivity. The following shows the extension of the four color-zones :f Retinal meridians. Red. Green. Yellew. Blue. Out. Out-up. Up. Up-in. In. In-down. Down. Down-out. Degrees. 21.2 16.7 17 28.4 43-6 33 14.2 16 Degrees. 21.6 16.4 16.9 29.2 42.6 31.6 14.2 15-6 Degrees. 35-6 334 3 i -9 Degrees. 35 32.8 32.2 A survey of these results reveals two interesting facts—a coinci¬ dence on the one hand of the retinal limits of sensitivity to red and to green, and on the other of those to yellow and to blue; a considerably wider extension of the yellow-blue zone than of the red-green zone. Hess also found that an increased saturation of stimulus gives an in¬ creased extension of zonal limits; he further demonstrated that the zonal limits are widened by an increase in the area of the color stimulus. *Tbis apparatus is figured in Titchener’s Experimental Psychology, Vol. I, Part 2, 1901, p. 20. fHess, 1 . c. S. 45ff- Of. also his tables, S. 49f. Hess expresses his results in cm.; for the sake of uniformity we have reduced his readings to degrees, (x cm. = about 2°, S. 46.) COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 29 Hess’s paper does not contain a theoretical discussion of his data, but Hering follows with a paper in which he shows that the theories advocated by Helmholtz and by Fick can not be reconciled with the results of Hess’s investigation.* Basevif employed a Masson disc, upon which were exposed his color stimuli in different degrees of brightness and saturation. He found blue to have the widest retinal zone, and green the narrowest, the rela¬ tive extensions being, green 1, red 1.14, yellow 1.19, and blue 1.32. Basevi’s adaptation experiments yielded the following results: The extension of the color zones is increased by dark-adaptation, but all are not increased in equal degree. The greatest increase occurs in the case of the yellow, then follow green, red, and blue in order. It follows from this that the peripheral sensitivity to all colors is increased by dark-adaptation. Basevi also performed a series of experiments to determine the influence of chromatic adaptation. After the retina had been exposed to each color for a time, each of his stimuli was presented in minimal brightness. The brightness was gradually increased until the color was recognized, the liminal brightness required for recognition being recorded in each case. The following results were obtained: With red-adaptation, green was the first color to be recognized, then came blue, then yellow, and finally red; with blue-adaptation the order was yellow, red, green, and blue; with green-adaptation, red, blue, yellow, and green; with yellow-adaptation red was the first to be recog¬ nized, then blue, green, and yellow. Unfortunately, Basevi made no determination of the color tones, or of the relative absorptions of the colored glasses employed in this experiment. HeggJ employed the Hering color-mixer to determine the four stable colors and to equate their white-values and their color-values. Then, in order that his subsequent perimeter experiments might be car¬ ried on more conveniently, he had a painter prepare exact duplicates of the four stimuli thus determined and equated. The parts of the peri¬ meter which were to serve as background in the investigation were *E. Hering. Ueber die Hypothesen zur Erklarung der peripheren Farben- blindheit, Graefe’s Archiv., XXXV, 4, 1889, S. 63-83. tV. Basevi. Sulla sensibilita della periferia della retina per la luce e per i colori, Annali di Ottalmologia, XVIII, 1889, pp. 41-52; Influenza dell’ adattamento sulfa isensibilita retinica per la luce e per i colori, Ibid., XVIII, 1890, pp. 475-481. tEmil Hegg. Zur Earbenperimetrie, Graefe’s Archiv., XXXVIII, 3, 1892, S. 145-168; La perimetrie des couleurs, Annales d’Oculistique, CIX, 3, 1893, PP- 321-347; Sur la periimetrie au moyen de pigments colores, Annales d’Oculistique, CXI, 1894, pp. 122-127. 30 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. painted in a gray color of the same brightness as the stimuli. His perimetric exploration of the retina gave the following results :* Meridian. Red. Green. Yellow. Blue. Out. Out-up. Up. Up-in. In. In-down. Down. Down-out. Degrees. 20 24 24 45 50 3 2 20 25 Degrees. 20 24 24 45 50 32 20 25 Degrees. 40 48 53 80 80 60 29 36 Degrees. 4 o 49 54 80 82 61 30 37 It will be seen from this table that Hegg found, in agreement with Bull and with Hess, that the limits of the red and the green zones coincide, as also do those of yellow and blue; and that the zone of the latter pair is wider than that of the red and green. Similar experiments performed with a smaller area of stimulus gave a lesser extension of the zones of all four colors (/. c., p. 343). Kirschmannf found in experiments with non-equated spectral stimuli that blue has the widest zone, and violet the narrowest; the order of the intermediate zones as found by this investigator is (in descend¬ ing order), yellow, orange, red, and green. Increase of luminosity of stimulus seems to have no influence upon the width of the zones, and increase of area of stimulus is followed by increased extension of color zone in some cases only. In a second series of experiments, Kirsch- mann employed colored papers representing a great variety of color tones, saturations, and brightnesses. 'The zones determined by means of these stimuli extend farthest out on the nasal and upper portions of the retina; their limits do not run parallel with each other, but intersect and re-intersect in a most, irregular fashion. It is quite impossible to tell from his determinations what is the relative extension of the differ¬ ent zones, since their relative widths differ upon different meridians. It would appear, however, that blue has the greatest extension and violet the least. The red and the green stimuli employed by Kirschmann did . *Ann. d’Ocul., CIX, p. 343. For the sake of uniformity, and in order to facilitate a comparison with the results of other investigators, we have referred Hegg’s readings to retinal meridians; in his paper they are referred to the corre¬ sponding meridians of the visual field. It will toe noticed that Hegg found the yellow-zone to toe slightly narrower than the tolue-zone. This state of affairs Hegg believes to be due to the greater refrangibility of blue light. f A. Kirschmann. Die Farbenempfindung in indirectem Sehen, Philosophische Studien, VII, 1893, S. 592-614. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 3 1 not give co-extensive zones, nor did his blue and yellow stimuli. Kirsch- mann sees in these results an argument against the theories of Helm¬ holtz and of Hering, but he fails to find an explanation for the phe¬ nomena which he reports.* Abneyf performed a great many experiments in which spectral colors were exposed in different brightnesses and at different visual angles. Employing as stimuli, red (670.5 jajx), yellow (589.2 pi/x), green (508.5 / CO 73 75 71 70 58 57 56 53 In-Up. ' U .ti L 58 56 65 62 40 44 40 39 Up. A +-» a 9 £ 0 59 56 52 49 43 40 45 40 Up-Out. a? 55 55 47 49 38 38 37 39 Out. O 80 41 <5 49 50 48 50 45 48 46 45 Out-Down... H - 44 44 39 42 40 37 40 36 * It should perhaps be mentioned that this series of determinations was made toward the close of the investigation. Before these experiments were entered upon, all five observers had had some months’ experience with color-stimuli applied to the horizontal meridian of the retina (the out and in meridians of the above table). It is probable that this preliminary train¬ ing brought about a refinement of color discrimination upon the "out” and “in” meridian, and that the zonal determinations of our table extend beyond the normal limits in these two directions. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 63 DISCUSSION OF RESULTS. Our investigation of the changes of color-tone in indirect vision has yielded results which are in harmony with the all but unanimous testimony of previous investigators. The contention of Woinow and Klug that yellow appears green in indirect vision has not been con¬ firmed, nor has Hellpach’s discovery upon the periphery of the absence of the sensation of yellow and of the presence of a “ gegenfarbige Zone.” Our determinations of the relative extension of the retinal zones are in agreement with those of all other investigators who have worked under standardized conditions of experimentation. Moreover, it has been established by the present investigation that the coincidence of the zones of each pair of “ stable ” colors which was previously found to be valid for the light-adapted retina, holds also for the dark-adapted retina. The phenomena which we have found to attend upon the con¬ tinuous stimulation of the peripheral retina, and those which have been shown to persist as an after-effect, upon the removal of the stimulus, are, we believe, for the most part, here pointed out for the first time, although they have, in part, been vaguely foreshadowed in the literature ever since the days of Troxler, Purkinje, and Aubert. So far as we have been able to discover, however, they have never been investigated in satisfactory detail. And the present paper can not hope to do more than to call attention to their existence, and emphasize the necessity of a more detailed investigation of their characteristics. As to our observation of the rapid fading of color from peripheral images, it is interesting to note that the same phenomenon was described by Troxler, just one hundred years ago. (Seepp. yi.) The existence of successive phases in this process of chromatic adaptation is sug¬ gested in the investigation of Landolt and Charpentier* and is discussed in the more recent work of McDougall.f That the phenomena of direct and of indirect vision are, at least to some extent, analogous in this regard, is clear from the familiar observation that the color may wholly disappear from the central image with continued fixation. The difficulty which we encountered in the production of after¬ images upon the peripheral retina was also experienced by Foerster,^ . . hcinclolt et Charpentier.. .Des sensations de lumiere et de couleur, dans la vision direote et dans la vision indirecte, Coraptes Rendus, LXXXVI 1878 PP« 495 ff- tw McDougalL The Sensations Excited by a Single Momentary Stimula¬ tion of the Eye, British Journal of Psychology, 1, 1904, p. 85. fFoerster. Ueber Hemeralopie, u. s. w. Habilitationsschrift, Hamburg 1 ^ 57 , S. 32. s ’ 64 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. by Aubert, 1 and by Franz. 2 Indeed, Franz found it impossible under the conditions of his experiments to arouse an after-image at a greater degree of eccentricity than 30°, and Foerster obtained but a very indis¬ tinct after-image when the peripheral retina (45 0 from the visual axis) was stimulated by a ray of direct sunlight. It seems, too, that the persistent after-effects which made them¬ selves felt in our experiments upon the peripheral retina are analogous in character with the after-images of direct vision. Attempts have been made by Exner, 3 Hess, 4 Basevi, 5 and others to determine how the retinal function is modified by local chromatic adaptation. 6 These investi¬ gations, which were for the most part confined to direct vision, estab¬ lish the law that fatigue by any color stimulus increases the retinal sensitivity to the complementary color, and that on subsequent stimula¬ tion a sensation of the complementary color tends to arise. Our state¬ ment regarding the latent or subliminal character of these after-effects finds confirmation in a demonstration which has recently been described by McDougall. 7 This investigator finds that when the central after¬ image has run its course and finally disappeared from view, it still exists in latent form, from which it may be revived by a restimulation of the retina. An analogous phenomenon was long since pointed out by Hering. 8 It must, however, be mentioned that if we accept the testimony of Aubert and of Franz as to the color of the peripheral after-image, our analogy breaks down. For both these investigators report that the peri¬ pheral after-image is of the same color as the central—that, e. g., the after-image of a purple stimulus is green, no matter at what part of the retina the after-image be aroused. That this statement is erroneous, we have succeeded in demonstrating by means of an experiment in which an intensive purple stimulus and a long exposure were employed. We found that the application of this stimulus to different regions of the retina gave the following after-images: Green at o°, blue-green at 25 °, 1 H. Aubert. Ueber das Verhalten der Nachbilder auf den peripherischen Theilen der Nethaut. Molesdhott’s Uniterauchungen, IV, 1858, S. 22off. 2 S. I. Franz. After Images, Psychol. Review, Mon., Supp. Ill, 1899, p. 29. 3 S. Exner. Ueber die Functionsweise der Netzhautperipherie und den Sitz der Nachbilder, Graefe’s Arohiv., XXXII, 1, 1886, S. 233ft. 4 C. Hess. Ueber die Tonanderung der Spectralfarben durch Ermiidung der Netzhaut mit homogenem Lichte., Graefe’-s ArChiv., XXXVI, 1, 1890, S. I2ff. 5 V.Basevi. Influenza dell’adattamento sulla sensibilita retinica per la luce e per i eolori., Annali d’Ottolmologia, XVIII, 1890, p. 480. 6 The work of Schon, Charpentier, and Peschel need not be discussed in this connection, since their results are expressed in terms of the influence of chromatic adaptation upon (the “ light sense.” Treatel and Landolt deal solely with the effects of general achromatic adaptation. 7 W. McDougall. Some New Observations in Support of Thomas Young’s Theory, etc., Mind, N. S., X, 1901, p. 55 . _ 8 E. Hering. Zur Lehre vom Lichtsinne, Wien, 1878, S. 129L COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 65 bluish at 40°, and dark gray (?) at 50°. The results of this experiment are fully in accord with the findings of Adamiik and Woinow,* who also report that the color of the after-images aroused at any retinal region is complementary to the color of the primary sensation produced at that region. Our results are further confirmed by the data contained in a more recent paper by Walther.f All of this goes to show that the phenomena which we have de¬ scribed in the foregoing pages as being characteristic of indirect vision are similar in kind throughout to the phenomena of direct vision. The whole retinal surface, with the exception of the macula and the blind- spot, is endowed with a similar function, to the extent, at least, that no region possesses a capacity which is wholly lacking in any other region. The color sensitivity of the periphery is unquestionably less acute than that of more central areas, and in consequence of this diminished sensi¬ tivity a constant stimulus may arouse different sensations at different regions. It can not, however, be said that any part of the normal retina, save the macula and the blind-spot, is wholly or even partially color¬ blind. For the whole manifold of sensation qualities which any region is capable of furnishing may, under appropriate conditions of stimula¬ tion, be furnished by every other region. Hellpach’s discovery that a yellow stimulus gives no sensation of yellow in indirect vision has not been confirmed by our results. Our experience with the stimulus which he has called yellow in his paper convinces us that Hellpach and the present writer differ in their employ¬ ment of color names. But it is difficult to understand how this differ¬ ence of terminology can account for his “ discovery.” For one can not comprehend how any part of the spectrum can, under normal condi¬ tions, appear more orange upon peripheral than upon paracentral re¬ gions of the retina. And even the most distinctly orange stimulus must appear yellow before it can pass over into gray in its progressive reces¬ sion from the visual axis.lj: * Adamiik and Woinow. Beitrage zur Theorie der negativen Nachbilder, Graefe’s Archiv., XVII, 1, 1871, S. 141 f. fAnthon Walther. Beobachtungen liber den Verlauf centraler und extra- macularer Nachbilder, Pfliiger’s Archiv., LXXVII, 1 and 2, 1899, S. 53-69. fThe recent testimony of Peters upon this point leaves no room for doubt as to what are the facts of the case. Peters, who, it may be mentioned, worked in the same laboratory as Hellpach, employed an exact duplicate of Hellpach’s yellow (Stimulus, and obtained results which (support our findings as against Hell¬ pach’s. Peters found (that the stimulus in question (which Hellpach had described as being composed of monochromatic light from the sodium line) was frequently identified as yellow-orange in direct vision. He reports further that it invariably appeared yellow upon the peripheral retina before passing over into gray. That is, it invariably appeared yellow when its luminosity was so weak as to produce any marked change of tone in indirect vision. (W. Peters. Die Farbenempfindung der Netzhautperipherie, u. s. w., Archiv fur die Gesamte Psychologic, III, 1904, S. 374 f-) 66 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. Hellpach’s “gegenfarbige Zone” is doubtless a product of the latent after-effects of stimulation (retinal fatigue), which we have described (pp. 57ff.). It will be remembered that in determining the position of the zonal limits, he moved his stimulus in from the periphery, each exposure being made at a point ten degrees nearer the visual axis than its predecessor. This procedure was continued until color was perceived; but since the steps had been too long to guarantee an accu¬ rate determination of the limits of the color-zone, he went back 20 degrees from the point at which color was first reported and readvanced over the same ground with shorter steps. Of course, this method pro¬ vided the most favorable conditions possible for the operation of the residual after-effects of stimulation. For his final determinations were invariably made upon a region which he had previously fatigued. It was inevitable that this method should yield the abnormal results which he reports. And if he had followed the same procedure in his explora¬ tion of the other regions of the retina, he would doubtless have found that the whole retinal surface, save the fovea and the blind-spot alone, is one vast “gegenfarbige Zone.” On the other hand, our own results justify the prediction that if he had given the retina an adequate rest before each stimulation, he would have found no trace of a “gegen¬ farbige Zone ” at any part of the retina. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 67 INTERPRETATION OF RESULTS. How are our results to be explained? Can they be brought into line with any or all of the theories of color-vision now in the field ? Can they contribute to an evaluation of the relative merits of the rival theories ? Any attempt to express our results in terms of physiological process reveals the possibility of four general principles of interpretation: (1) It may be held that the phenomena which have been found to be characteristic of indirect vision are wholly due to the peculiar objective conditions which must, from the nature of the case, attend the stimula¬ tion of the peripheral retina. (2) On the other hand, it may be held that these phenomena are the result of a peculiarity of structure or of chemi¬ cal composition of the peripheral retina. (3) It may be conceived that the phenomena in question are to be referred to a simultaneous co¬ operation of the factors mentioned under 1 and 2. And (4) all of these views may be discarded as inadequate; one may conclude, instead, that the phenomena in question can not be explained in terms of a modification of retinal process at all, and conceive that they are to be referred to a cerebral or other central process whose nature it is im¬ possible to envisage or describe. (1) The first view either conceives the retinal surface to be uniform in structure and composition throughout its whole area, or regards those differences which may be held to occur as being of negligible sig¬ nificance. It refers the transitions of color-tone to the physical form of the visual organ, and the spatial arrangement of its parts, emphasiz¬ ing the fact that the retinal surface occupies such a position relative to that of the pupil and of the lens, that its outlying parts possess less favorable conditions for vision than its central region. The advocate of this view need not assume that the color-sensing substance with which the periphery is supplied is in any wise different from that which is found at the center. For the defective refraction of oblique rays of light, and the non-perpendicular position of the plane of the pupil to that of the path of the incident light naturally and necessarily give rise to a less bright and less well-defined peripheral image. In short, a given pencil of rays constitutes a lesser physiological stimulus in indi¬ rect than in direct vision, for the simple reason that in the former a part of the light fails to gain access to the pupil, while the part which does enter is imperfectly focused upon the retina. This view as here formulated has perhaps never been advocated by any single writer; but it is a composite statement of the positions held by Albini on the one hand, and on the other by a group of investigators 68 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. composed of Raehlmann, Landolt, Lamansky, and possibly Aubert. There can be no doubt that the peripheral image is relatively faintly illuminated, and it may be granted that it is also characterized by ill definition. But one finds it difficult to see how these differences in objec¬ tive conditions of stimulation can account for all of the phenomena of indirect vision. For if the phenomena in question are a product of the lesser luminosity of the peripheral image, one would expect to find a thoroughgoing parallelism between the changes of tone which occur when an image moves across the retina and those which occur when an image occupying a constant retinal position is varied in brightness. That the analogy between these two series of changes is anything but perfect has frequently been demonstrated. Nor can the problematic influence of defective refraction save the theory. For Albini himself has shown that the progressive correction of refraction in indirect vision does not run parallel with the different refrangibility of the colored lights employed. Indeed it seems probable that increased brightness of image was the determining factor in extending Albini’s color-zones, and that refractive correction had nothing to do with the matter. Moreover, the fact that the keenness of the peripheral “ light sense ” is undimin¬ ished, or even increased, would seem to indicate that the imperfect optical properties of the visual organ have been over-emphasized. (2) The second view conceives the histological structure or chemi¬ cal constitution of the peripheral retina to be different from that which obtains at the center, and brings this structural or chemical differen¬ tiation into relation with the difference between the functions which have been established in the two cases. The elaboration of this concep¬ tion has taken different forms in the hands of different theorists. (a) Maxwell believed that the absorption which results from the pigmentation of the yellow-spot is sufficient to account for the transi- tons of color-tone which he observed. Maxwell’s principle is, of course, powerless to account for changes in tone which occur beyond the limits of the macula. , ( b ) Helmholtz conceived that of the three visual substances but two are distributed over the whole retinal surface. The red-sensing substance is absent from the periphery, which is therefore red-blind. This conception was supported and extended in different directions by certain of his disciples. But the later Helmholtzians—and subsequently Helmholtz himself—concede its inadequacy. (c) Closely allied with the earlier Helmholtzian view is the theory which posits an integrity of structure of the whole retinal surface, but assumes that an impairment of function has taken place in the sensitive COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 69 substance of the periphery. This view is advocated by Schon and by Fick and is ultimately adopted by Helmholtz as representing his final position. We have elsewhere (pp. 39f.) shown cause for refusing to accept it. ( d ) According to the theory of Mrs. Ladd-Franklin, the visual substance is to be conceived as a product of a progressive development, in which three stages may be differentiated. In its primitive form the photo-chemical substance is decomposable by light of all sorts—the conscious product of the decomposition being an achromatic sensation. In this form the substance occurs only in the rods, and this supposition coincides with the fact that the extreme periphery of the retina con¬ tains rods almost exclusively. In its second stage of development, where it occurs only in the cones, the visual substance is differently decompos¬ able by long-waved and by short-waved light. Here arise the sensa¬ tions of yellow and of blue, besides the achromatic series. This is the stage of development which is represented by the “ dichromatic ” color zone of the normal retina, i. e., the zone in which, with moderate con¬ ditions of stimulation, blue and yellow are the only colors perceived. In its completely developed form the yellow-producing substance has reached a still higher stage of development and is now capable of being separately decomposed by erythrogenic (red-producing) and chloro- genic (green-producing) rays of light. This form of the photo¬ chemical substance occurs in considerable quantity only in the central and paracentral regions of the retina (hence called by Mrs. Ladd- Franklin, the “tetrachromatic zone”). The normal retina thus con¬ tains visual substance in all three stages of development; and the propor¬ tion of the less highly developed substances increases with increase of distance from the center. This theory has the advantage of being cast in an evolutionary mold, and of accounting for the relative subjective imperfection of the peripheral retina in evolutionary terms. It connects this imperfection with the relatively undeveloped structure which is known to be char¬ acteristic of the periphery, and with the fact, established by Ramon y Cajal, that the rods are undeveloped cones. The hypotheses of this theory readily fall into line with all of the facts revealed by the present investigation. The differently decompos¬ able substances are so distributed as to render it plausible that the retina should contain three concentric zones, and that these zones should vary in extent with variable conditions of stimulation. The retinal process which underlies the fading of peripherally seen colors through yellow or blue to gray, is rendered easy of envisagement by the hypothesis 70 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. that the photo-chemical substrates of the three visual processes represent different degrees of complexity of development, and hence presumably also represent different degrees of tenacity of function. The rapid adaptation and the persistent after-effects of the peripheral retina may be traced to an ad hoc hypothesis of a peculiar degree of chemical sta¬ bility of partially dissociated molecular groups. The seeming contra¬ diction between the presence of a bluish tinge in the stable red and the stable green and the absence of that tinge from the specific sensations furnished by the erythrogenic and chlorogenic substances, disappears when we remember the influence of the yellow pigmentation of the macula.* ( e ) The Hering theory assumes the existence of three sorts of visual substance, each of which has its own retinal distribution, and its own specific function. The whole retinal surface is supplied with all three substances, but their distribution is such that any given region contains a richest supply of white-black substance, and a scantest amount of red-green substance. These hypotheses adapt themselves to the coincidence of the zonal limits of the pairs of colors, because the substance which is active in the sensing of a color is also supposed to mediate the sensation of its complementary. The lesser extension of the red-green zone is referred to the relatively circumscribed distribution of the red-green substance. In common with all other theories it must trace the variability of zonal extension to the fact that the variation of a stimulus of constant tone, produces a variation of intensity of physi¬ ological action. The fading out of the peripheral colors through yellow or blue to gray is accounted for by the assumption that the retina con¬ tains a richer supply of white-black substance than of yellow-blue, and of yellow-blue than of red-green. The relatively rapid adaptation of the peripheral retina is referred to its relatively scant supply of chro¬ matic substance—a hypothesis which also accounts for the lesser satura¬ tion of colors seen in indirect vision. The persistent duration of the after-effects of peripheral stimulation is expressed physiologically in the statement that the restoration of chromatic equilibrium is more sluggish upon peripheral than upon central regions. Whether this physiological characteristic can be brought into relation with the rela¬ tively undeveloped state of the peripheral retina is not clear. (3) The third view occupies a mediating position between the first and the second. It has never attracted a following in the literature, nor *Mrs. C. Ladd-Franklin. Eine neue Theorie der Lichtempfindung. Zeit- sohrift fur Psychologic und Physiologic der Sinnesorgane, IV, 1893, S. 211-241; On Theories of Light Sensation, Mind, N. S., II, 1894, pp. 473-489; Baldwin’s Dictionary of Philosophy and Psychology, II, 1902, pp. 792ff. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 7 1 can a strong case be made out in its behalf. For while it is doubtless true that the periphery possesses less favorable optical conditions, still one can not understand how these can furnish even a partial explana¬ tion of the visual phenomena which are here to be accounted for. (4) The fourth conception is wholly different from any of the others. It represents an attitude of despair, in that its advocate con¬ fesses his failure to bring his facts into relation with any retinal process which can be conceived in analogy with the phenomena or principles of physics, of physiology, or of chemistry; and feels himself obliged to invoke the aid of a mysterious cerebral or other central process whose nature he also fails to characterize. Such a view as this might win the temporary adherence of the scientist who temporarily despairs of find¬ ing a more definite or more promising envisagement of the process of color vision; but it can scarcely hope to be accepted excepting as a tem¬ porary resting-place, or as a last resort to be chosen when all others fail. Such a view has been suggested by Landolt and Charpentier, and is mentioned as a tentative hypothesis by von Kries. It is too vague and ethereal to offer a promising field for discussion. 72 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. SUMMARY. The essential features of our results may be summarized as follows: (1) In moving across the retina, the images of colored objects of moderate size and luminosity pass through certain regular series of transitional tones. On the basis of these changes of color-tone we may conceive the retinal surface to contain three concentric zones—an ex¬ treme peripheral zone upon which all stimuli of moderate area and luminosity appear colorless; an intermediate zone where they appear yellow or blue (two exceptions will be mentioned later), and a central zone where red and green appear. (2) The position and extent of the color zones are not fixed, but variable. Their area in any given case depends upon the momentary condition of retinal adaptation, and upon the brightness and saturation of the stimuli employed. (Other investigators have established the fact that zonal extension is also a function of the character of the back¬ ground, the condition of optic refraction, and the magnitude of visual angle of stimulus.) Hence the absolute and relative extension of the zones may be varied at will. It is possible, for example, to choose a violet stimulus which can be recognized as violet at the extreme peri¬ phery, and it is equally possible to choose another stimulus of the same tone of violet which can not be recognized as violdt at the center of the fovea. (3) Of all possible colors, there are four and only four which undergo no change of tone in indirect vision. These are a purplish- red (non-spectral), a yellow (about 570 MM), a bluish-green (about 490 /*/*) > and a blue (about 460 /*/*) * (4) When stimuli representing these four stable colors are equated in white-value and in color-value, the retinal zone upon which the red is recognized coincides with that of the green, and the zone of the yellow coincides with that of the blue. The extension of the yellow- blue zone is considerably wider than that of the red-green zone. (5) When a color stimulus is applied to an eccentric region of the retina, the color rapidly fades out of the image. The fading process follows a gradual course, and passes through a regular series of transi¬ tions. Here, too, the order of change is through yellow (for stimuli of long-waved tones) or blue (for stimuli of short-waved tones) to gray. If the stimulation be sufficiently intensive or sufficiently long-continued, *These numbers represent the wave-lengths at the center of the spectral bands transmitted by our stimuli. A more accurate description of the composition of the stimuli will be found on p. 60. COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 73 the complementary blue or yellow appears as a final stage. All of these changes occur in the presence of the stimulus. (6) The stimulation of the peripheral retina is attended by char¬ acteristic after-effects. These may be described as being rudimentary after-images. They are analogous with after-images in that (a) they possess a complementary character; but they differ from after-images in that ( b ) they are (or at least were under our experimental condi¬ tions) wholly latent and unconscious (or sub-liminal). Their presence was felt, however, in the changed quality of the sensation aroused by subsequent stimulation, (c) A prominent characteristic of these after¬ effects is their persistent duration. They can be got rid of only by resting the eye for a considerable period after each stimulation. ( d ) As to their spatial attributes, our experiments justify only the general state¬ ment that they seem to extend over a wider retinal area than did the images which occasioned them. (7) There seems to be no doubt that Hellpach’s zone of comple- mentariness is an artifact, and that its discovery is wholly due to the experimenter’s failure to avoid retinal fatigue in his explorations. The peripheral retina is readily fatigued, and the fatigue-effects here persist with extraordinary tenacity. This characteristic is also present, though in lesser degree, at every other part of the retinal surface. When a color-stimulus which has already fatigued any region of the retina is reapplied to the same or an adjacent region before the retina has fully recovered from its previous stimulation, the resultant sensation tends to appear either colorless or in a tone which is complementary to that of the primary sensation aroused by the same stimulus at that point.* This phenomenon has been abundantly demonstrated in this and in numerous other investigations; it is unquestionably the same phenome¬ non which Hellpach reports. Hellpach is in error, however, in sup- *It is, of course, incorrect to say that these fatigue-products are comple¬ mentary to the “objective Farbe” of the stimulus as seen in direct vision. (See Hellpach, 1 . c., S. 537.) Such a characterization is true only in case the stimuli employed represent stable tones. In every case the tone of the “fatigue sen¬ sation ” aroused at a given region is complementary to the primary or “ non¬ fatigue ” sensation aroused at that region. Thus a purple stimulus is attended by green after-effects only at those regions where the purple of the stimulus is recog¬ nized. Upon those regions where this stimulus appears yellow, subsequent stimuli seem to be tinged with blue. This is in accord with Hellpach’s finding that red, orange, and yellow all appear bluish upon the “ gegenfarbige Zone.” Strangely enough, he reports that green appeared yellow-reddish and purple appeared yellow-greenish upon this outermost zone. This irregularity can be explained only on the assumption that Hellpach’s green and purple stimuli were more in¬ tensive than the others. 74 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. posing that this complementary function is a normal characteristic of the non-fatigued retina; he is no less mistaken in supposing that it is peculiar to the periphery alone. The same phenomenon can be made to appear at every part of the retinal surface. It is invariably absent from periphery and center alike, when care is taken to delay the appli¬ cation of the stimulus until the retina has fully recovered from the effects of previous stimulation. (8) Our results are in accord with the Hering and Franklin theories of color vision. They can not be reconciled with any other theory with which the writer is acquainted. BIBLIOGRAPHY. Abney, W. de W. The Sensitiveness of the Eye to Light and Colour. Nature, XLVII, 1893, pp. 538-542. Colour Vision. New York. (No date.) 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