LEWISES LIBRARY MEIMCAL SCHOOL SMFORD R. GIFFORD LIBRARY THE PHYSIOLOGY OF VISION BY THE SAME AUTHOR In Leather Envelope- Case, Price 2os. net. CARD TEST FOR COLOUR BLINDNESS This novel test for the detection of Colour Blindness is designed to meet the frequently expressed need for a simple and portable, but thoroughly efficient test, for use in cases where Dr. Edridge-Cireen's Colour Vision Lantern is not available. The test is fully described on pp. ].5(J-l(jO of the present book LONDON : (i. BELL AND SONS, LTD. THE PHYSIOLOGY OF VISION WITH SPECIAL REFERENCE TO COLOUR BLINDNESS 1 JL> BY F. w. "EDRIDGE-GREEN M.D., F.RC.S. Oculist London Pensions Boards ; late Chairman Ophthalmic Board, Central London Medical Boards, National Service; late Member International Code of Signals Committee ; late Hunterian Professor of the Royal College of Surgeons, etc. ; Inventor of Colour Percep- tion Spectrometer, Colour Perception Lantern the Official Test of the Navy ; and Bead Test, the Test of the National Service LONDON G. BELL AND SONS, LTD. YORK HOUSE, PORTUGAL ST., KINGSWAY, W.C.2 1920 DEDICATED TO PEOF. E. H. STARLING AN IDEAL HEAD OF A LABORATORY G4 ( J'-r> PREFACE THIS book is the result of numerous researches on vision and colour- vision, the principal original papers of which are given at the end of the volume, and present the subject in a new aspect. It will be noticed that each section of the subject has been given from the point of view of new facts apart from any theory and so is available for an explanation on any theory. The subjects of vision and colour- vision have been viewed too much from the point of view of theory and the primary assumption is made that the theory is true when this can be easily shown not to be the case, as for instance long papers on induction when there is no induction and the classification of colour blindness as red blindness, green blindness, etc., when this classification is so meaningless that a man may be classified by one observer as a case of complete red blindness and by another as a case of complete green blindness. It is probable that it is for this reason that it is so extraordinarily difficult to establish a new fact. It took me twenty years to establish such simple facts as vii viii PREFACE that certain colour-blind men can pass the wool test and that when a certain portion of the spectrum is isolated it appears monochromatic. I must, therefore, express my indebtedness to those who have examined the facts and especially to Professors Starling and Bayliss. In this book I have attempted to give a simple compre- hensive account of form and colour-vision, which will make clear the present state of knowledge on the subject and also indicate the direction in which further research is required. A great deal of the uncertainty which has existed in the past on these matters has been due to the fact that both physicists and physiologists who have written upon this branch of science have had a tendency to accept as the basis of their theories supposed facts, which in a large number of cases have proved not to be facts. The physicists in particular have shown a reluctance to experiment anew. Further, many workers unacquainted with the difficulties of the subject have made use of imperfect apparatus, such as coloured papers, which reflect light corresponding to very ill denned regions of the spectrum, others, as I have often been able to show, have vitiated their experiments by the admission of stray light. Experiments on colour- vision need quite as much care and rigorous definition of con- ditions as those in any other branch of science. The investigation of the problems of vision requires a know- ledge of physics, physiology, ophthalmology, and psy- chology. A want of knowledge of any of these branches PREFACE ix of science may entirely destroy the value of an experi- ment or theory. This book is necessarily very largely concerned with my own work on the subject, extending over the past thirty years. It is gratifying to be able to record that the theories put forward, which met with a good deal of opposition in their inception, are now generally accepted by modern physiologists ; I have, nevertheless, been particularly careful to give in all cases the full experi- mental grounds on which they rest. Anyone who wishes to construct a fresh theory is thus provided with the facts which he must explain, and sufficient details are given to enable him to repeat, if he so desire, my experiments. Where I have rejected the results of former experiments I have given my reasons for believing that the methods were erroneous or that facts had been overlooked. These and other things convince me that a general book on the subject is needed, and I sincerely hope that it may help students who realise the wide imperfections of the old theories but have not the time to hunt through the work scattered in various papers for the information which they require. I believe that the general reader, too, will find much in this branch of science to interest him. The practical importance of a true understanding of colour-vision is obvious. I will only instance that without a knowledge of the theory of the subject it is impossible to devise tests which will prevent dangerously colour-blind men from being employed as lookouts and x PREFACE signalmen. As an example of this the old wool test based on the trichromatic theory, in addition to re- jecting many normal sighted persons, allows over fifty per cent, of dangerously colour-blind to escape detection. I am not aware of any attempt on the part of the rapidly diminishing number of supporters of the trichromatic theory to explain the above fact and it is therefore an anomaly that the theory should still be taught in physics text books. Those who are interested in the physical aspects of the subject should read the very important paper by Dr. R. A. Houstoun in the Philosophical Magazine, Sept., 1919. The laws of colour contrast are of the utmost utility to designers and kindred craftsmen. It gives me much pleasure to express here my thanks to Sir Frederick Mott, Profs. A. D. Waller, W. Halli- burton, W. M. Bayliss, E. H. Starling, A. W. Porter, Sir Ronald Ross, Dr. A. Lynch, Dr. R. A. Houstoun, Mr. T. H. Bickerton, Dr. E. N. da C. Andrade, Lord Moulton, and many others for the interest which they have taken in my work. F. W. EDRIDGE-GREEN. CONTENTS CHAPTER PAGE I. THE EXCITATION OF THE VISUAL APPARATUS - 1 II. SPECIAL POINTS IN THE ANATOMY OF THE RETINA 10 III. THE DIOPTRICS OF THE EYE - 15 IV. THE ACCOMMODATION OF THE EYE - 21 V. THE PHYSIOLOGY OF THE IRIS 26 VI. DEFECTS OF THE EYE AS AN OPTICAL INSTRUMENT 29 VII. THE ACTION OF LIGHT ON THE RETINA - 41 VIII. THE ORIGIN OF VISUAL IMPULSES - 50 IX. LIGHT AND DARK ADAPTATION 56 X. VISUAL ACUITY 59 XI. POSITIVE AND NEGATIVE AFTER-IMAGES - 64 XII. THE TIME RELATIONS OF VISUAL SENSATION - 72 XIII. VARIATIONS IN LIGHT SENSATION 77 XIV. VARIOUS VISUAL PHENOMENA- 80 XV. BINOCULAR VISION - 106 XVI. SUMMARY - 134 xii CONTENTS CHAPTER PAGE XVII. THE SENSATIONS CAUSED BY SIMPLE AND MIXED LIGHTS - 136 XVIII. THE SIMPLE CHARACTER OF THE YELLOW SEN- SATION - 141 XIX. METHODS OF EXAMINATION OF THE COLOUR SENSE 147 XX. HEXACHROMIC VISION - 168 XXL HEPTACHROMIC VISION 173 XXII. COLOUR BLINDNESS 176 XXIII. THE EVOLUTION OF THE COLOUR SENSE 213 XXIV. TRICHROMIC VISION AND ANOMALOUS TRICHRO- MATISM - 218 XXV. THE POSITIVE EFFECT OF STIMULATION OF THE EETINA ON SURROUNDING EEGIONS - 226 XXVI. SIMULTANEOUS COLOUR CONTRAST - 230 XXVII. SUCCESSIVE CONTRAST- 248 XXVIII. COLOUR ADAPTATION 253 XXIX. THE THEORY OF COLOUR VISION 263 XXX. OBJECTIONS TO OTHER THEORIES OF VISION AND COLOUR VISION 267 LIST OF PAPERS - - 274 INDEX ...... - 278 CHAPTER I. THE EXCITATION OF THE VISUAL APPARATUS. We can only have cognisance of the external world as our senses and faculties inform us of its existence. Any defect, either in the sense organ or perceptive centre, by preventing the perception of certain classes of sensations, has the same effect as if the physical stimuli giving rise to the sensations did not exist. It is impos- sible to explain to a man who has been born blind the nature of sight ; he does not feel his loss in the same way as a man who has once been possessed of vision. The Doctrine of Specific Sense Energy. Johannes Miiller made a great advance in the physio- logy of the organs of sense when, adopting the views of Bell, he formulated his laws of specific sense energy. He showed that the sensations that we experience of light, colour, sound, heat, etc., are the special properties of the sense organ and its cerebral associations and not a function of the external stimuli which caused those sensations ; for instance, the sensation of light which is caused by stimulation of the cerebro-retinal appara- tus is a special function of this apparatus, and any 2 PHYSIOLOGY OF VISION stimulus acting upon the eye can only give rise to a sensation of light when this is conveyed through the optic nerve. Mechanical, thermal, or electrical stimuli applied to the retina or optic nerve give rise, if they produce an effect, to a sensation of light. In the same way any of these stimuli applied to the auditory nerve, if they give rise to any sensation at all, give rise to a sensation of sound. When the same stimulus falls upon two different sense organs, it gives rise in each to the sensation peculiar to that organ ; for instance, the rays of the sun falling upon the hand give rise to a sensation of heat through a stimulation of those nerves which cause when stimulated a sensation of heat. It follows from this that sensations only exist in ourselves, and that the sensation only bears a resemblance to the stimulus as a written symbol bears to the thing it describes. Without the eye the world would be in darkness, without the ear in silence. There are undoubtedly special physical stimuli which give rise to light and sound respectively, but there are closely allied physical stimuli which are not perceived by us at all ; for instance, photographs can be taken in a room which appears absolutely dark to us. These photographs by ultra-violet or infra-red light have special characters of their own. This shows that physical stimuli which are not perceived by our senses are capable of doing important work. There are essential physical differences which are not detected by the sense organ ; for instance, we cannot distinguish PHYSIOLOGY OF VISION 3 between polarized and non-polarized light by the eye. In the future, it may be found that some of the physical stimuli causing the sensation of light have very different characters but agree in causing a sensation of light when acting upon the eye. Those stimuli for which the sense organ is specially constructed are called adequate stimuli, as, for instance, light in the case of the eye and sound of the ear. Other stimuli capable of producing a sensation are called inadequate stimuli. A corollary which has become attached to that of the specific energy of the senses is that nerve impulses are similar in all nerves, and that, therefore, any differences must exist at the peripheral and central ends of the nerves. There is no evidence for this hypothesis, which seems in the highest degree improbable. There is no evidence to show that a nerve cannot transmit impulses of varying rate and length. We have an example of how this is accomplished in the case of sound by the telephone. It would not be necessary that the impulses should be in the same order of frequency as the physical stimuli causing the impulse, but they might bear some definite relation to them. We know, however, very little at present of the nature of nerve impulses. We now have to consider how the physical stimuli affect the sense organ, and how the sensations vary with different people. If we compare a number of persons with regard to their sensations, we find that these differ in a remarkable 4 PHYSIOLOGY OF VISION manner. For instance, one man will declare that a room which is brilliantly lit up with red lights is abso- lutely dark ; another will declare there is nothing to be heard when a bat or a mouse is screaming ; another will state emphatically that there is no difference in taste between pheasant and mutton, or that port and sherry are exactly alike except for colour. There are innumer- able varieties and degrees in these differences of sensation. As the sense organ has been developed in the process of evolution to respond to certain physical stimuli, it is probable that the differences found between persons in their capacity of differentiating between the physical stimuli affecting one of their sense organs corresponds to an earlier state in the evolutionary process. Have we any means of ascertaining the lines on which evolu- tion has proceeded ? We know that the physical stimuli which affect our sense organs differ in their physical characters, and each set of physical stimuli can be arranged in a series ; for instance, the waves causing the sensation of sound can be arranged according to their size from the lowest bass note to the highest treble, or waves which cause the sensation of light from the lowest perceptible red to the highest perceptible violet. We should, therefore, expect that the sense organ would be evolved so that it would discriminate between those physical stimuli which were physically most different. We should also expect that the sense organ would be developed so that the range of sensitive- ness to the physical stimuli at the ends of the series PHYSIOLOGY OF VISION 5 was increased, that is to say that the length of the physical series which affected the sense organ became increased. The differences found in the varying sensations of different persons fall into two distinct classes namely, inability to distinguish between physical stimuli occu- pying neighbouring positions in the physical series, and inability to perceive physical stimuli occupying positions at one or both ends of the physical series. The appear- ance of a physical series to any person may be called the psycho-physical series for that person. A psycho- physical series is a physical series as it appears to the mind. A. Physical Series. A physical series consists of various physical stimuli arranged in a series. For instance, the waves which give rise to the sensation of sound or those giving rise to the sensation of light may be arranged in a series from the largest to the smallest. We also know that the series is continued both above and below the range which is capable of giving rise to a sensation. A physical unit is the smallest conceivable portion of a physical series. B. Psycho-Physical Series. A psycho-physical series is a physical series as it appears to the mind. It has a definite commencement, a definite termination and certain definite units. A 6 PHYSIOLOGY OF VISION spectrum forms an admirable instance of a psycho- physical series. An absolute psycho-physical unit con- sists of a portion of the psycho-physical series which contains physical units which the observer is incapable of discriminating between, as, for instance, those rays which are included in a portion of a spectrum which appears monochromatic. Apart from light there are many other physical stimuli which are able to excite a sensation of light. A blow on the eye will cause a sensation of light. Pressure on the eye in the dark will cause a sensation of light in the corresponding part of the field of vision. This has been designated a phosphene. Pressure on the front of the eye will cause the appearance of the star-figure which is described later and the rapidly moving and interlacing currents. Visual sensation has also been caused by electric currents, and by placing the eyes in a magnetic field. As it is necessary for men and animals to discriminate between adjacent objects, relative difference becomes much more important than actual difference. The physical differences around us are continually changing, and the variations in these physical conditions have to be specially ascertained by physical measurements. The variation in the amount of light falling upon the eye during the day is very considerable, at one period of the day black print may reflect more than twelve times the amount of light reflected by white paper at another period of the day. We are not conscious of PHYSIOLOGY OF VISION 7 this difference, and the paper looks white and the print black at both periods. If in a dark room a candle be placed so that a shadow is thrown by an opaque object on a screen the shadow is plainly marked and visible. If, however, sunlight be allowed to fall upon the screen as well, or the light from a powerful electric lamp, the shadow becomes invisible and the screen appears uniformly lighted. The perceptible difference in lighting between two adjacent surfaces has been ascertained to be about T ^ of the total amount of light in ordinary conditions. A certain amount of light is necessary before a sensa- tion is evoked, the amount varying according to the state of the eye. A simple illustration will show how the same physical stimuli falling upon the eye in different conditions may produce opposite results. If a piece of ultramarine blue paper be placed on one side of a photometer and be illuminated by an ordinary incandescent electric lamp, it will be found that it can be exactly matched by a piece of orange-brown paper illuminated by daylight on the other side of the photometer. If the ultra- marine blue paper be taken out and examined by the same electric light which was used for the photometer it will be seen that it is a decided blue. Therefore when the blue paper is seen isolated, it appears of a colour complementary to that of its appearance when the surrounding parts of the retina are stimulated by the same light. It will be seen, therefore, that the retina 8 PHYSIOLOGY OF VISION must be considered as a whole and that the actual effect of any external stimuli upon it must be considered, when all other stimuli are carefully excluded. Nature is not concerned with the fact that one object appears blue and another green, but is concerned with the power of the animal to discriminate between stimuli for the purposes of that animal. Those creatures low in the scale of evolution which possess eyes of a rudimentary character are made aware that there is danger near when a shadow is thrown upon them. As evolution proceeds and each sensory cell is connected with a special nerve cell so does the power of localization in space increase also the power of differentiating between adjacent stimuli. The distance between two cones represents the power of visual acuity. On examining the external surface of the retinae of young monkeys, I found that the cones were much larger and much fewer to the square millimetre than those of an adult man. It would appear from this that man can discriminate between objects which would not be possible to a monkey. The physiology of vision may be conveniently divided into three sections : (1) The dioptrics of the eye or the means by which images are formed on the retina. (2) The means by which the light forming the image on the retina is changed into visual impulses. (3) The resulting perceptions. It seems incredible that the cones could be directly stimulated by light. It is so much more probable that PHYSIOLOGY OF VISION 9 the stimulation of the cones is indirect and photo- chemical. The sensitive ends of the cones are situated in a fluid, which is kept in its place by the external limiting membrane on one side and a corresponding membrane covering the pigment cells on the other. It is easy to see, therefore, that if this fluid were photo- chemical, we have a ready explanation of the formation of visual impulses, by the stimulation of the cones by the products of the photo-chemical decomposition of this fluid. In the sensitive layer of the retina there are not only cones but rods which are found distributed round the cones except in the fovea. These rods contain a photo-chemical substance, the visual purple. There is. therefore, a substance present which is able to sensitise the fluid round the cones by diffusing into it. This view explains all the ordinary phenomena of vision, including a large number of facts which were previously inexplicable. There is also, as stated by Nagel, 1 not a single fact pointing to the view that the rods are per- cipient elements. This is the only theory which will explain why there is no qualitative difference between the foveal and para-foveal regions. It also explains the fact that an after-image can change its relative place in the visual field. 1 Physiol. des Menschen, vol. iii. p. 107. CHAPTER II. SPECIAL POINTS IN THE ANATOMY OF THE RETINA. The retina is the membrane which contains the expan- sion of the optic nerve : it diminishes in thickness from behind forwards, being thickest, about -4 mm. in the region of the yellow spot, about half that thickness in the equator and about a fourth at the ora serrata or indented anterior portion. The Layer of Rods and Cones. The elements which compose this layer are of two kinds, called from their shape rods and cones. Each consists of an outer and inner segment. The rods are about -06 mm. long and -002 mm. in diameter. The cones are about -035 mm. long and about -006 mm. in diameter. The outer segments of the rods are imbedded in the pigment cells of the hexagonal layer. Krause has estimated that there are seven millions of cones and 130 millions of rods in the human retina. Sulzer finds only about half this number of each. Sulzer estimates the number of fibres in the optic nerve as about half a 10 ANATOMY OF THE RETINA 11 million. It will be seen, therefore, that even if the cones alone be the terminal percipient elements, in the peri- pheral parts of the retina many cones must be connected with a single nerve fibre. The outer segments of the rods contain a rose-coloured substance, the visual purple. The Macula Lutea and Fovea. This is the most important part of the retina, and differs considerably in structure from the remainder. The yellow spot is so called on account of the yellow colouring matter which pervades the four or five inner layers. Ghdlstrand is of opinion that the yellow colour of the yellow spot is due to a post-mortem change. Apart from the evidence of the pigment which can be obtained during life, I have frequently seen the yellow spot bright yellow in the retinae of monkeys examined immediately after death. It is somewhat elliptical in shape, and measures about 2 mm. in diameter. The long diameter of the ellipse is horizontal. In the centre of the yellow spot is a small pit, the fovea centralis, which measured in a retina I examined about -5 mm. The bottom of the pit I found to measure about -1 mm. Only cones are to be found in the fovea, and they are much longer and thinner than in other parts of the retina. This particu- larly applies to the outer segments, which thus present a larger surface for photo-chemical stimulation. The 12 PHYSIOLOGY OF VISION other layers of the retina are much thinner at the fovea than elsewhere, though their thickness gradually increases until in the outer parts of the yellow spot the retina is at its thickest. The layer of ganglion cells is especially thickened at the edge of the fovea. The nerve fibre layer disappears as a distinct layer at the edge of the fovea, the fibres joining the central processes of the FIG. 1. Schema of Structure of Human Retina (Greef). ganglion cells. As the line of the pigment cells remains level and the cones are much longer, the external limiting membrane dips in and thus forms an external fovea. This I found to be -3 mm. in size in the specimen I examined. ANATOMY OF THE RETINA 13 The intervals between the rods and cones and the hexagonal pigment cells are only partially filled by the processes of the latter, the remainder is filled by a liquid into which the visual purple diffuses. There is in the embryo a distinct space between the hexagonal pigment cells and the remainder of the retina. This is filled with fluid, and is the remains of the cavity of the primary optic vesicle. I find four depressions or canals which lead into the larger depression of the external fovea. These canals appear to have smaller branches, and serve to conduct the visual purple into the part of most acute vision. In certain conditions where there is obstruction of the outflow at the back of the eye these canals form a star figure. The same star figure can be seen entoptically. The space between the rods and the cones is in direct communication with the lymphatics of the optic nerve, and this is probably how the waste products escape. Schwalbe has shown that this space may be filled by injecting coloured fluid under the sheath which the optic nerve derives from the pi a mater. An examination of the accompanying figure shows the difference in the connections of the rods and cones with the ganglion cells. It will be noticed that whilst that of the cones is direct, the rods terminate in rounded knobs, many of which are connected with one ganglion cell. Transverse cells also connect groups of rods. It 14 PHYSIOLOGY OF VISION will be seen that for purposes of location in space the rods appear to have a very unsatisfactory anatomical arrangement, any satisfactory method of discrimination appearing impossible on this formation. If, however, the anatomical structure be regarded from the point of view of the distribution of the visual purple, it is exactly what we should expect. CHAPTER III THE DIOPTRICS OF THE EYE. The dioptric portion of the eye is an optical apparatus constructed so that a correct image of an external object may be formed on the retina. It is similar to a camera obscura. and consists of a number of approxi- mately spherical surfaces approximately centred round an optic axis and separated by various media. It has not been finally settled whether the optic axis passes through the fovea. according to some observers it does, whilst others state that it falls a little to one side of it. Rays of light falling upon the eye and subsequently impinging upon the retina have to traverse in succession the anterior surface of the cornea, the substance of the cornea, the posterior surface of the cornea, the aqueous humour, the anterior portion of the capsule of the lens, the anterior surface of the lens, the various layers of the lens, each of which has a different refractive index, the posterior surface of the lens, the posterior portion of the capsule of the lens and the vitreous humour. It will be seen, therefore, that excluding the capsule of the lens, the rays of light have to traverse four surfaces separated 15 16 PHYSIOLOGY OF VISION by five media : the four surfaces being the anterior surface of the cornea, the posterior surface of the cornea, the anterior surface of the lens and the posterior surface of the lens. The five media are air, the substance of the cornea, the aqueous humour, the substance of the lens and the vitreous humour. This optical system can be simplified still further and reduced to three curved surfaces and four media. When rays are refracted by a medium bordered by two spherical surfaces which are concentric to each other the medium and the posterior surface may be ignored and the rays may be considered as passing directly from the anterior surface into the second medium. As the posterior surface of the cornea is approximately concentric with the anterior surface, the substance of the cornea and its posterior surface may be neglected. It has been shown mathematically that when a number of spherical surfaces are centred on an optic axis and separated by various media, the system can be repre- sented by two ideal surfaces, each of which has its own principal point and nodal point. In order to construct such a system it is necessary to know : 1 . The refractive indices of the media. 2. The radii of the spherical surfaces. 3. The distance of the surfaces from each other. The cardinal points of the eye can be ascertained from these data. DIOPTRICS OF THE EYE 17 Position of the Cardinal Points. The cardinal points of this schematic eye are the position of the principal foci, both nodal points, and the two principal focal points. Listing accepted the following values as true : 1 . The refractive index of air - 1 2. The refractive index of the aqueous humour - 1-337 3. The refractive index of the lens - 1454 4. The refractive index of the vitreous humour 1 -337 5. The radius of curvature of the cornea - 8 mm. 6. The raclius of curvature of the anterior surface of the lens - - 10 mm. 7. The radius of curvature of the posterior surface of the lens 6 mm. 8. Distance of the front surface of the cornea to the anterior surface of the lens - 4 mm. 9. Thickness of the lens - 4 mm. He deduced from these data the position of the cardinal points as follows : 1. The first principal focus is 12-8326 mm. in front of the anterior surface of the cornea. 2. The second principal focus is 14-6470 mm. behind the posterior surface of the lens. 3. The first principal point is 2-1746 mm., and the second principal point is 2-5724 mm. behind the anterior surface of the cornea. 4. The first nodal point is '7580 mm., and the second nodal point is -3602 mm. in front of the posterior surface of the lens. Some of the values taken by Listing are at variance with the calculations of other observers. Matthiessen l 18 PHYSIOLOGY OF VISION 1. The Refractive Indices of the Media. These have been variously estimated by different observers. The following are some of the latest : Substance. Index of Refraction. Author. Cornea - 1-3771 Matthiessen 1 Aqueous Humour 1-3374 Hirschberg 2 Capsule of Lens - 1 -3599 Outer layer of the Lens 1 -3880 Middle layer of the Lens 1 -4060 Nucleus of the Lens - 14170 The Whole Lens - 14371 Matthiessen l 1-4215 Treutler 3 Vitreous Humour - 1 -3360 Hirschberg 2 2. The Radii of the Surfaces, (a) The Cornea. The radius of the anterior surface of the cornea is about 7-8 mm., which is the mean of numerous measurements. The radius of the posterior surface of the cornea is about 6-2 mm. (b) The Lens. The radius of the anterior surface of the lens is about 10 mm. and that of the posterior surface 6 mm. 3. The Thickness of the Refracting Surfaces. (a) The Thickness of the Cornea. This is given very differently by different authors. Blix gives it as from 482 to -668 mm. and Tscherning finds it 1-15 mm. (b) The depth of the Anterior Chamber. This is about 3-6 mm. 1 Pflnger's Arch. vol. xix. p. 543, 1879, and vol. xxxvi., 1885, 2 Zentralbl. f. d. mediz. Wissensch. 1874. 3 Klin. Monatsbl.f. Auqenheilk. 1902 DIOPTRICS OF THE EYE 19 (c) The Thickness of the Lens. This is about 3-6 mm. JSTagel x taking the following data estimates the cardinal points of the schematic eye as below : The refractive indices, air = l, aqueous humour = 1-337, the lens = l-437, and the vitreous humour = 1 -337 ; the radii, anterior surface of the cornea =1- 8 mm., anterior surface of the lens =10 mm., posterior surface of the lens =6 mm.; the thickness of the media; anterior chamber =3- 6 mm., lens=3 : 6 mm. The first principal point lies 1-75 mm. behind the anterior surface of the cornea, the second principal point 2-09 mm. behind the anterior surface of the cornea or 5-11 mm. in front of the posterior surface of the lens. The first principal focus is 13-75 mm. in front of the cornea, the second principal focus lies 22-79 mm. behind the anterior surface of the cornea. The first nodal point is 6-95 mm. and the second 7-29 mm. behind the anterior surface of the cornea. Both principal points of the eye lie so close together that we can for all practical purposes substitute a point midway between the two in place of both : the same applies to both nodal points. When the complicated system of the eye is thus reduced to one spherical surface and two media we have the reduced eye, and from it we can calculate the size and position of images formed upon the retina. This simple system consists of a spherical surface with a radius of 5 mm. between a medium with a refractive 1 Physiol. des Menschen, 1905, vol. iii. p. 46. 20 PHYSIOLOGY OF VISION index of 1 in front and one of a refractive index of 1-33 behind. The nodal points of this simple system lie as in the actual eye near the posterior surface of the lens. The first focal distance of this system is 15 mm., the second 20 mm. Formation of the Retinal Image. An inverted image of an object is formed upon the retina by the optical system which has just been de- scribed. This inverted image may be seen at the back of an excised eye of an Albino rabbit through the semi- transparent coats of the eye-ball. The size of the retinal image may be calculated, provided that we know the size of the object and its distance from the cornea. CHAPTER IV. THE ACCOMMODATION OF THE EYE. We are not able to see distinctly at the same time objects which are situated at different distances from the eye. The accommodation of the eye is a term applied to the power of the eye which enables it to see objects distinctly at various distances. When the eye is at rest and emme- tropic, rays coming from an infinite distance, that is parallel rays, are focussed upon the retina. When the eye is accommodated for a near object and the rays falling upon it are diverging, the anterior surface of the lens becomes more convex and brings the rays to a focus more in front of the retina than when the lens is in a passive condition. The Mechanism of Accommodation. Though all are agreed that the anterior surface of the lens becomes more convex during accommodation, there are numerous theories as to how this is brought about. Helmholtz's theory of the mechanism of accommodation is that which is most widely accepted, and is as follows. In a condition of rest, that is during negative accommo- dation, the lens is kept in a somewhat flattened condition 21 22 PHYSIOLOGY OF VISION by the tension of the Zonule of Zinn. In positive accommodation, that is when the eye is focussed for near objects, the ciliary muscle (longitudinal fibres) contracts, pulls forward the choroid and relaxes the Zonule of Zinn, and the lens on account of its elasticity becomes more convex on its anterior surface on account of the relaxation of the Zonule of Zinn, which keeps it flattened. The posterior surface is supported by the vitreous humour, and therefore the anterior surface protrudes forward. Thomson Henderson's J theory of accommodation is based on two facts : (1) The ciliary muscle is composed of non-striped fibres. Non-striped fibres do not contract into activity but into a phase of rest, i.e., stomach, bladder, etc. (2) The zonular fibres in their course from the lens to the ora serrata present a curvature. A curvature in a non-rigid structure like the zonule cannot exist unless supported. He maintains that whilst the changes in the lens curvature in accommodation are brought about by slackening of the zonule, yet these results are not produced by contraction of the ciliary muscle but by its relaxation. The longitudinal fibres of the ciliary muscle are inserted opposite the point of greatest curvature of the 1 Ophthalmoscope, Sept. 1912. ACCOMMODATION OF THE EYE 23 zonule, and therefore when contracted into their phase of rest keep the zonule taut by maintaining and sup- porting the curvature, i.e., when the eye is at rest as during sleep. The circular fibres act as a sphincter ciliaris. 'In accommodation an associated action of contraction of the circular fibres with relaxation of the longitudinal lowers the zonular curvature and so reduces the tension on the lens. In negative accommodation the opposite takes place, the longitudinal fibres, contracting into a phase of rest, pull upon and increase the zonular curva- ture, and the circular fibres relax. The fact that the circular fibres of the ciliary muscle are strongly developed in hypermetropic eyes and either absent or very ill-developed in myopic eyes is in favour of Henderson's view. Most of the other theories of accommodation have been disproved by observation and experiment. Proof that the Anterior Surface of the Lens alters its Curvature during Accommodation. Purkinje-Sanson's Images. If light be allowed to fall upon the eye through two triangular apertures placed one above each other in a piece of cardboard three pairs of images will be seen. During negative accommodation three pairs of images will be seen. The first pair, which is the brightest of the three, is formed by the anterior surface of the cornea, which acts as a convex mirror. This pair is of medium 24 PHYSIOLOGY OF VISION size. The second pair, which is of medium brightness, is the largest, and is formed by the anterior surface of the lens. The third pair, which is of medium size and brightness, is inverted and formed by the anterior surface of the posterior portion of the capsule of the lens, which acts as a concave mirror. There are therefore two pairs of erect images formed by the cornea and anterior surface of the lens respectively and one pair of inverted images formed by the capsule of the lens acting as a concave mirror. If the person on whom the observation is being made be now told to accommodate for a near object it will be noticed that though no change takes place in the images formed by the cornea and very little in those formed by the capsule of the lens, the pair formed by the anterior surface of the lens becomes smaller and approach each other as well as approaching the pair of images formed by the cornea. This proves that the anterior surface of the lens has become more convex, as the size of an image formed by a convex mirror depends upon the radius of curvature of the mirror, being largest when the radius of curvature is largest. The Effect of the Accommodation upon the Cardinal Points of the Schematic Eye. The following data being taken ; refractive indices of the aqueous and vitreous humours 1-3365. refractive index of lens 1-4371, radius of anterior surface of cornea 7-829, radius of anterior surface of lens in negative ACCOMMODATION OF THE EYE 25 accommodation 10, in positive accommodation 6, radius of the posterior surface of the lens in negative accommodation 6, and in positive accommodation 5-5. Helmholtz x estimates the cardinal points in millimetres as follows : Accommodation. Negative. Positive. Focal distance of lens - 50-617 39-073 Position of the First Principal Point 1 -753 1 -858 Second 2-106 2-257 First Nodal Point 6-968 6-566 Second 7-321 6-965 First Principal Focus 13-745 12-132 Second 22-819 20-955 The last six distances are given for positions behind the anterior surface of the cornea, except the first principal focus, which is situated the distance given in front of the cornea. 1 Physiol Optik, p. 140. CHAPTER V. THE PHYSIOLOGY OF THE IRIS. The iris has two special functions : 1. It acts like a stop in any piece of optical apparatus, thereby diminishing spherical aberration. 2. It increases or diminishes the amount of light which enters the eye. When it is contracted the cone of light which has its base at the iris is smaller than when it is dilated. The iris is provided with two muscles, a sphincter and dilator pupillae. Nerves of Iris. The constrictor fibres of the iris are supplied by the third nerve, and pass through the lenticular ganglion and to the eye through the short ciliary nerves. The dilator fibres are supplied by the sympathetic root of the lenticular ganglion and by the ophthalmic branch of the fifth, the latter reach the eye through the long ciliary nerves. The contraction of the pupil through the influence of light is a reflex act, the optic nerve being the afferent nerve and the third nerve being the efferent and the centre being in the brain, the exact positions not having 26 PHYSIOLOGY OF THE IRIS 27 been finally determined. After division of the optic nerve no contraction takes place when light falls upon the eye, and the same is true when the third nerve is divided. When light falls upon the eye both pupils usually contract to the same extent. This is called the con- sensual reflex. When the contraction due to light is removed dilatation takes place through the sympathetic, the constrictor influence being removed. The Argyll-Robertson Pupil, In this condition the pupil contracts to accommo- dation and not to light. It is a common symptom of locomotor ataxia. Movements of Iris. 1 . The pupil dilates : (a) On removal of light from the eye. (b) Accommodation for distant objects. (c) When the aqueous humour is in excess. (d) In certain emotions, as, for instance, fear. (e) In dyspnoea. (/) Upon violent exercise. (g) On stimulation of certain sensory nerves, as, for instance, those of the sexual organs. (h) On poisoning by various drugs and in certain diseases. (i) Through the local action of certain drugs, as, for instance, atropin and euphthalmin. 28 PHYSIOLOGY OF VISION 2. The pupil contracts : (a) When light falls on the eye. Schirmer states that no difference in contraction is found from 100 to 1000 meter candles. (b) When the eye is accommodated for near objects. (c) When the eyes converge. (d) When the aqueous humour is deficient. (e) In poisoning by various drugs and in certain diseases. (f) Through the local effect of certain drugs, as, for instance, physostigmin and muscarin. CHAPTBE VI. f DEFECTS OF THE EYE AS AN OPTICAL INSTRUMENT. In order that an object in the field of vision may be clearly seen, it is necessary that all the rays of light coming from a point of the object should, after passing through the refracting media of the eye, be brought together at another point at the external surface of the retina. Every deviation from this will cause corre- sponding defects in the image which is formed. It is upon the fulfilment of this condition that the perfection of definition of any optical instrument depends. When the light from a luminous point strikes the eye it forms a cone, of which the base is situated on the cornea and the apex at the luminous point. These rays will be refracted by the cornea and the light will enter the eye through the iris. As only the light which passes through the aperture in the iris can affect the retina, a new cone will be formed with its base at the pupil. After refraction by the lens this cone will be brought to a focus. If this focus correspond with the external surface of the retina the luminous point will be seen as a 29 30 PHYSIOLOGY OF VISION single point, because at the focus it will be again only a single point. It is obvious that if the focus be formed either in front or behind the retina instead of a point there will be a small circle on the retina. This is called a circle of diffusion. We can with the aid of a convex lens easily observe the phenomena of circles of diffusion. Prick a pin-hole in a piece of black cardboard and place a light behind the cardboard. We can with the lens focus the light coming from the pin-hole on a piece of white paper. It will be noticed that only in one position will there be a clear image of the pin-hole on the paper. If the lens be moved too far away from the paper or placed too close to it, the point will be replaced by a small circle of light, which varies in its size according to the extent which the lens is distant from its correct focussing position. It follows from this that we are not able to see clearly at the same time objects which are situated at different distances from the eye in the field of vision. In order to convince himself of this fact, the reader should look through a veil at a book situated at a distance of two or three feet ; he will find that he is unable to see the meshes of the veil and the printing on the book at the same time. When the image is formed in front of the retina instead of on it through the axis of the eye being too long the condition is called myopia. When the image is formed behind the eye through its axis being too short the condition is called hypermetropia. Emme- tropia is applied to the condition when parallel rays DEFECTS OF THE EYE 31 coming from a distance are brought to a focus on the retina without an effort of accommodation. The power of accommodation gradually diminishes as age advances on account of the lens becoming less elastic. At the age of ten with an emmetropic eye objects can be clearly seen at a distance of 7 cm. from the eye. After this the nearest point of distinct vision recedes further and further from the eye until at the age of seventy-five all accommodation is lost. When the near point has receded further than 22 cm. from the eye the condition is called presbyopia, and spectacles with convex lenses are required to bring back the near point to 22 cm. There are very many variations from the figures which I have given above ; the accommodation may fail much earlier or persist much later. I had a patient who was sixty-three when she consulted me, because she was apparently becoming short-sighted and could not recognise her friends across a street without making a special effort. She did not complain of head- ache, difficulty in reading or any symptom referable to the eyes and had never had any trouble with her eyes. On examination I found there was hypermetropia to the extent of two dioptres that is to say, a lens of two dioptres placed in front of the eye would bring the image of an object at a distance to a focus on the retina. The power of accommodation was therefore required in order to see any object distinctly, and therefore the apparent short-sightedness had occurred through temporary relax- ation of the accommodation. This case is an example, 32 PHYSIOLOGY OF VISION of which there are so many, of the existence of a defect accompanied by a compensating superiority which remedies the defect. There is another defect which is very common, that is astigmatism. If we have a curved surface which is equally curved all round, light coming from a point will be brought to a focus at a point, but if one meridian have a different curvature to the other their focal distances will be different and the rays will no longer be focussed at one point. The bowl of a spoon is an example of an astigmatic surface. We are, however, chiefly concerned with the defects of an emmetropic eye. I say emmetropic instead of normal, because an eye may have normal refraction and still possess many defects. In any optical instrument there are two things which have to be avoided as much as possible. They are chromatic aberration and spherical aberration. Both these defects exist to a considerable extent in the eye. It is curious that Euler objected to Newton's conclusion that achromatism in lenses was impossible because, as he said erroneously, the eye was achromatic. It is very easy to demonstrate that the eye is not achromatic. If we look at a spectrum in a spectroscope we find that it is impossible to have the red and the violet in focus at the same time, we are apparently short-sighted for the violet in comparison to the red, and we shall have to push the eyepiece of the spectroscope further in when viewing the violet. As a matter of fact, a normal DEFECTS OF THE EYE 33 sighted person is short-sighted in violet light. If certain test types which can be read easily at twenty feet but at no further distance be illuminated with blue-violet light it will be found that it is no longer possible to read them. This blue-violet light can be obtained most easily by screening a light with a blue glass in combination with a blue-green glass, which cuts off the red rays transmitted by the blue glass. It is also easy to notice the same condition on viewing a spectrum projected on a screen from a sufficient distance. When the red end is clearly visible as a rectangle the violet end appears blurred and irregular. Most persons must have noticed that a purple light rarely appears as a uniform colour, but either as a red light surrounded by a halo of violet or a violet light surrounded by a halo of red. A purple glass is particularly useful in order to demonstrate these phenomena, nearly the whole of the middle of the spectrum is absorbed whilst the red and violet at the extremes of the spectrum are allowed to pass. If we take the screen with a pin-hole in it and having placed the purple glass behind and also a source of light, we shall see that the pin-hole will either appear as a red dot surrounded by a halo of violet or a violet dot surrounded by a halo of red according as we accom- modate for the red or the violet. There is only one position in which the pin-hole can be seen as purple, and that is when the eye is accommodated for a point midway between the red and the violet. In these circumstances the pin-hole appears larger and not so 34 PHYSIOLOGY OF VISION well defined. All these phenomena can be explained on the view that violet light is brought to a focus sooner than red light that is to say, when the image for red is formed on the retina the image for violet is formed in front of it. We can demonstrate this with our purple illuminated pin-hole and a non-achromatic lens. It will be noticed that when we focus the rays coming from the pin-hole on a white screen that the screen has to be brought nearer the pin-hole in order to obtain the violet image than the red. Chromatic aberration has to be taken into considera- tion when dealing with all problems of colour. I have made mosaics of small pieces of coloured cardboard, using two colours only in each mosaic, the change in colour of the pieces forming the mosaic is very noticeable. It is necessary, therefore, when observing the phenomena of simultaneous contrast to make use of pure spectral colours only. The emmetropic eye appears to focus chiefly for the central or green rays. This may be demonstrated in the following way. If two sentences be painted on a black ground, the letters being formed by red and blue spots intermingled, one sentence being in red spots and the other in blue, an emmetrope will declare on looking at these from a distance that he sees only a confused mass of red and blue spots. This shows that the eye of the emmetrope usually focusses for the green rays namely, those which are intermediate between the red and violet. I have shown DEFECTS OF THE EYE 35 that in this position chromatic aberration is less evident than in any other. If, however, he be rendered slightly myopic by placing a weak convex lens in front of one eye he will at once read the sentence formed by the red dots. If, on the other hand, a weak concave glass be placed in front of his eye so as to make him hyper- metropic he will read the sentence formed by the blue dots. In the same way an uncorrected myope will read the red sentence, because the red rays, being less refracted, form the clearest image for him. The hyper- metrope will read the blue sentence best because the blue and violet rays being most refracted give him the clearest image. I find that most emmetropes call the purple light of my lantern purple ; whilst those who are myopic call it red ; those who are hypermetropic call it blue. Monochromatic Spherical Aberration or Astigmatism. In addition to chromatic aberration there exists in optical instruments in which lenses form a component part a second form of aberration, spherical aberration. The reason of this is that light of one colour emitted from a point is only brought to a focus approximately by an ordinary lens. There exists, however, certain curved surfaces which are called aplanatic, which reunite in a single point light emitted from a point. The curve for light coming from an infinite distance is an ellipse. It is obvious that when there is spherical aberration in a system which is situated round an axis the circle 36 PHYSIOLOGY OF VISION of diffusion which is formed round the image of a lumin- ous point will be a spot of which the greatest intensity will be situated in the axis. The monochromatic aberrations which are found in the eyes are not symmetrical around an axis, in fact they are asymmetrical and of a kind which should not be found in any well-made optical instrument. It will be seen, therefore, that the term spherical aberration when applied to the eye does not include all cases of mono- chromatic aberration, as there are other causes than defects in the curvature of the surfaces. The term astigmatism covers all causes. The irregular astigmatism caused by the lens may be seen by making a pin-hole in a piece of black cardboard. The cardboard should then be held just beyond the far point of the eye, a convex lens being used if necessary to bring the far point near enough to the eye. It will be noticed that there will be either several images of the pin-hole or there will be a star-shaped figure with four to eight rays. This is the cause of the star-shaped appearance of stars and the rays which are seen round street lamps viewed from a distance. Tears and the secretion of the Meibomian glands also cause unequal refraction. When tears are present they form a watery lens which is concave vertically and convex horizontally ; this causes the appearance of long rays. If we now have a luminous object instead of a point the images of diffusion will be formed in the same way, the bright parts combining to form an image. Most DEFECTS OF THE EYE 87 eyes can see two and some many more. For instance, I can in certain circumstances see images which are too numerous to count of an electric light filament. This is the monocular diplopia or polyopia. After removal of the lens a point no longer appears as a star-shaped figure, though any astigmatism due to varying curvature of the cornea remains. I find that the best method of observing the figure due to the stellate structure of the lens is to observe a number of lights arranged in a parallel series across a large open surface. The lights on the opposite shore of a lake which is not too large answer admirably for this purpose. When I close one eye and view the street lights in this manner each light has eight rays pro- ceeding from it, forming a star with eight rays. The pattern of each is exactly the same, showing conclusively that the star is not due to a retinal defect but to some cause which can affect the images of all the lights. It will be noticed that when a tear or any foreign matter passes over the cornea the images of all the lights will change in pattern. It will also be found that the images with the left eye have not exactly the same pattern as with the right. If we observe a concentric series of black and white circles each about one millimetre in breadth at an equal distance apart at a distance well within the power of accommodation, we shall see certain rays which appear light and defined. If we examine these rays we shall see that the lines forming the black and white circles 38 PHYSIOLOGY OF VISION are definite there whilst the adjacent portions are ill- defined and grey. Moving the figure from the eye also moves the position of the radiating lines. If we now accommodate the eye as for a distant object there will appear on the figure eight or ten sectors separated by fine black lines. I find that in addition to the appearances mentioned when I look at the disc so that the lines are not clearly visible the central area appears brighter, the brightest portion forming an ellipse round a central dark portion. Diffraction in the Eye. Diffraction is caused in the eye by the pupil as a whole, by any irregularities in the borders of the pupil and by any irregularities or small objects in the media of the eye in the course of the rays. The pupil acts like any small circular aperture, and the images of diffraction formed by its edges are circles alternately light and dark diminishing in brilliancy from within outwards. The images of diffraction formed by irregularities of the edge of the iris are fine rays brightly coloured. We can see these by looking through a very small aperture with irregular edges. It will be noticed that when the object containing the aperture is turned on its axis in a circular direction the fine rays will move round in the same way. Defective Transparency of Media. Any defect in the transparency of the media of the eye impairs the image. As may be seen entoptically. DEFECTS OF THE EYE 39 various cells, small portions of membrane, etc., are to be found in most eyes. The media of the eye may also transmit certain rays imperfectly, as, for instance, the blue when the lens is yellower than usual. The blood vessels are also situated in the path of the rays, with the exception of the non-vascular central portion of the retina. Defective Centering of the Surfaces. The various refracting surfaces are rarely absolutely correctly centred, but the deviation is usually so slight as not to cause much impairment of the image. The defects which I have described produce much less effect on the accuracy of vision than would be supposed for the following reasons. Any particular state which is due to internal and not external causes is soon ignored, as, for instance, the blood vessels of the retina, in fact we have to take special means in order to see them. We judge of differences in the appearances of the images as caused by differences in the stimuli produced by external objects. As we tend to ignore phenomena produced by internal causes I find that as a rule older men have much greater difficulty in seeing some new subjective phenomenon than younger ones. If, however, we pay special attention, some subjective phenomenon or defect becomes increasingly evident, until at last it may become very troublesome. There is another anatomical condition which has to be considered. The sensitive portion of the retina must 40 PHYSIOLOGY OF VISION not be considered as a uniform surface, but as a series of points which are placed further apart towards the periphery. The feeble parts of the image will not be intense enough to stimulate a cone, and any stimulation which is caused will be neglected. In nearly all dis- persion and diffraction images the centre is much more intense than the periphery. CHAPTER VII. THE ACTION OF LIGHT ON THE RETINA. The optic nerve is not directly sensitive to light, hence the existence of the blind spot in the field of vision. This blind spot occupies about 6 in the field of vision in a position corresponding to the optic disc. The appearance of Purkinje's blood vessel figures shows that the percipient layer of the retina must be behind the layer containing these vessels. H. Muller l found by calculating the positions of the shadows from their projection upon a scale that the rod and cone layer was the percipient layer of the retina. The following objective effects of light upon the retina have been recorded : 1. Bleaching and regeneration of the visual purple. 2. Alterations in the microscopic appearances of the retinal elements after staining. 3. Alterations in the chemical re-action of the retina. 4. Movements of the retinal elements. (a) Phototropic of the pigment epithelium. (6) Contraction of the cones under the influence of light. 5. Electrical changes. 1 Sitzungsber. u. Verhandel. d. physik.-med. Ges. Wurzburg, 1852 u. 1864. 41 42 PHYSIOLOGY OF VISION 1. Bleaching and Regeneration of the Visual Purple. In the outer limbs of the rods a purple substance is found which is sensitive to light. This purple is sensitive to monochromatic as well as to white light. It is bleached most rapidly by the greenish yellow rays, those to the blue side of these coming next, the least active being the red. This visual purple is found exclusively in the rods. Several observers had noticed that the colour of the rods was red or rose-red, and in 1876 Boll l made the important discovery that this colour was bleached by light. (1) Colour of the Visual Purple. The name visual purple would hardly convey to most persons a correct idea of the colour of visual purple. If the retina of a frog or a rabbit which has been previously kept in the dark be examined it will be seen to be pink or rose colour, which could easily be mistaken for .blood. The term pink or rose would much better describe the colour than purple. The visual purple of fishes inclines more to violet than that of mammals. The maximum of the absorption curve for fishes is found at A540/WM, that for mammals at X50(W. Kiihne stated that the visual purple in the first change of bleaching became yellow. I have never been able to see this yellow, the Colour has always bleached without changing in hue. This yellow colour must have been 1 Ber. d. Akad. Berlin, 1876. THE ACTION OF LIGHT 48 due to the admixture of blood, as when the whole retina is allowed to bleach it becomes more of an orange colour, but if a portion of the external segments of the rods be separated on a glass plate and allowed to bleach, it will be noticed that there is no qualitative change in the colour. Abelsdorff and E. Kottgen l find that with apes, rabbits and fregs the purple bleached without changing to yellow. Throughout the bleaching, the absorption maxi- mum remained in the same place, the absorption and the colour of the solution remaining qualitatively unaltered. (2) The Distribution of the Visual Purple. The visual purple is found exclusively in the rods. It does not follow from this, however, that it is not to be found in regions of the retina in which there are only cones, because the external segments of the rods are dipped in a fluid, which surrounds both the external segments of the rods and cones. The visual purple can therefore diffuse into this fluid and become distributed to every part of the outer layer of the retina. 1 2 have found the visual purple between but not in the cones of the fovea. When the retina was first examined the fovea was the reddest part of the whole retina. Kiihne also found the visual purple in a fluid form in one case, but he did not recognise the significance of the observation. He writes 3 : "In one of these eyes 1 Zeitschr. f. Psychol. u. Physiol. d. Sinnesorgane, 12, S. 161. 2 Trans, of the Ophth. Soc., 1902, p. 300. 3 The Photochemistry of the Eetina and on the Visual Purple, by W. Kuhne, translated by M. Foster, 1878, p. 34. 44 PHYSIOLOGY OF VISION which had been laid open in the dark for an hour, I saw to my great surprise the whole of the retinal mass flooded by a clear purple solution which, when poured upon a plate, exhibited the same behaviour as to light as the mass itself." This refers to a retina of a shark. (3) The Bleaching of the Visual Purple. The visual purple is bleached by monochromatic as well as by white light. Kiihne found that the visual purple was bleached most rapidly by yellow-green rays then by green, blue, green-yellow, yellow, violet, orange, red, in the order mentioned. With red light the bleaching is very slow. (4) The Regeneration of the Visual Purple. The visual purple is regenerated by the pigment cells of the retina, and this will take place in an eye which has been removed from the body, the bleached retina being again laid on the pigment cells. Victor Bauer 1 finds that not only is the visual purple decomposed and regenerated in daylight but that light is plainly a stimulus for its regeneration. In fact, he finds with a suitable light that an intensity can be found by which the regeneration of the purple colour in an eye which is exposed to light is clearly more rapid than in an eye which is kept in the dark. The experiments were made with frogs and white rabbits. Two similar animals were taken and their visual purple bleached by exposure to 1 Pfluger's Archiv., 1911, S. 490. THE ACTION OF LIGHT 45 direct sunlight, one was then placed in a dark room and the other left with its eyes exposed to light of moderate intensity, either daylight or the light from a glow-lamp. When the retinae of both were examined and compared, it was found that the purple colour was more definite in the eye which had been kept in the light. (5) Chemical Characters of the Visual Purple. It is soluble in a solution of the bile salts. The purple fluid thus obtained re-acts to light in the same way as normal visual purple. (6) Optograms. Kiihne found that he could take photographs with a rabbit's eye by means of the visual purple. A window with its bars was focussed on a rabbit's retina. This was left from two to seven minutes. Then on examina- tion the parts of the retina corresponding to the light parts of the window were bleached, whilst that corre- sponding to the bars of the window and frame was only slightly affected. This image was then fixed by drying, which greatly retards the bleaching of the visual purple. (7) Fluorescence of the Decomposition Products of the Visual Purple. The rod and cone layer of the retina fluoresces strongly in ultra-violet light. The purple itself is not the cause of this fluorescence, because the bleached retina fluoresces more strongly than the unbleached. It is not the 46 PHYSIOLOGY OF VISION substance of the rods which fluoresces, but the decom- position products of the visual purple. 2. Alterations in the Microscopic Appearances of the Retinal Elements after Staining. The appearances after staining have been found to be different with a light and dark adapted eye. For instance, the cones have been found with the same stain to be green in the light adapted eye and yellow in the dark adapted eye. 3. Alterations in the Chemical Reaction of the Retina. The ordinary reaction of the retina is alkaline ; this is changed to acid by the action of light. 4. Movements of the Retinal Elements. (a) Phototropic of the Pigment Epithelium. When an eye has been exposed to light, processes of the pigment epithelium are thrown out between the rods and cones until in the maximal position they are restrained by the external limiting membrane. This maximal position is reached after five or ten minutes exposure to bright sunlight. When the eye has been kept in the dark the processes of the pigment cells are retracted into the body of the cell ; this dark position of the pigment cells is only reached after the eye has been kept for two hours in total darkness. The most re- frangible portion of the spectrum causes a much more marked forward movement than the less refrangible THE ACTION OF LIGHT 47 portion ; red light in particular is very weak in this respect. (b) Contraction of the Cones under the Influence of Light. Van Genderen Stort l found in the light-adapted eye of the frog that the cones were situated close to the external limiting membrane, whilst in the dark-adapted eye the cones were extended and were found to be quite close to the pigment cells. Engelmann 2 found that the movement of the cones, like that of the pigment cells, is dependent on the nervous system. 5. Electrical Changes. (a) Current of Rest. An eye connected with a galvanometer by means of non-polarisable electrodes shows a positive current, that is a current passing in a direction from the fundus of the eye to the cornea. Waller 3 regards this so called current of rest as a current due to the mechanical injury of the eye. (b) Current of Action. When light falls upon the eye, there is usually a short latent period amounting to 0-2 of a second, then an increase in the positive current, the amount depending on the degree of illumination. A small negative varia- tion at the commencement is often found. Waller and 1 Onderzoek. Physiol. Lab. Utrecht (3), 9, 145. 2 Arch.f. d. ges. Physiol, 35, (1885). 3 Quart. Journ. of Exp. Physiol., 1909, p. 170 48 PHYSIOLOGY OF VISION Gotch find that light of all wave-lengths causes effects of a similar character, dependent upon the degree of illumination, that is to say, all produce like white light an increase in the positive direction. When the light is shut off there is a still further increase in the positrv e current, that is to say, the effect of darkness after light- is very similar to the effect of light after darkness. Gotch x finds that : (1) Green light evokes a response characterised by a comparatively short latency, and that this reaches the highest maximum. (2) Red light evokes a response of the same sign, but characterised by much longer latency, and that whilst it also reaches a high maximum, it falls a little short of the green effect. (3) Violet light evokes a response of the same sign characterised by a latency shorter than that caused by red but longer than that caused by green, and especially characterised by its smaller amount. Einthoven and Jolly 2 find the latency in the photo- electric reaction of the frog's eye is in complete agree- ment with the latency of light perception in the human eye. The period of latency is much longer with weak than intense light. The effects of light on the retina are easily explained in accordance with the theory that the visual purple is 1 Journal ofPhysiol, vol. xxi. no. 1, p. 26, March 29th, 1904. 2 Quarterly Journal of Experimental Physiology, 1908, p. 409. THE ACTION OF LIGHT 49 the visual substance. The contraction of the cones and the movement of the processes of the pigment cells prevent as far as possible the decomposition of the visual purple in the liquid surrounding the cones. In darkness it is necessary to have as large an area of stimulation for the cones as possible, and also to promote as far as possible the free distribution of the visual purple. CHAPTER VIII. THE ORIGIN OF VISUAL IMPULSES. Very little is known at present of the exact nature of visual impulses and of their origin. The position where visual impulses first arise has been found through calculations based upon the Purkinje blood vessel shadows being projected upon a graduate scale in varying positions of the light. This has been ascertained to be the external third of the retina, that is to say in the layer of rods and cones. On my theory of vision it will be seen that we have a thin layer of photo-chemically sensitive fluid corresponding to the sensitive plate of the camera. The visual purple is in every respect suitable for the visual substance, and would have been accepted as such, had not Kiihne ascertained that it was not present in the cones. As only cones are present in the fovea, the region of most distinct vision, it was not considered essential to vision. This difficulty is entirely obviated by the theory of the relative functions of the rods and cones which has been propounded. Many physiologists have tried to assign different functions to the rods and cones, but all these theories have failed 50 ORIGIN OF VISUAL IMPULSES 51 because all the functions which were said to be the exclusive property of the rods have been found only gradually diminished in the fovea. For instance, Von Tschermak, Hering, Hess, Garten, and I have found the Purkinje phenomenon, the variation in optical white equations by a state of light and dark adaption, the colourless interval for spectral lights of increasing intensity, the varying phases of the after image, in the fovea, only gradually diminished. The complete absence of any, qualitative change between the foveal and .extra foveal regions is a very important fact in support of the hypothesis that the visual purple is the visual substance. There may be other photo-chemical substances in the retina, but there is not the slightest evidence that such is the case. We could, of course, split the visual purple into innumerable simpler photo-chemical substances, each of which had its own absorption curve, having its maximum in some particular part of the spectrum. It is difficult to say at present exactly how the visual purple acts as a stimulus transformer, but this is because so many plausible hypotheses immediately occur to us. It is very probable that light acting upon the visual purple is according to its wave length absorbed by particular molecules, the amplitude of their vibrations being increased. These vibrations may cause corre- sponding vibrations in certain discs of the outer segments of the cones, which seem especially constructed to take up vibrations. We know that when light falls on the retina it causes an electric current. We know how the 52 PHYSIOLOGY OF VISION telephone is able through electricity to convey waves of sound, and something similar may be present in the eye, the apparatus being especially constructed for vibrations of small wave length. The current of electricity set up by light may cause the sensation of light and the vibrations of the atoms or molecules the sensation of colour. The theory of Nernst as to the origin of nervous impulses may be applied to visual impulses. He holds that nerve impulses are caused by the accumulation of ions against a membrane. The decomposition of visual purple may set free ions which, impinging upon a membrane covering some portion of the cones may set up a visual impulse. In all vital processes there is a condition of katabolism or chemical change in the protoplasm and an anabolic or building up process in which the protoplasm is restored to its normal state. We have therefore to con- sider two definite processes in the visual purple namely, a breaking down of the visual purple photo-chemically by light and its restoration by the pigment cells and rods. Under ordinary conditions of light and during the whole of the day time, the visual purple is con- tinually being bleached and reformed. It is obvious, therefore, that when the eye has been kept in the dark and is then exposed to light an observation taken immediately will not be comparable with one taken a few seconds afterwards, because in the first observation we have only to consider the katabolic change, whilst in the second observation the anabolic change has to be ORIGIN OF VISUAL IMPULSES 53 considered as well, as the visual purple has to be reformed for subsequent seeing. There appears to be very little evidence in ordinary circumstances of this anabolic process. The retina, therefore, corresponds to a layer of photo-chemical liquid in which there are innumerable wires, each connected with a galvanometer. When light falls upon a portion of this fluid the needle of the galvano- meter corresponding to the nearest wire is deflected. The wires correspond to the separate fibres of the optic nerve, and the galvanometers to the visual centres of the brain. It may be well here to deal with the objections which have been raised to the visual purple being the visual substance. The chief objection namely, that it is not present in the fovea has already been disposed of by finding that although it is not present in the cones it is present around them. The other objections have on examination very little weight, and are mostly based on erroneous statements. They are that animals such as frogs, naturally possessing the pigment, continue to see when their visual purple has been absolutely bleached, as it may be by prolonged exposure to strong light, and that the visual purple is entirely wanting in some animals which see very well. On the first point it may be well to quote Kuhne. He writes : ' Without direct sunlight I have never succeeded in a room in obtaining frogs' retinae free from visual purple, not even when I allowed the animals to leap about before the window the whole day long." My contention is that in those 54 PHYSIOLOGY OF VISION cases in which the retinae were bleached by sunlight and the frogs were still able to see, there was sufficient visual purple or its decomposition products for vision, but not enough for external recognition. The experimental conditions are in these cases so difficult as to make it quite impossible for anyone to say that no visual purple was present. I have again and again examined the retinae of animals which contained visual purple, as, for instance, monkeys, and in many cases failed to detect any even when the animal has been kept in the dark for a considerable period before the retina has been examined. The same applies to the second objection, and where animals have been stated to have no visual purple or no rods, subsequent observers have found both. It was stated formerly that the invertebratae do not possess visual purple, but it has been shown that in the eye of the butterfly there is a photo-chemical sub- stance which is rapidly bleached and reformed. The bat was formerly stated not to possess visual purple, but Trendelenburg % has written two papers on the Sehpurpur auf die Fliedermaus. It was formerly stated that a tortoise possesses only cones, but on examining the retina of a tortoise I found the rods and cones as definitely marked and distinct from each other as in man. Even if there were no visual purple the argument fails, because there might be some other means of stimu- lating the cones. It is again interesting to quote Kiihne on this point : " According to all appearance the visual purple in a bird's eye is deficient in proportion as the ORIGIN OF VISUAL IMPULSES 55 retina is provided with other more stable means of absorbing coloured light, I mean the coloured globules of the cones. This is at least the case in the nocturnal and predaceous birds, and is entirely true as regards the pigeon and the fowl." It would seem likely that the photo-chemical processes necessary for vision must have an elaborate nervous mechanism which could only arise from the retina. The peculiar arrangements of the rods seem to indicate that these are the elements involved. Kuhne writes : ' It must not be forgotten that the retinal epithelium of the choroid has been proved to be a very important physiological or chemical constituent of the retina, or, stated shortly, as a purpurogenous gland, the cells of which can scarcely fail to possess a very peculiar compli- cated innervation. Now I do not see that irritable fibres can arise from other source than from the nervous mass of the retina." It does not appear to have occurred to Kuhne that the rods were the nervous elements for which he was seeking. He looked for some direct connection between the rods and the pigment cells. We know that in other parts of the nervous system it is sufficient to have contact between two cells without direct continuity. CHAPTER IX. LIGHT AND DARK ADAPTATION. The importance of the change which takes place when a person goes from the light into a dark room has only in the last few years been recognised. During the day and in a bright light the eyes are in a state of light adaptation, that is to say they are adjusted so as to be able to see in a bright light. When the light is feeble a different state of the eyes is required, and this state is called that of dark adaptation. Most are aware of the difficulty of seeing when we go from a brightly to a dimly lighted room. A certain amount of time is required before we can see objects in the room, but on remaining in the room the eyes become more and more sensitive to light until we are able to see quite clearly objects which were invisible when we first entered the room. This increase of sensibility by the state of dark adaptation has been shown by Piper l to vary very considerably with different persons. Various observers have noticed an increased sensi- bility of the eye through the exclusion of light for 1 Zeitschr. f. PsychoL d. Sinnesorg. 31, S. 161. 56 LIGHT AND DARK ADAPTATION 57 mechanical and electrical stimuli and for ultra violet, Eontgen and Becquerel rays. On the theory of vision which I have given many explanations of dark adaptation are possible, all of which are consistent with the facts. The accumulation of the visual purple in the liquid surrounding the cones may be sufficient, the liquid becoming more and more sensitive to light as the percentage of visual purple in it increases, or some additional substance may be added to the fluid making it more sensitive to light. This may be caused by the withdrawal of light from the eye or by hormones formed because of the reduction of light on other parts of the body. Again the withdrawal of light may cause a mechanical arrangement which is specially favourable to photo-chemical decomposition by a weak light. Perhaps all these causes are in operation. We know that when the eye is in the dark the processes of the pigment cells with their pigment are retracted and the cones are also pushed further away from the external limiting membrane. This would increase the size of the space and also the area of the surface of stimulation of the cones. In the case of a light adapted eye the opposite is the case, the pro- cesses of the pigment cells with their pigment being pushed forward and the cones being retracted against the external limiting membrane. This would have the effect of isolating impressions and preventing the free flow of the visual purple, thus requiring a greater stimulus. We know that light acts as a stimulus to 58 PHYSIOLOGY OF VISION the regeneration of the visual purple, and therefore though more is used up more is formed. When we go from a dark to a light room, the dazzle and excessive sensation of light are just what we should expect through an accumulation of photo-chemical substance. CHAPTEK X. VISUAL ACUITY. If two small points of light fall upon the eye, unless these are a certain distance apart they will appear as a single point of light. The region of the fovea is that at which two points can be distinguished with greater ease than any other part of the retina. The limit of visual acuity is usually given as 50 ", though with strong contrast some observers have been able to distinguish two points when even closer together than that. Two points which are easily distinguished as distinct when seen centrally rapidly become one when viewed with the peripheral part of the retina. Dor gives the diminution of the visual acuity from the fovea to the periphery of the retina as follows : The centre of the fovea being counted as 1, at 5 outwards the visual acuity is J 10 i 55 55 55 55 | f> 15 55 55 55 55 3\> To "V 55 5? 5U 30 ^ " 55 55 TOT) * 4-0 i 5? jj j? 55 "2"(TT5"* 60 PHYSIOLOGY OF VISION The Influence of Stimulation of the Periphery of the Retina upon Vision with the Centre. (1) If we look at two small isolated stars of equal magnitude either may be made to disappear by looking fixedly at it, whilst the other remains conspicuously visible. The phenomenon is most marked on a dark night, and when the star looked at is in a portion of the sky comparatively free from other stars, and when only one eye is used. On a very dark night a considerable number of small stars, occupying the centre of the field of vision, may be made to disappear, whilst stars occupy- ing other areas of the field of vision are plainly visible. (2) Other lights or objects, when small and with dark surroundings, as, for instance, a piece of white Cardboard on black velvet, may be made to disappear in a similar manner. (3) No change can be observed if a very bright light, a group of stars, or a uniformly illuminated surface be made the subject of the experiment. (4) If in a dimly lighted room a piece of black velvet about 3 feet square be fastened upon a door and in the centre of the velvet a pin be inserted so that the head faces the observer, the head of the pin is a conspicuous object surrounded by the black velvet. If it be looked at fixedly with one eye it will disappear, and after a few seconds the whole of the black velvet and door will disappear, and the visual field becomes considerably contracted, the wall-paper on either side of the door appearing to unite. VISUAL ACUITY 61 (5) The darker the surroundings, the brighter will be the light which is made to disappear. When these experiments are made with a lantern, the result is very startling. One moment we are looking at a bright light, and the next have the sensation of having become quite blind, the sensation of absolute blackness being greater than can be obtained in any other way. Just before the light disappears, it appears to pulsate, appearing and disappearing as if a rose diaphragm were shut and opened before it, except that the re-appearance of the light is always from without inwards. The same appear- ance may be seen with the stars on a dark night. The chief essentials in making a bright light disappear are to use only one eye, and to have the surroundings of the light absolutely dark. In my experiments which I made at night I used a specially constructed lantern, which I viewed through four open doors, so that all extraneous light could be as far as possible excluded. (6) If a small light be viewed in a dark room with one eye, the eye will continually move. If, for instance, a white spot be made on dark paper, it will be found that the eye has much less tendency to move when the white spot is surrounded by a number of other white spots, and in these circumstances will also be seen brighter. The facts of visual acuity may be explained on the theory of vision as follows : The images of a series of parallel white and black lines will fall upon the retina and decompose the photo-chemical liquid there. 62 PHYSIOLOGY OF VISION o o o o c c FIG. 2. Fig. 2 is a drawing which I have made from a micro- scopical preparation of the external surface of the fovea. It will be seen that the cones are arranged in nearly parallel lines but with a slight curvature inwards. Each cone is separated by a space which is about the diameter of a cone. The images of two parallel black lines are repre- sented on these cones. It will be seen that the cones will be stimulated according to the degree of photo-chemical decomposition of the liquid surrounding them. M. Schultz gives the diameter of a cone in the fovea as from '0020 -'0025 mm., H. Muller '001 5 -'0020 mm., Welcker '0031 -'0036 mm. With Listing's schematic eye a visual angle of sixty seconds corresponds to an image of '00438 mm. on the retina, though the diffusion circles make the image cover an area ten times as large, but the intensity of the image of a point of light is greatest in the centre of the circle and diminishes towards the circumference. It will be seen that in order that two points of light should be distinguished as two instead of one, at least one cone must intervene between those which are stimulated by the points of light, that is to say, a cone which is not stimulated intervening between two which are stimulated gives rise to a sensation of black- ness. It will be seen from Fig. 2 that a series of parallel black and white lines will be seen either as a chess board pattern or if larger as beaded. The fact that this is so was pointed out by Purkinje. The result of light VISUAL ACUITY 63 impinging upon the retina in the space between two cones would result in the stimulation of both these cones equally if the image of the point of light were at an equal distance from both, but one would be stimulated more than another in proportion to the distance from it of the image of the point of light. On the older view- namely, that the cones were directly stimulated by light- there should be evidence of absence of sensation when the light falls upon the intervening spaces between the cones. No evidence of this has been adduced. CHAPTER XL POSITIVE AND NEGATIVE AFTER-IMAGES. Positive and negative after-images are designated as such, in the photographic sense : that is to say, in the positive after-image the light parts correspond to the light parts of the object and the dark parts correspond to the dark portions of the object. In the negative after-image the reverse is the case, and the light parts of the object are seen as dark and the dark parts are seen as light. A positive after-image is seen very well in the morning ; if on awaking the eyes be directed towards a window and then immediately closed and covered with the hand, a positive after-image is seen of the window. If the window be subtended by a white cloud the light parts of the window appear rose-coloured and the bars dark. If a coloured object be fixated for ten to twenty seconds and then the eyes be directed towards a uniform white surface a negative after-image is seen of approximately complementary colour. In ordinary circumstances a period of short duration and subsequent occlusion of the light favours the development of the positive after-image, whilst a period of long fixation and subsequent stimu- 64 AFTER-IMAGES 65 lation by light favours the development of the negative after-image. 1. Separation of the Positive and Negative After-images. Both eyes must be carefully covered with the hands for several minutes whilst the right eye is directed forwards so that when the hand is removed a small triangular piece of white paper on a large sheet of black paper is visible. The small piece of white paper should be illuminated by the sun. On removing the hand and then immediately replacing it so that the piece of paper is visible for the shortest possible period of time, a tri- angular positive after-image of the paper is seen ; this first appears a dazzling white, then becomes violet, which gradually becomes more and more purple, and then fades away without becoming negative, and lasting for a period of from 8 to 12 seconds. On letting light through the lids the image is seen as dark (negative). If, when the after-image has existed for two seconds and when particular note has been taken that there is no other after-image in the field of vision, a sharp quick jerk be given to the head, down and up, the after-image will apparently move upwards in the field of vision. It will no longer be triangular but appear as an irregular circle with portions detached. If now the fingers be moved very carefully so that a certain amount of light enters the eye through the lids a clear cut triangular image will be seen in the field of vision in the original position of the after-image and below its secondary 66 PHYSIOLOGY OF VISION position. The image appears as a dark triangular portion on the reddish-yellow field formed by the light through the eyelids. Whilst the positive after-image has moved the negative after-image is found in the original position. 2. Two Negative After-images from one Light Stimulus. If the above-mentioned experiment be repeated and the positive after-image be allowed to remain in its secondary position for two seconds before allowing light through the eyelids two negative after-images will then be seen, one triangular in form and very faint corre- sponding to the primary position of the after-image and one much darker .and irregular in shape corresponding to the secondary position of the after-image. 3. Blending of Positive After-images. If on awaking, one eye (the other being covered) be directed towards three windows in a line and separated by portions of wall and the eye be immediately closed and covered with the hand and after one or two seconds the head be shaken from side to side, the light parts of the after-image will blend into one uniform rectangle of light, the dark parts disappearing. On allowing light to enter through the eyelids a negative after-image of the windows is seen in the original position occupied by the positive after-image. If, however, the head be immediately shaken after the perception of the positive after-image the negative after-image only appears as AFTER-IMAGES 67 an ill-defined portion corresponding to the blended after-images. 4. Subjective Effects of Distributed Photo-chemical Material. If a positive after-image be obtained for one eye and the light immediately excluded, on shaking the head the after-image is apparently distributed over the retina. I have then found that four definite subjective appear- ances may be seen, though it is rare to see more than one of these at the same time. (a) The Star-figure or Portions of it (cf. Fig. 10). In this case when the centre is seen the whirling in this portion described on page 90 is very noticeable. When the outer part is seen this forms a wide-meshed network of bright lines. (b) Cone Figure (cf. Fig. 7). The appearance corre- sponds to the outer aspect of the cones of the retina that is to say, numerous bright points in the centre and rather larger circles separated by wider intervals exter- nally. 1 (c) Checkered Figure. The greater part of the visual field may appear as if covered by small squares alter- nately light and dark like a chess board ; these squares in the centre are very small, not much larger than the points just described, only they appear square instead of circular. (d) Brush Figure. A number of short bright lines 1 When red light is used, the points are all bright red. 68 PHYSIOLOGY OF VISION all over the visual field may be seen. These lines have a length about six times their diameter. All are pointed forwards like a brush viewed from the bristle side, the lines are not, however, parallel to each other except in different regions of the visual field. They all point forward, but at different angles in different regions. 5. After-image of Mosaic of Red and Blue Squares on a White Ground. If a mosaic of pieces of red and blue cardboard of about a centimetre square be pasted on a white ground and then a positive after-image be obtained in the usual way, it will be noticed that the positive red after-image disappears much more quickly than the blue. The colours also disappear more rapidly than the light effects, the mosaic being surrounded by white appears as a positive after-image dark on a white ground long after the colours have disappeared. No detail is seen in the mosaic, only a uniform dark rectangle. In this method the period of light stimulation being as short as possible it is difficult even if bright sunlight be used to obtain a complementary coloured after-image. The after-image of the whole, including the white surround, often becomes purple. This is particularly noticeable when the eye has been covered with the hand for a little longer than usual before making the experiment. When a spectrum on a screen is made to disappear suddenly it will be noticed that the colours do not all vanish at once. The red disappears before the blue. AFTER-IMAGES 69 This explains the fluttering heart illusion. When a mosaic similar to that mentioned above is moved from side to side in a bright light, preferably sunlight, the blue appears to slide over the red. This is explained by the fact that the blue positive after-image is more persistent than the red. There is another illusion in connection with this mosaic which I have not seen previously mentioned. When the mosaic has been moved from side to side two or three times, the whole of each line formed by the red and blue squares appears as if tilted forwards at an angle of about 30. It appears as if we were looking at a series of troughs, each formed by a line of blue and red squares, instead of a plane surface. 6. Negative After-image retains its Shape and does not move. If a negative after-image of a coloured light, e.g. red, be obtained in a dark room the blue-green after-image moves with the eye, but does not change its relative position in the field of vision. On shaking the head it disappears, but on keeping still it reappears by degrees at exactly the same point and is of exactly the same shape as before, it gradually disappears without changing colour. 7. Star Figure in After-image of Spectral Red. When a negative after-image of spectral red has been obtained the star figure in red (cf. Fig. 10) is often seen in the centre of the blue-green negative after-image. ?0 PHYSIOLOGY OF VISION 8. Certain Phases of the Positive After-image. If two rectangular strips of white paper about three inches long and a third of an inch wide be placed on a piece of black velvet and separated by a distance of an inch, definite positive after-images may be obtained of the two strips by viewing them with one eye, the eye being directed to a point midway between the two strips of paper, the other being closed and covered with black velvet, for the shortest possible time the eye being simply opened and closed. Two clear cut positive after-images will first be seen ; these rapidly become blurred and gradually approach each other, the central portions of each appearing to bulge towards each other and to combine first ; the upper and lower portions disappear first, the two after-images gradually combine in the centre of the field of vision, the last phase being a white circular blur, which slowly disappears with a whirlpool movement. It will be noticed that the after- images do not become negative. THE NEGATIVE AFTER-IMAGE OF BLACK. The negative after-image of black is green, not white. This can be seen very easily by regarding intently a black object and then looking at a white one, as at the snow. The best black object is a hole from which no light can be reflected. The fact that the after-image of black is green gives an explanation of a pretty little toy which excited much interest a few years ago, AFTER-IMAGES 71 Benham's top. This consists of a disc divided into two semi- circles, one white and one black : on the white half there are black lines at intervals and at different distances from the centre. When the top is turned so that the black half precedes the white one the black lines nearest the black half appear red. This red is the contrast colour to the green which is seen as the after- image of the black semi-circle. The after-image of black varies with the intensity of the light employed : in a feeble light it is white, raise the illumination it becomes yellow, raise it again it becomes yellow-green, increase it further it becomes pure green, try the experi- ment in the sunlight and it becomes blue-green. CHAPTER XII. THE TIME RELATIONS OF VISUAL SENSATION. It lias not yet been ascertained whether there be a stimulus of sufficiently short duration not producing an effect upon the eye. An electric spark which has a duration of a very small fraction of a second produces a very definite effect upon the eye. The Effect of a Stimulus of Short Duration. If a bright object be rapidly moved in an absolutely dark room or if a bright object be shown for a fraction of a second, six definite stages will be seen before the visual field resumes its normal appearance. (1) The primary image. (5) A tertiary image. (2) A dark interval. (6) A dark phase which (3) The secondary image. lasts until the visual (4) A second dark interval. field resumes its normal appearance. The secondary image has been the subject of much discussion. It was first observed by Purkinje. 1 It appears in an interval of about one-sixth to a quarter of a second after the commencement of the primary 1 Purkinje, Physiol d. Sinne, vol. ii. p. 110. 72 TIME RELATIONS OF SENSATION 73 image. Von Kries has stated that the secondary image is not to be found at the fovea. But this appears to be due to his method of experimentation, as a retardation of the image would produce exactly the same effect. Hess * points out that in the foveal region the secondary image is bent outwards. (See Fig. 3.) I quite agree with Hess that the secondary image can be seen in the fovea, and this can be demonstrated easily in the following way : If a strip of white paper be moved backwards and forwards" over a dark background in a dimly lighted room it will be noticed that after a dark interval the piece of white paper is followed by the secondary image, which becomes bent outwards when the central portion of the field of vision is reached. Hess also finds that the secondary image is seen with red light. The following method 2 shows still more conclusively how the recurrent image can be seen in the foveal region. A series of small electric lights should be arranged in a straight line in a dimly lighted room and the observer be situated at such a distance that the image of the 1 Pfluger's Arch., Bd. 95, 1903. 2 Journal of Physiology, vol. xlv. Nos. 1 and 2, page 73. 74 PHYSIOLOGY OF VISION centre light falls upon the foveal regions of his retinae. On closing one eye and covering that eye with the hand the centre light is carefully observed whilst another person turns out all the lights simultaneously ; these are allowed to be visible for about a second. Four definite stages will be noticed, that is to say, 1. Con- tinuation of the sensation. 2. Period of darkness. 3. The recurrent image. 4. Period of darkness (negative after-image), in which the details of the objects, such as the outline of the filament, can be noticed. In the second stage both the light and the dark parts of the object appear dark and no details are observable. It will be seen that the recurrent image is more marked in the foveal region than anywhere else, in fact, it might be missed in any other part. The above phenomenon can be seen very easily with the electric advertising signs which are so common and in which a number of illuminated letters appear and disappear at short intervals. The fourth stage lasts longer than the third and the fifth much longer still. Hess gives fifteen seconds as the duration of the six stages, the last ten seconds are occupied by the sixth stage. The curving of the secondary image can be explained by a retardation of the sensation in the foveal region. Hess lays particular stress upon the fact that the phases, especially the three bright and three dark phases, are similar for the region in which there are only cones to that in which rods are found. TIME RELATIONS OF SENSATION 75 The Effects of Intermittent Light. When light falls upon the eye the primary image occupies a certain definite time. If two stimuli fall upon the eye at an interval so short that the primary effect of the first stimulus has not passed away before the second stimulus impinges upon the retina, the result is a continuous sensation. This is the fact upon which cinematography is based. When the interval between the two stimuli is not sufficiently long flicker is noticed, and the appearance or disappearance of this flicker depends upon certain conditions. (1) Influence of the intensity of the light. If an intensity of light which requires flashes to succeed each other at 18*96 per second be taken, then at 4 times this intensity Baader finds that the rate will have to be 24*38 per second, for 18 times 29'84 ; for 193 times 41-31 ; for 1800 times 50*24 per second. (2) Variation of the surroundings causes a change in the rate at which flicker disappears. 1 (3) T. C. Porter 2 has shown " that the apparent luminosity of two flickerless discs under the same illumination may be precisely the same, and yet the angle of the black sector may be different in the two discs ; indeed, one disc may be entirely white and the other may have a black sector by no means small under some conditions of illumination, and these 1 tSherrington : Journal of Physiology, 1897, vol. xxi. p. 33. 2 Contributions to the study of Flicker, Proceedings of the Royal Society, A, vol. Ixxxvi. p. 502, 1912. 76 PHYSIOLOGY OF VISION two may, to the eye, form a perfect match when flickerless." (4) Flicker disappears sooner from the central than the peripheral parts of the retina. Exner l and Char- pentier 2 also found that the size of the retinal image was a factor in the disappearance of flicker. When the size of the retinal image was increased the point of fusion was raised. Billarminoff 3 found a higher fre- quency necessary for fusion on the nasal than on the temporal half of the retina with certain kinds of light. 1 Sitzungsberichte der K. Akademie der Wissenschaften, Wien, 1868* Band Iviii. Abth. 2, S. 601. 2 Archives d'Ophtalmologie, Paris, 1890, tome x. p. 340. 3 Archivfiir Ophthalmologie, 1889, Band xxxv. Abth. 1, S. 25. CHAPTER XIII. VARIATIONS IN LIGHT SENSATION. Purkinje Phenomenon. Purkinje stated that if a red and blue of equal intensity be diminished in the same proportion, the blue appears much brighter than the red. This is easily noticed as twilight approaches, red flowers become dark or black whilst blue flowers are conspicuously visible. The same change is noticeable in the colours of a stained glass window. If a spectrum be gradually diminished in intensity the point of maximum luminosity gradually shifts from about the region of the D Line to the blue side of the E Line with a spectrum of very diminished intensity. Konig gives A535 for the weakest light, X615 for the strongest. Bering 1 finds that the state of adaptation of the eye is the chief factor in the production of the Purkinje phenomenon. He finds that the intensity of an equally bright red and blue may be reduced without altering the relative luminosity. The luminosity of the equally bright red and blue may be left unchanged, and yet the Purkinje phenomenon will appear if the eye be dark 1 Arch.f. d. qe,s. PhysioL 1895, S. 519. 77 78 PHYSIOLOGY OF VISION adapted. He lays particular stress upon the alteration in the saturation of the colours. The matching of spectral colours with white light will vary in the results according to the state of dark adapta- tion of the eye. ThePurkinje phenomenon is a photo-chemical phenome- non, and is found with other photo-chemical substances. All know that we can recognise red light at night when the eye is dark adapted, and I find this is the case with the extreme periphery of the field of vision. The Purkinje phenomenon is found for localised areas of the field of vision, that is to say, when the eye is fixated upon a black object this region becomes locally dark adapted and the Purkinje phenomenon is found with it. If we look at a red and a blue object through a pinhole, taking care that the eye does not become dark adapted, if the red object be brighter than the blue in ordinary circum- stances it still appears brighter through the pinhole. In the Centre of the Field of Vision. A hole of about 2 cms. (in diameter) is made in a door between two dark rooms ; the hole is filled with two glasses, one red and one blue, placed one above the other so that half is red and half blue, the red being decidedly in ordinary conditions the brighter of the two ; a lamp with an obscured glass moveable on a bench is arranged immediately behind the two glasses. When the lamp is close to the glasses the red is decidedly brighter than the blue in all positions of the eye. When the lamp is VARIATIONS IN LIGHT SENSATION 79 moved a certain distance away from the glasses, it will be noticed that, whilst the red is the brighter of the two when the image falls on the fovea, on moving the eye so that the image falls peripherally the red is seen very dark or black and the blue is seen as a bright white light. On moving the lamp still further away it will be noticed that the blue will be seen the brighter of the two even with the fovea. This experiment reconciles the state- ments of those who declare that the Purkinje pheno- menon is to be found in the fovea with those who declare that it is to be found in the periphery and not in the fovea ; results depend on the intensity of the light employed. Simultaneous Contrast. There are two kinds of simultaneous contrast, simul- taneous luminosity contrast and simultaneous colour contrast. Simultaneous Luminosity Contrast. When the same grey is placed upon black and upon white it appears darker upon white than it did upon the black. When a black square is placed upon a white surface and regarded intently without moving the eyes it will be noticed that the black appears to become lighter. A number of fine white particles appear to invade the black surface and make it become lighter and lighter. Simultaneous Contrast is most marked where the contrasting surfaces touch, and appears to be due to an exaggerated perception of relative difference. CHAPTER XIV. VARIOUS VISUAL PHENOMENA. 1. Appearances in the Field of Vision due to Peculiarities of the Yellow Spot. 1. Loewe's Experiment. If we look at a clear white surface through a solution of chloride of chromium, which is of a celadon green colour, we shall see a purple spot in the centre which varies in size and shape with different persons. On careful examination it will be found to consist of three portions, the centre corre- sponding to the fovea appears as a bright purple disc, the middle corresponding to the non-vascular portion of the yellow spot appearing as a dark green ring, and lastly Loewe's ring corresponding more or less accurately to the outer feeble yellow portions of the yellow spot, which appears as a purple ring surrounding the central portion and possessing a diameter twice or three times as large. 2. Apparent Size of Regions of Yellow Spot in Field of Vision. The region corresponding to the yellow spot occupies a considerable area in the field of vision. I find that the visual angle which any portion subtends can be easily measured by projecting the after-image of an so VARIOUS VISUAL PHENOMENA 81 object occupying a known visual angle on the sky above a house. A comparison is easily made in both cases with portions of the house. If an observer look straight at the cloudless sky, preferably about three hours before the sun sets, for about ten to twenty seconds, a disc or an ellipse with the long diameter horizontal, and which is to me of about nine degrees, appears in the sky. Inside this is a central darker portion of about two degrees in FIG. 4. each diameter, and this surrounds a central brighter portion which corresponds to the point of direct vision. This is about 40 to 50 minutes in diameter and is circular in form. (See Fig. 4.) Helmholtz x gives a disc of the diameter of about 40 to 50 minutes as the apparent size of the fovea in the field of vision and the diameter of the aureola, which corresponds to the non-vascular portion of the yellow spot and is the portion where the yellow is most intense, 1 Physiol. Optik, p. 567. F 82 PHYSIOLOGY OF VISION as about two degrees. This is correct for me. The remaining portion which corresponds more or less exactly to the remainder of the yellow spot is Loewe's ring. 3. Appearances of Central Region with Different Inten- sities of Light. Exner 1 points out that with feeble light the central region is seen as a dark disc surrounded by Loewe's ring, and that as the intensity is increased this region becomes brighter and brighter until it becomes more luminous than its surroundings, the fovea being specially noticeable as a bright disc of greater intensity than any other part. The appearance of Loewe's ring varies with different persons and in different conditions. Loewe saw the ring circular and so do I, but Helmholtz saw it rhomboidal. I see all three portions circular in the conditions I have just mentioned and every part is darker than the sur- rounding sky, the region corresponding to the fovea being the brightest of the three. I have seen after looking at the sky the central portion of the yellow spot appear as a bright yellow spot, with a dark centre. It was circular in form, and on one occasion lasted for ten minutes before disappearing. When the light is feeble the foveal region is not seen and the central dark portion is larger and Loewe's ring is smaller. The shape is then more often oval, rhom- boidal or irregular. Fig. 5 shows the appearance to my right eye on a dark night with a cloudless sky. The dark centre was surrounded by a lighter portion. Bright 1 Ctrlb.f. d. Med. Wssnsch. S. 594, 1868. VARIOUS VISUAL PHENOMENA 83 lines then came from the periphery and proceeded in an irregular manner to the centre. When these lines reached the centre each broke up in a very similar way to a rocket and left the surrounding portion lighter than before. This continued until the yellow spot region became lighter instead of darker than the ground. FIG. 5. I have seen the yellow spot region largest and darkest when on waking in the early morning I have directed my eyes to the white ceiling. The whole of the centre portion of the field of vision appeared as a black disc surrounded by luminous circles. These circles then interlaced and encroached on the central black portion, which disappeared from without inwards. There are several ways in which the region of the yellow spot may be seen bright on a dark ground. Helm- holtz l has seen it on waking in the morning and looking at a dark background after having first exposed the 1 Optik Physwl. S. 569. 84 PHYSIOLOGY OF VISION eyes to the light from a large window. A disc of dazzling brightness is seen of the size of the non- vascular aureola of the yellow spot. I find that the region corresponding to the non- vascular portion of the yellow spot can be seen very well as a bright spot on a dark ground in the following way : If one eye be closed and the other directed to the sky through a deep red glass, after ten or twenty seconds the central portion will appear purple instead of red. FIG. 6. If the eye be now closed and covered with the hand so as to exclude all light, the field of vision appears green, with the exception of the centre, which is seen as a bright red spot, much brighter than the rest of the field. The green gradually invades this red from without inwards until the whole field of vision is one uniform green and the centre becomes of similar brightness to the parts surrounding it. (See Fig. 6.) Fig. 7 shows a sector of one of the subjective appear- ances of the central region as seen by me on awaking VARIOUS VISUAL PHENOMENA 85 in the morning. The centre has a spotted appearance, the circles being larger at the periphery, with gradually increasing black intervals. Outside the macular region the spots of light are further apart and less denned. The retina after a night's rest is in a most favourable state for entoptic experiments. On awaking in the morning and then directing the eyes to a white ceiling the region corresponding to the yellow spot is marked out as an oval black patch and light appears to invade FIG. 7. this patch from without inwards. Helmholtz concludes from this that light is perceived an instant later in the central as compared to the peripheral portions of the retina. Helmholtz attributes this observance to Max- well, 1 but as a matter of fact Maxwell said exactly the opposite. He said : " If we look steadily at an object behind a series of bright bars which move in front of it, we shall see a curious bending of the bars as they come up to the place of the yellow spot. The part which 1 Report of Brit. Assoc. ii. p. 12, 1856. 86 PHYSIOLOGY OF VISION comes over the spot seems to start in advance of the rest of the bar, and this would seem to indicate a greater rapidity of sensation at the yellow spot than in the surrounding retina." I agree with both of these apparently contradictory statements. 4. Yellow Spot Region Seen on Opening an Eye. I find that if one eye be directed to the sky whilst the other eye is closed and covered with the hand, when the second eye is opened the region corresponding to the yellow spot is seen as a much lighter spot. This phenomenon is seen very well at night when there is a clear sky. 5. Bright Spots Seen in the Field of Vision. I have several times noticed on stooping on a sunny day that bright spots have appeared in the field of vision. These have formed a circle which gradually contracted, each spot becoming smaller and brighter as the centre was reached. The appearance exactly corresponded to Fig. 7 seen from without inwards. II. Entoptic Appearances of the Yellow Spot and the Blood Vessels of the Retina. The yellow spot and the blood vessels of the retina may be seen ent optically in several ways. Purkinje described the first three methods. 1 1. Illumination through Sclerotic. If by means of a lens of short focus a light, as intense as can be endured without discomfort, be concentrated on the sclerotic 1 Btr. z. Knntn. d. Sehens, S. 89, 1819. Neue Btr. S. 115, 117, 1825. VARIOUS VISUAL PHENOMENA 87 as far as possible from the cornea whilst the eye is directed towards a dark surface the vessels of the retina will be plainly visible on a yellowish red background. The finest capillaries can be seen, and it will be noticed that these are absent in the centre of the field of vision, the vacant space being bordered by the loops of the capillaries. Whilst the portion of the field of vision corresponding to the rest of the retina appears of a uniform illumination, that in the centre where there are no capillaries is brighter, and has an appearance similar to that of the fovea under the microscope, in which the cones are seen as a series of small round circles, when the retina is viewed from its external side. (See centre of Fig. 7.) Helmholtz describes the entoptic appearance of this central non- vascular portion as being similar to that of shagreened leather. If the light be moved upwards on the sclerotic whilst the eye is kept fixed the images of the vessels appear to move in the same direction whilst the central non-vascular portion appears to move slightly in the opposite direction. Helmholtz, after showing in a very complete manner how the shadows of the arteries cause the effects which are seen, remarks that the appearance of the central portion is undoubtedly not produced in the same manner. It will be noticed that the vascular tree encroaches on the central portion on the side opposite the light, whilst above and below it only touches it ; on the side nearest the light there is an interval between the two. All the appearances remain the same whether the light be at the internal 88 PHYSIOLOGY OF VISION or external angle of the eye. The reason of this is that the vascular system is situated anteriorly to the portion of retina which gives rise to the spotted appearance. The portion of retina which gives rise to this appearance exactly corresponds to the non- vascular portion of the retina. When I have repeated this experiment, and I have found the light from an acetylene lamp concen- trated on the sclerotic answer admirably, the central portion has first appeared dark, but in addition to the vessels concentric bluish-violet coloured waves are seen. These waves appear as bluish-violet coloured circles of light which roll inwards from the outer part of the field of vision. They occupy the whole circumference, and appear as steadily diminishing circles. Each succeeding circle reaches further towards the central portion of the field of vision until one touches it. It then appears to break up into a star-shaped figure and becomes much brighter. This is then replaced by the spotted appear- ance already mentioned. J. H. Nuel x and L. Wolfiberg 2 have come to the conclusion from careful measurements that the appear- ance of the central portion corresponds to the mosaic formed by the cones of the fovea. 2. Illumination through Cornea. The second method of seeing the blood vessels of the retina is similar to the first, only the light is allowed to enter the eye through the cornea. The observer being in a darkened room and the eyes being directed forward, a candle is moved 1 Arch, de Biol T. iv. 1883. * Arch. fur. Augenhl xvi. 1886. VARIOUS VISUAL PHENOMENA 89 backwards and forwards either at the side or above the eye. The fine vessels and capillaries are not seen so well in this method as in the other two. In the central portion several observers have seen a bright disc circular or elliptical in shape, others have been unable to see the whole of the disc. In ordinary circum- stances I only see a portion of the disc, it is bordered on its outer side, the side nearest the light, by a dark crescent and external to this is a bright crescent which is much brighter than any portion of the interior of the disc. When the light moved upwards the bright crescent also appears to move upwards. When I move the light much more quickly and with a circular movement I am able to see the whole of the disc. The appearance then often changes to that seen in the first method, this appearance being generally pre- ceded by the pale bluish- violet circles. I often find that the central portion appears dark and not light. I find that the central portion appears dark with red light, whilst with green light it appears bright and spotted, the rest of the field also appears brighter. 3. Illumination through Pinhole. The third method of seeing the retinal vessels consists in viewing a large uniformly illuminated surface, as for instance the sky, through a pinhole aperture which is moved rapidly backwards and forwards in front of the eye. The capillaries are very clearly defined, dark on a light ground. In the centre there is a portion free from 90 PHYSIOLOGY OF VISION vessels and bounded by the convex side of the loops of the capillaries. 4. Illumination when the Retina is Specially Sensitive. I find that I can see the retinal vessels very clearly if on opening my eyes in the morning on awaking I immedi- ately direct them to the white ceiling. The larger vessels appear very black and distinct. The central portion of the field of vision appears as an oval black FIG. 8. patch and light appears to invade this patch from with- out inwards. The effect is only momentary. III. Currents Seen in the Field of Vision not due to the Circulation. 1. Currents Seen with One Eye Partially Covered. If one eye be partially covered with an opaque disc whilst both eyes are directed forwards in a not too brightly illuminated room and special attention be paid to the VARIOUS VISUAL PHENOMENA 91 covered eye an appearance of whirling currents will be seen with this eye. (See Fig. 8.) These currents appear to be directed towards the centre, and have a very similar appearance to a whirlpool. On closing both eyes all the portion in which the whirling currents are seen appears as dull purple. These currents cannot be due to vessels, because we know that the centre of the retina, corresponding to the point where the greatest movement is seen, is free from vessels. The appearance is also very different from that of the movement of blood in vessels. The experiment succeeds best if the eyes have been previously exposed to a fairly bright light. An opaque disc in a spectacle frame suffices admirably, a certain amount of light being allowed to enter the eye from the periphery. 2. Currents Seen in the Light with One or Both Eyes Open. It is easy to see the currents at almost any time on regarding fixedly a not too brightly illuminated surface. I find that it is better to use only one eye, but they can be seen with both eyes open. The first appearance which is always vis- ible to me is a star -shaped figure corresponding to the region of the fovea. (Fig. 9.) When seen with one eye this star has eight rays. It is due to the structure of the lens. There appears to be rapid circular movement behind this star that is, the FIG. 9. 92 PHYSIOLOGY OF VISION movement appears to be further off in the field of vision like a top spinning, or a Catherine wheel, the rays of the star remaining always visible and stationary. The movement is from left to right with the right eye and FIG. 10. from right to left with the left eye. The field of vision then becomes dark, and the movement spreads until it covers the whole region corresponding to the yellow spot, and the currents in this outer portion form a network with wide meshes. (Fig. 10.) The currents seem to proceed from four main places of entry, two horizontal VARIOUS VISUAL PHENOMENA 93 and two vertical. The movement is at first slow and then gets more and more rapid, especially in the centre. 3. Currents Seen in the Dark. Currents can be seen in the dark which correspond in their general character to those seen in the light. The whirling in the centre is usually very noticeable. Generally pale bluish-violet circles form in the periphery, and these gradually con- tract and advance on the centre of the field of vision. When the circle reaches the centre it breaks up into a star-shaped figure and becomes much brighter. This is then succeeded by another contracting circle. 4. Currents Seen through Yellow-green Glass. I see these currents with yellow-green glass when the eye has become fatigued by looking through the glass, the whole field becomes dark and the whirling currents are seen. The general form remains the same. Fig. 10 should be compared with the drawing given by Exner 1 as the figure seen entoptically with red light. It is evident that he saw the same figure, but he does not say anything about movement, this is probably due to the fact that the figure is very fugitive with red light. The position of the currents in the outer part of the field of vision seems to change continually 5. Currents Seen with Intermittent Light. If when regarding a rotating disc composed of black and white sectors we note the time when the fine flicker is most marked and keep the eyes steadily fixed on the disc, the field of vision often becomes dark red and we see a i Pfl&ger's Arch. i. S. 375-391, 1868. 94 PHYSIOLOGY OF VISION number of interlacing currents forming the figure shown in Fig. 10. 6. SuMen Cessation of Currents. The currents are usually very evanescent, but I have seen them at times continuously for several seconds. Occasionally they will all stop at once and the appearance is like Fig. 1 1 , three cornered spots with rounded ends being seen at the Fio. 11. junction of the currents. These currents, whilst preserv- ing the same general form, seem to change their path continually. External objects are not visible in the portion of the field of vision in which the currents are seen. 7. Effect of the Currents on an After-image. The currents carry the visual quality, colour and brightness of the region from whence they come into the after-image. They also tend to move the after-image towards the centre, thus if we have two similar after-images, one VARIOUS VISUAL PHENOMENA 95 situated in the centre and the other a short distance from the centre, the one external to the centre may be carried into the centre and combine with the one already there. The way in which the currents encroach on an after- image may be seen in the following way : One eye being shut and covered up, the other is directed to the bright blue sky, whilst the open fingers are moved rapidly before it. On shutting the eye and covering both the yellow spot region is marked out as a rose-coloured oval. There is a sensation of rapid and whirling movement on the outside of the oval. As the movement encroaches on the rose oval this disappears from without inwards, the last movement to be seen being the whirlpool move- ment like a top spinning in the centre. 8. Effect of Movement of 'the Eyes on the Currents. The currents, especially the broad ones which are found in the outer part of the field of vision, are affected by the movements of the eyes. It would be thought that a phenomenon so striking and so easily seen as the currents in the retina would have been observed by many, but this is not the case ; from the description given by Helm- holtz it is evident that he saw them when repeating Vierordt's experiment with intermittent light. 1 He differs from Vierordt in considering that the appearance is due to lymph corpuscles in the blood instead of the circulation in the retinal vessels, but he apparently con- fuses several distinct phenomena. Ferree 2 finds, and I agree with him, that the streams carry with them the 1 Physinl. Optik. p. 533, 2 The Amer. Journ. of Psychol. p. 484, 1908. 96 PHYSIOLOGY OF VISION visual quality, colour and brightness of the background from which they come. Ferree considers that eye movement is the chief or sole factor in the formation and direction of these currents, but I see them quite clearly with my eye quite still, and the whirling in the centre is not appreciably different when the eye is moved from one point to another. 9. Effect of Moving Material on Centre of Retina. On opening one eye on awaking in the morning and looking at the ceiling the central portion is seen as an irregular, circular, rhomb oidal or star-shaped black spot. On closing the eye again a bluish violet circle appears at the periphery or middle of the field of vision, contracts, and then after breaking up into the star-shaped figure and becoming brighter disappears, to be followed by another contracting circle. If the eye be opened when the star figure has formed in the centre it will appear as a bright rose-coloured star, much brighter than any other part of the field of vision. If, however, we wait till the star has broken up and disappeared before opening the eye, it will be found that only a black spot is seen in the centre. 10. Time of Interval between Contracting Circles. I have timed the contracting circles with a stop-watch and find that the interval between two is very irregular. They may follow each other regularly at intervals of one or two seconds and then cease. They are not apparently synchronous with the pulse or respiration, and they still go on moving when I hold my breath, VARIOUS VISUAL PHENOMENA 97 11. Effect of Light on Size and Colour of Currents. When the light which is entering the eye is diminished a broad current narrows to a thin line. The circles in these circumstances contract to thin lines which are at an angle to the circumference of the circle and are not joined together. The colour of the circles and currents is with very dim light bluish white, with more light bluish violet, and as more light is added the colour becomes redder and redder until it is finally rose. 12. Effect of Light on the Star Figure. The central part of the star figure is seen with dim light. If more light is allowed to enter the eye the star figure changes into a rhomboid, exactly as it did when seen by pressure and the pressure was increased. The currents and star can be seen very well in a dim light, as for instance that of a white blind illuminated by moonlight. On moving the open fingers before one eye, the laby- rinth of whirling currents is well seen with the star figure in the centre. The movement continues for several seconds after the hand has been removed from the eye. 13. Effect of Moving Material on Appearance of Small Lights. If a small light be looked at in a dark room, as for instance that coming through the smallest diaphragm of my colour perception lantern, which represents a 5| inch bull's-eye railway light at 1000 yards, care being taken not to move the eye, the contracting bluish violet circles will be seen. The colour of the circles is the same for white light or any coloured light. When it reaches 98 PHYSIOLOGY OF VISION the centre the light brightens. If the circles stop the light disappears. 14. Illusion of Moving Light. In many cases in repeating the above experiment the light will appear to move. This is particularly noticeable with red light, but may be seen with any other. The light appears to move until it apparently comes close enough to be grasped by the hand when it is really 20 feet off. I have the impression the whole time that I am looking straight at the light when it is really falling upon a peripheral part of the retina. The other closed eye is still directed straight at the light, and on opening this eye two images of the light are seen, which rapidly coalesce, the peripheral image joining the central one. The light appears to move as if some substance went downward by gravity : when the head is upright the image appears to move upwards, when bent to one side the image appears to move in the opposite direction, but appears to approach during its movement. 15. Phenomena Seen on Closing one Eye. If in the morning on awaking I look at the white ceiling with both eyes the black spot appears and disappears as usual. If I then close one eye, bright curved lines appear in the peri- phery of the field of vision and the centre becomes bright. IV. Appearances due to the Pigment Cells of the Retina. 1 . Visibility through Intermittent Light. Charpentier l states that if the fingers be slightly separated and then 1 Compt. Rendns, xcii. pp. 335-357. VARIOUS VISUAL PHENOMENA 99 when the other eye is shut the fingers moved rapidly backwards and forwards whilst the gaze is directed towards the blue sky, the field will be covered with dark purple -violet hexagonal figures, there being a light interval between each. I agree with Wolfflberg that these figures correspond to the hex- agonal pigment cells. They cover a much greater area of the field of vision than corresponds to the fovea and have a diameter at least four times as large as that of the circles seen in the region of the fovea. The following experiment shows almost certainly that the appearances are due to the pigment cells : Whilst my right eye was fixed on a gas flame shielded by an opal glass globe I moved from side to side in front of the eye a piece of black cardboard with a vertical slit of an inch long and a quarter of an inch wide. On moving this moderately quickly the whole gas globe appeared pure green instead of yellow and covered with hexagonal figures, each with a brighter circular spot in it. Between these hexagons were little red curved lines in which there appeared to be rapid movement. These red lines were between but did not encircle the hexagons. Increasing the rapidity of the movement of the cardboard did not alter the appearance of the phenomenon. (See Fig. 12) 2. Visibility when the Retina is Specially Sensitive. I can also often see hexagons of the same size on 100 PHYSIOLOGY OF VISION awaking in the morning, but they are light with a dark interval between each. V. The Effect of the Blood in the Vessels of the Retina. It occurred to me that if we could suddenly be made conscious of the absorption of the green rays by the blood, since the cones in the vascular area are influenced only by those rays that pass through the blood, the colour of the absorbed rays would appear as a contrast effect in the field of vision corresponding to the non- vascular portion of the retina. The effect can be shown in two ways. 1. If we look through a blue-green glass at a uni- formly illuminated white surface, as for instance a white cloud in the sky, for about 30 seconds, on removing the glass the whole of the field of vision appears rose, with the exception of the centre corresponding to the central non- vascular portion of the retina ; this appears bright green. The colour of the green is pure green and not a blue-green similar to the colour of the glass. It will be noticed that there is a brighter por- tion forming the star figure already described (Fig. 10). The colour of the surrounding field of vision is rose and not red, the complementary of the blue- green glass. As the colour seen is very similar to that of the visual purple it might be thought that this was the cause of the phenomenon. The green area, however, corresponds to the non-vascular portion of the retina and not to the rod free portion, and VARIOUS VISUAL PHENOMENA 101 I have never seen this green colour in conditions in which visual purple phenomena become evident. 2. On looking at a sky which is obscured by a white fog for 20 seconds with one eye, the other being closed and covered with the hand : on removing the hand from the covered eye after opening it, the centre of the field of vision is seen as pure green, the star-shaped figure being seen as in the previous experiment. The surrounding part of the field of vision does not change colour, but appears white, as it did before opening the covered eye. The light transmitted through a white fog contains a preponderance of red rays, as may be seen by viewing a light or the sun through it : both appear much redder than usual. VI. Visual Phenomena caused by Pressure on the Eye, If gentle pressure be made on the front of one eye with the palm of the hand a star-shaped figure with four rays is seen, bright on a dark ground. It is similar in every respect to the central part of Fig. 10. When both eyes are covered with the hands and more prolonged pressure is made on the right with the palm a rhomboidal figure is seen. This is formed through the filling up of the spaces between the rays of the star, and the points of the rhomboid correspond to the extremities of the rays of the star. In addition bright lines are seen moving in a whirling fashion from the periphery towards the centre. The appearance is exactly the same as that of the currents I have already 102 PHYSIOLOGY OF VISION described. It is evident that Helmholtz saw the pheno- mena as I do, as his description 1 will apply admirably to the currents and star figure I see on prolonged pressure. VII. Various Effects of Intermittent Light. If we note the effects of intermittent light upon the retina it will be found that the phenomena which are seen apparently change. All are phenomena which have been described, but they are seen at different times. For instance, if the open fingers be moved rapidly before one eye whilst the vision is directed towards a white cloud in the sky, we may see in succession some or all of the following, but not necessarily in the order given. 1 . Loewe's ring, Maxwell's spot and the fovea. 2. The appearance of the blood vessels of the retina. 3. The vascular network due to the capillaries of the retina. The non- vascular portion is seen as a uniform white circle surrounding a granular disc. 4. Appearances due to the yellow pigment of the yellow spot. 5. The star figure and currents. 6. Appearances due to the pigment cells of the retina. 7. The eight-rayed star due to the lens. VIII. Effect of Blow on the Eyeball. I was suddenly thrown off my bicycle by riding over an object in the road and struck the outer side of my right eyeball on the ground. A bright rose-purple light 1 Physiol. Optilc, ii. Aufl. S. 237. VARIOUS VISUAL PHENOMENA 103 was visible in the corresponding part of the field of vision for several hours after the accident. IX. Influence of a Light on Surrounding Regions of Retina to that which is Stimulated. If a light be looked at steadily with one eye when it has dark surroundings incapable of reflecting any light, it will appear to be surrounded by a halo made up of numerous rays of the same colour as the light. If the light be white the rays look like thin rivers running towards the light. Each point of these rivers becomes alternately rose and green, that is to say a rose-coloured particle appears to be followed by a green one, which is again followed by another rose particle. On covering the eyes with the hands after shutting them a pure green patch is seen corresponding to the light. This is surrounded by rose corresponding to the rays. The rose gradually encroaches on the green, which does not change colour. On opening the eyes a bright rose patch is seen, which does not change colour but gradually fades away. X. Red Spot in Field of Vision. When in darkness with the hands over the eyes I often see a small circular red spot in the centre of the field of vision. When the attention is fixed upon it, it swells out and becomes colourless. XI. The Appearance of the Blind Spot in the Field of Vision. On waking in the morning and viewing the ceiling with one eye I have seen the blind spot as a clearly 104 PHYSIOLOGY OF VISION defined black area with the images of the blood vessels of the retina spreading from it as branching black lines. This appearance can also be seen when intermittent light is used before one eye. The fact is important as showing that the sensation of black can be obtained in a region in which there are no sensitive elements in the retina. Interpretation of the Phenomena. On the theory given I explain these phenomena in the following way. The currents seen are currents caused by the liquid sensitized by the visual purple flowing into the external fovea. When there is visual purple in the fovea this is the most sensitive part of the whole retina, but when there is none there time must elapse before it can diffuse into the spot. I have three specimens prepared by C. Devereux Marshall showing the retinae of monkeys from the outer side and the arrangement of the rods and cones. The appearance of the cones of the fovea is exactly the same as the entoptic appearance of this central portion. The cones appeared as circles arranged in lines nearly at right angles to each other with a slight curve towards the centre of the fovea. I found the same with a human fovea, only the cones were smaller than in the case of the monkey. On examining the external surface of the retina of a monkey there appeared four slight depressions leading to the larger depression of the external fovea. These depressions correspond to the four main branches seen in the subjective phenomena, and would appear to VARIOUS VISUAL PHENOMENA 105 be channels to allow of the easy flow of the visual purple. It occurred to me that if this were the case we should obtain evidence of them in cases where the outflow from the retina was obstructed, as by tumour. I find that this is the case, the star-shaped figure given by Sir Victor Horsley in his paper on tumour of the frontal lobe l is almost exactly the same as that seen subjectively. All the drawings are for my right eye. The web-like appearance seen subjectively corresponds to the cone distribution of the retina as viewed from its outer side, the portions occupied by rods appearing as dark spaces. The yellow pigment in the yellow spot on the theory I have given should have a similar function to the yellow screen in photography. 1 Brit. Med. Journ. p. 556, 1910. CHAPTER XV. BINOCULAR VISION. When the two eyes are used together, vision is much better than with one eye alone. In this chapter we are concerned with the details of the differences in vision with two eyes over that with one. The Visual Fields. The whole of the sensations received from the external world by one eye is the visual field for that eye. The visual fields for both eyes are not the same, and the visual field for each eye varies with certain conditions, as for instance, the size of the pupil and the position of the eye. The visual field for both eyes together is much larger than for one alone, and the central portions of the fields of both eyes overlap, therefore in the visual fields for both eyes certain objects are only seen with the right eye, others only with the left, and those occupying the centre with both. Corresponding Points. If the retinae of both eyes be placed one over the other so that the foveae and similar quadrants correspond, 106 BINOCULAR VISION 107 then all those points which lie over each other in both retinae are called corresponding points. When the corresponding points of the two retinae are stimulated by light, the stimulus in each case is usually referred to the same portion of the field of vision. It will be noticed that the foveae of the two retinae are corresponding points and that the blind spots are not. When in ordinary circumstances both eyes have been directed to an object which is then seen singly, one of the eyes is moved with a finger, two images of the object are immediately seen instead of one, because the images of the object fall on non-corresponding points of the two retinae. A special function .of the movements of the eye is to bring the object looked at upon corresponding points of the two retinae. The eye moves in the orbit in a similar way to a ball and socket joint. The movements of the eye are carried out round a centre of rotation which has been ascertained to be 13*5 mm. behind the anterior surface of the cornea. This is not quite 2 mm. behind the geometric centre of the eye, which, however, is not a sphere. When the eye is directed to a fixed point a line drawn from this to the centre of rotation and the centre of the fovea is called the visual axis. When the eyes are directed straight forward, the visual axes are parallel but on looking at any near object the visual axes become 108 PHYSIOLOGY OF VISION convergent, the convergence increasing in proportion to the nearness of the object. The corresponding points have been ascertained in a number of different ways, and it is found that they vary slightly with individuals and are not exactly represented by one retina being placed over the other. Purkinje 1 marked out the corresponding points of the two retinae by means of pressure phosphenes. Helmholtz gives the corresponding points and lines in the visual field as follows : 1 . The fixation points of both visual fields, especially that corresponding to the middle of the foveae. 2. The horizon for both eyes. 3. The vertical meridian to the horizon. 4. Points in the vertical line which are at equal distance from the corresponding points of the horizon. 5. Points in the horizon which are at equal distance from the fixation points. 6. Points of both visual fields which both in height and breadth have a similar angle. Horopter. The horopter is a line or surface formed by the points in the field of vision, the images of which would fall on corresponding points of the retinae. 1 Physiol. d. Sinne, vol. i. 142 f. 1819. BINOCULAR VISION 109 The Movements of the Muscles of the Eye. The eye is moved inwards outwards upwards downwards inwards and upwards inwards and downwards by the Rectus internus. Rectus externus. f Rectus superior. \0bliquus inferior, f Rectus inferior. (Obliquus superior. [Rectus internus. | Rectus superior. [Obliquus inferior. [Rectus internus. < Rectus inferior. outwards and upwards outwards and downwards I Obliquus superior. [Rectus externus. I Rectus superior. [Obliquus inferior. [Rectus externus. I Rectus inferior. I Obliquus superior. Listing's Law. It will be noticed that in addition to movements of the eyes upwards and downwards, outwards and inwards, or in any oblique direction there is another possible movement of the eye, and that is one similar to a cart wheel moving on its axis. Listing's law is that when the eyeball moves from the primary position to a secondary position the angle of rotation in the secondary position is the same as if the eye had been moved round a fixed 110 PHYSIOLOGY OF VISION axis perpendicular to both the first and the second positions of the visual line. If this were not the case images would fall on non-corresponding points of the two retinae. The experimental proof is usually given as follows : If on a wall there be placed a sheet ruled with a number of horizontal and vertical lines and an after-image of a cross made by a vertical and horizontal line at one of the angles be looked at, it will be found that the after-image of this cross will also consist of a vertical and a horizontal line. It will be found that the vertical line of the after-image will correspond with the vertical lines on the paper and the horizontal line of the after-image will correspond with the horizontal lines on the paper as the eyes are moved up and down over the paper. When double images fall upon corresponding points they combine to form a single image just as if the images had been received from one object. If two coins be taken and placed on a table so that the heads of each correspond, double images of each can be obtained by looking at an object further in the distance, two of these images can be made to combine by varying the positions of the coins and the object which is being looked at. We shall then have three images of the two coins, two single and one combined image. The eyes usually move together, even when one of them is blind. This is not invariably the case, as is shown by the experiment of the moving light given on page 98. BINOCULAR VISION 111 Binocular Projection. In binocular vision the visual lines are apparently shifted on either side half an interocular space, so that the visual line now occupies a position midway between the two eyes, the visual line being moved for the left eye half an interocular space to the right and that of the right eye half an interocular space to the left. In binocular vision we see as if we had one eye occupying a position midway between the two eyes. This is apparent on looking out through a pair of spectacles ; we appear to be looking through one large glass with its centre midway between the two eyes, the internal portions of the rims of both glasses seem to have dis- appeared. If with the nose closely pressed against a looking-glass we look away as if into the distance we apparently see one eye in the median position looking at us out of the glass. Binocular Contrast. The perception of relative difference is as important in binocular vision as it is in monocular vision. This accounts for the predominence of contours and sharply marked black or white lines in binocular vision. Double Images. As the two foveae are corresponding points, it follows that if the eyes be directed to an object in the distance and at the same time an intervening object be regarded, it is not possible for the intervening object to fall upon 112 PHYSIOLOGY OF VISION corresponding points. A simple example of this is the case of a gun which is sighted for a certain object : the sight must either be used for the right eye or for the left. It follows from this that a large number of double images are formed, which would be very trouble- some if they were noticed in ordinary life. As a matter of fact, it is difficult to see these double images without specially looking for them. The chief reason for this is that one eye is principally used in any visual act. One eye usually has a preponderating influence but, in most cases the eyes are used alternately, and it is often difficult to ascertain to which eye the image which is seen belongs without closing one of the eyes. If a numbered scale be placed at a distance of fifteen feet from the observer and on a table midway between it and the observer be placed an object with a sharp triangular point, the fact mentioned above can be illustrated. If the observer look at the triangular point with both eyes open, he will see that it is quite easy for him to make it correspond with a particular figure on the scale. At the same time he will not see any trace of a double image even when this is carefully looked for ; at least this is true for a large number of persons. If, however, the dominant eye be closed, the triangular point no longer corresponds to the previous figure on the scale but springs up in another portion of the field of vision. From this it appears that a large number of double images are in ordinary circum- stances suppressed. Fig. 14 illustrates the formation of double images. BINOCULAR VISION 118 Both eyes are supposed to be directed to point A, and this point forms an image on both retinae at corresponding points namely, the centres of the foveae of both eyes. Bl and B2 and Cl and C2 show the positions on the retinae of the points B and C whilst the point A is being observed. It will be noticed that the images fall on disparate points of the retina. The centre portion of the figure shows the projection in space of the double images. Though double images are seen when one eye is moved with the finger whilst both are directed at one object, the essential point in the formation of the double images is that the whole apparatus is dislocated whilst the eyes are open and that images are falling on non-corre- sponding points. One eye may be moved independently of the other without the formation of double images or change in their localisation in space. If an after- image be formed and the eyes closed and covered with the hands, the after-image is seen single and not double when one eye is moved independently of the other with the finger. Of course in this case the retinal stimulation is the same, the corresponding points of both eyes being stimulated as before, the only change being in the move- ment of one eye. The improvement in the perception of relative distances with two eyes over one is shown by Hering's ball experiment, in which small balls of various sizes are dropped in front or behind a vertical thread ; with two eyes it is very easy to tell whether the balls are dropped in front or behind the thread ; it is very difficult with one eye. 114 PHYSIOLOGY OF VISION Retinal Rivalry. When two dissimilar pictures are combined in a stereoscope there is a tendency for one or other of them to prevail in the field of vision. A continued alternation takes place, first one is seen almost entirely, then a combination of the two and then the other. This will even occur when such a very marked object as a thick black cross is combined stereoscopically with a network of black lines on a white ground. At one moment the black cross will be seen on a white ground, then a black cross on a network and then the network on a white background, the black cross having entirely disappeared. The same will be found in combining horizontal and vertical lines, and even when a broad black horizontal stripe is combined stereoscopically with a broad black vertical stripe on a white ground. When the two black stripes are combined and the centres are fused, these appear black, but the adjacent portions of the cross appear much lighter, increasing in darkness at points further away from the black centre. One or other of the arms of the cross will become lighter and tend to disappear or actually disappear at intervals. When a white pattern on a black ground and a similar pattern in black on a white ground are combined stereoscopically, the field does not appear a uniform grey, but an object in relief, either black on a white ground or white on a black ground will be seen alternately and successively, and varying combinations of the two. When an object outlined in black is combined stereoscopically with the BINOCULAR VISION 115 object represented entirely in black it appears almost the same as if two outlined figures had been combined, the centre appearing almost as white as the surrounding background. The dominance of one eye or the other is particularly noticeable when stereoscopic pictures of a simple object painted in different colours, as for instance red and blue, are placed in the stereoscope. The object is usually seen in one or the other colour and an alternation takes place, the object may first appear blue, then purple, and then red. Again, if two pictures be combined in a stereoscope in which one is quite different from the other, as for instance, a cage on one side and a bird on the other, it will be noticed that the two pictures are not completely fused as they would be by combining the picture of the bird with that of the cage. The bird will undoubtedly be seen within the cage, but the bars of the cage will be indistinct or missing where they would if they were fused touch the bird or run right across it. The same is found with many even with horizontal and vertical lines. A series of vertical lines being placed on one side of the stereoscope and a series of horizontal lines on the other, in the combined picture the horizontal or the vertical lines will predominate and an alternation will take place. Single Vision with Disparate Points. When images of two objects fall upon widely disparate points double images are formed, but when there is 116 PHYSIOLOGY OF VISION only slight disparation a single image is formed. The amount of disparation which will allow of single vision has been estimated by Volkmann. 1 He combined stereoscopically two sets of parallel lines; both lines of the first pair were at a fixed distance from each other, and of the second pair one was fixed and the other moveable, so that the distance between these two lines could be varied at will. (See Fig. 13). The important fact in this experiment is that when the lines are fused with disparate points their position in space appears to be different ; one line appears to be raised above the other in the stereoscopic combination, and we have an appearance of stereoscopic relief. Wheatstone was of opinion not only that we can have single vision with disparate points, but that we can have double vision with corresponding points. This has been denied by many observers, but is still a subject of discussion. He showed that if a thick vertical line be drawn on one sheet of paper and a thin vertical line with a thick line running diagonally across it on another sheet of paper (see Fig. 15) : when these are placed in the stereoscope the thick diagonal and vertical lines combine into one and a thin line is seen running vertically through it. The important point in this experiment is that the thick line appears to stand out from the paper. Spearman also holds the same opinion, which of late years has been steadily gaining ground. He shows that if two figures, like Fig. 16, be combined in the stereoscope two definite 1 Arch.f. Ophth. 1859, Bd. v. Abth. 2, S. 1. BINOCULAR VISION 117 F's will be seen. I find that I can see these F's for several seconds at a time, though there is a tendency for the thin vertical line on the right to move, and we have the curious spectacle of an object moving plainly backwards and forwards although all the time the place of retinal stimulation remains unchanged. If objects seen by corresponding retinal points necessarily appeared in the same direction, then, when the thin lines of the left of A and B respectively combined into L, the other two thin lines would necessarily appear as JL : for the distance between the two vertical thin lines is equal to the distance from the left extremity of the left hori- zontal thin line to the centre, not the left extremity of the right horizontal thin line. The Perception of Relief. Though the perception of relief is much more complete when both eyes are used, there is undoubtedly perception of relief with only one eye. When the visual axes are parallel and objects in the far distance are being regarded, the visual fields of both eyes are almost exactly similar, and the perception of relief is the same with one or both eyes together. Objects appear in relief when viewed by an electric spark, and in this case there is no time for any movements of the eye so that the eyes are brought to converge on any particular object. The perception of relief either monocularly or binocularly is practically the same for one or both eyes when the objects are beyond a certain distance from the eyes. 118 PHYSIOLOGY OF VISION The perception of relief in these cases depends almost entirely upon the light and shade of the object and the judgment formed therefrom as to whether a solid or plain object is being observed. I have on many occasions been quite unable to decide whether a lantern slide was being shown or a projection by means of an epidiascope. Coloured photographs of crystals, for instance, have had such a very solid appearance, just as if the actual objects were being shown. It is the same with many of the coloured pictures which are exhibited by means of the cinematograph, the appearance of relief is as striking as it is when pictures are viewed in the stereoscope. In the perception of monocular relief the isolation of the object is important, a photograph viewed through a magnifying lens with one eye appears in higher relief than without the lens, and this appears still more striking when viewed with one eye through a stereoscope when the picture occupies the greater part of the field of vision ; the relief is even more marked when both eyes are open and one is. viewing a sheet of white paper which is placed on the other side of the stereoscope. Wheatstone l states that when a perspective of a building is projected on a horizontal plane, so that the point of sight is in a line greatly inclined towards the plane, the building appears to a single eye placed at the point of sight to be in bold relief, and the illusion is almost as perfect as in stereoscopic relief. Two different images are undoubtedly necessary for l Phil. Trans. 1838, p. 381. BINOCULAR VISION 119 binocular perspective. If two horizontal wires be looked at with the head erect from a distance of about twenty yards when they are about five feet from each other the images are the same for each eye, and it is difficult to say how far apart the wires are. When the head is turned to one side the difference in position is at once evident. When the visual axes are not parallel, the visual fields become more and more dissimilar in proportion to the nearness of the object and the convergence of the visual axes. If for instance a cube be looked at when it is placed on a table at a distance of three feet from the eyes, and so that one side of the cube is only visible with the right eye, it is quite obvious that the visual impres- sion for the right is quite different from the left, as may be seen by carefully observing the cube whilst first the right eye and then the left is closed. Now if the cube be regarded in relation to the two pictures it will be noticed that the picture formed by one or other eye has a dominant influence in relation to the appearance of the cube as viewed binocularly. This is particularly notice- able with regard to surrounding objects. If, for instance, the exact point of correspondence between one corner of the cube and some noticeable portion of some other object situated some six inches away be noted carefully, it will be found that the correspondence of the two will be for one eye only, as can be seen by closing first one eye and then the other. It will be found that the double images for one eye have been entirely suppressed, and it is difficult to see them even with the most careful 120 PHYSIOLOGY OF VISION observation when the objects are not too far apart. A point which will immediately strike the observer when he closes first one eye and then the other whilst viewing the two objects is that the striking appearance of depth and distance disappears when only one eye is used. It now becomes very difficult to say with any degree of certainty how far the two objects are apart. The perception of relief and depth therefore seems to be intimately connected with single vision with disparate points. The perception of binocular relief is, however, inde- pendent of double images and the stimulation of disparate points, provided that the object presents images to the two retinae similar to those which are presented by an object in the field of vision. This can be shown by taking a pair of stereoscopic photographs in which the point of sight is at the centre of each and cutting them verti- cally in two, and then having pasted the left half of the left photograph on the left side and the right half of the right photograph on the right side on white or black cardboard at an appropriate distance so that there is no overlapping when placed in the stereoscope, a picture in striking relief is obtained when combined together in the stereoscope. In this case it will be noticed that there is no portion common to both fields of view. In each case the overlapping portion is combined with white. It seems probable that this is how binocular vision takes place in ordinary circumstances. If an object in high relief, as for instance a vase or the face of a BINOCULAR VISION 121 person be viewed at a short distance and one particular point fixated, it will be noticed that the right eye domi- nates the right side of the field of vision and the left eye the left side. The image seen is almost entirely that of the right eye for the right side and that of the left eye for the left side, as may be proved by noticing the relations of surrounding objects and closing first one eye and then the other alternately. The same is found when pictures are carefully examined in a stereoscope. If the relations of two objects be carefully noted, as for instance the exact distance of the spire of a church from an adjoining house, when this is markedly different in the stereoscopic pictures for the right and left eyes, it will be found that the distance noted binocularly corresponds for either one of the two pictures, but is not a position which would correspond to a fusion of the two. The conditions in which fusion takes place will be shortly described. In ordinary circumstances a large number of double images are completely suppressed. Double images appear when the mind is not able to project externally an object which would produce the two pictures which are falling upon the two retinae. This is strikingly shown by Fig. 19. If these be combined in the stereoscope it will be found that first two of the squares combine into one and double images are seen of the two others. Then suddenly both pairs of squares will combine to form two squares, one square appearing much nearer and ^mailer than the other, but like it situated in the median line. 122 PHYSIOLOGY OF VISION When the two halves of stereoscopic pictures are combined in this way it will be noticed that they blend perfectly and that there is no retinal rivalry. (See Fig. 18.) When in the stereoscopic photographs viewed in the ordinary way there is some incongruous object, as for instance a flaw in the photograph or two objects which do not correspond, the perception of relief is actually better with the divided photograph. If a careful comparison be made between stereoscopic pictures viewed in the ordinary way and the halves of stereoscopic pictures as described above, it will be found that when there is any superiority in the ordinary stereo- scopic photographs it is when there is a fusion of a line or lines in the foreground. In some cases the perception of relief is much better with the divided photographs and with objects which appear in very strong relief, as for instance in some of marble columns and stalactites in a cave, that is to say rounded surfaces. In many of these cases it will be found that the ordinary stereoscopic photographs are very hard to fuse. It would appear therefore that the perception of such objects in relief in ordinary binocular vision took place, not by fusion, but by suppression of the right eye image on the left side and the left eye image on the right side ; the two halves of the image being combined at the point of fixation. The inversion of relief produced by the pseudo- scope can be seen with the divided photograph. It will be noticed that in the case of a rounded object, as for instance a sphere or rounded column which is BINOCULAR VISION 123 viewed binocularly, only a solid object could give rise to the two images which are formed upon the two retinae irrespective of the duplication of impressions, that is to say when only the right and left halves are considered. The mind therefore projects the image of an object, capable of producing both impressions, in the field of vision. Binocular vision itself, apart from any double images, tends to give rise to a perception of solidity and relief. If two photographs taken from the same negative be placed one on either side of the stereoscope a striking appearance of relief is obtained, though the photographs are not stereoscopic photographs and no double images are formed. If a stereoscopic photograph of a scene be placed in the stereoscope and viewed in the ordinary way the usual striking appearance of relief is obtained. If now, whilst the eyes are still looking at the photograph, a sheet of white cardboard be dropped over one of the photographs so as to entirely occlude this photograph and only present a white surface to one eye, a striking appearance of relief remains. This fact may be empha- sised even more by altering the experiment in the following way : If the cardboard be placed so that only half of one photograph be occluded, a careful comparison can be made between the upper and lower halves of the combined visual field. It will be noticed in many cases that the appearance of solidity is nearly as great in the portion which is being viewed with only one eye as with that which is being viewed with both. It will be noticed 124 PHYSIOLOGY OF VISION that when there is one picture in the stereoscope and a white blank on the other side, that the appearance of solidity is greatly diminished when the eye receiving the impression of the white surface is closed. It is probable that the more striking appearance of solidity of objects in the distance when viewed with both eyes than when viewed with one must be due to the duplica- tion of similar impressions, as when the visual lines are parallel or nearly parallel the difference between the photographs on the two retinae for objects in the distance which occurs on account of the interocular interval is very small. Straub has shown that perception of depth can be obtained with one eye by stimulating adjacent points successively. The influence of the movements of the eye on the perception of relief has been the subject of much dis- cussion, but numerous observers have shown that the perception of relief is quite independent of the movements of the eyes, as indeed is shown by the stereoscope. Binocular Fusion. It is obvious that the fusion of images on the retina does not account for binocular vision. Where images of the same object fall upon corresponding points these are fused and appear single ; this is particularly the case for images falling upon the centres of the two foveae. When the images do not fall upon corresponding points there is suppression and selection. In certain cases BINOCULAR VISION 125 images which fall on non-corresponding points may be fused, but the appearance of the object is quite different from that which is found when images fall on correspond- ing points. If stereoscopic pictures of a truncated cone be placed in a stereoscope, a very definite picture of the truncated cone in high relief is obtained. Let us suppose that the truncated cone is placed so that the smaller end faces the observer. On comparing the stereoscopic pictures it is seen that they are quite different for the right and left eyes, the small circle appears with the right eye to be much further away from the base than it does with the left eye. If the pictures be carefully examined in the stereoscope when viewed binocularly it will be noticed that there has been a complete fusion of the two images. There has been much discussion as to whether there really is fusion in these cases or whether each point of the image is combined successively whilst other points are seen double. There can be no doubt that there is actual fusion, when double images appear the perception of solidity greatly diminishes. Without experience no one could tell in many cases the figure into which two stereoscopic pictures will combine, as for instance two slanting daggers which combine into one dagger which apparently stands out from the paper ; or two other daggers of the same shape but of different size which combine into one which appears to stand out diagonally across the paper. It may be noted that the images on the two retinae of the object as seen would be similar 126 PHYSIOLOGY OF VISION to the two stereoscopic pictures. (See Fig. 17). It would appear therefore that fusion takes place when the mind is able to project externally the images of an object in a position which would give rise to the images on the two retinae similar to the two stereo- scopic pictures. In those cases in which there is fusion of two halves of stereoscopic pictures presented to each eye it is obvious that the same mental process takes place, because in this case no double images are formed. If the stereoscopic pictures representing a truncated cone be divided, the small circle being divided exactly in the centre or three circles arranged similarly, two or three perfect circles will be seen when combined stereoscopi- cally in the manner previously described. It might be thought that the explanation of fusion in binocular vision was to be found in a common cerebral centre for each pair of corresponding points, and indeed this was the view of many of the older physiologists. There are, however, many facts which are quite incon- sistent with this view, particularly those ellicited by the researches of Sherrington, 1 Macdougall, 2 and Hart- ridge. 3 The following facts are against the view of a common cerebral centre for both corresponding points. Objects do not appear brighter or only slightly brighter with both eyes than when viewed monocularly. If there were a common cerebral centre the results should 1 Journal of Psychology, 1904, p. 26. 2 Brain, 1911, p. 371. 3 Jonrn. of Physiol 1915, p. 47. BINOCULAR VISION 127 be the same as if one eye were stimulated with a light of twice the intensity. In certain conditions the additional stimulation of one eye with light not only causes no sensation of increased brightness but the reverse This is illustrated by Fechner's paradoxical experiment. One eye is directed to an illuminated surface, as for instance a sheet of white paper, whilst the other eye is closed. If the closed eye be suddenly opened behind a sheet of smoked glass, the field of vision will appear distinctly less bright. In this case the additional stimulation of the second eye with the light which passes through the smoked glass adds nothing to the sum of the stimulation. If an after-image be formed in one eye and allowed to die away, the eyes being shielded from all light, this after-image may be revived one or more times by momen- tarily allowing light to enter the eye. This may in certain circumstances be accomplished by allowing light to enter the other eye, but the effect is not nearly so marked as when light is allowed to enter the eye which is primarily the seat of the after-image. When one or both eyes are stimulated with an inter- mittent light of a given intensity, the disappearance of flicker is almost exactly the same for one or both eyes together or alternately on corresponding points of the two retinae. The well-known facts of binocular rivalry and fusion are against the view of a common centre ; we have every grade of fusion from complete fusion to the entire 128 PHYSIOLOGY OF VISION suppression of one image. The fact that single vision is not confined to corresponding points but disparate points may be united in certain circumstances and the union of images falling upon disparate points causes an appearance of depth. The Predominance of Contours is an important factor in binocular fusion. If there be placed in a stereoscope a white field for one eye and a black field with a white vertical stripe running across it for the other, in the combined fields there will be seen a white stripe bordered by black lines gradually fading off into white. The remainder of the combined field of the black and white will be almost as white as the white stripe. Macdougall l explains this as due to the reciprocal re-enforcement of corresponding points with inhibition of adjacent points. The parts of the field which are bright to both eyes pre- dominate over the parts which are bright to one eye only. The parts which are bright to one eye only are partly or completely inhibited. This may also be illustrated by combining a white field with a small black circle in the centre on the other. If the circle be small it appears fully black in the combined field, that is to say the white element in the circle is inhibited. If the circle be made gradually larger a point will soon be reached when only the peripheral parts of the circle will appear black, the centre appearing dull grey ; the centre of the circle will appear whiter and whiter as it is increased in size. 1 Brain, 1911, p. 384. BINOCULAR VISION 129 Theories of Binocular Perspective. Wheatstone's Theory. Wheatstone discovered that two slightly dissimilar pictures, dissimilar in the same way as two retinal pictures of the same object, produced when presented to each eye separately and at the same time a visual effect similar to that of the solid object or objects which were represented. He invented the stereoscope in order to combine these pictures. His theory was that in viewing a solid object two slightly dissimilar pictures were formed on the two retinae and that the mind united or fused them into one. Brucke's Theory. Briicke declared that there was really no mental fusion of two dissimilar images. His view was that in regarding a solid object the eyes are in incessant and constant motion, and that the observer combines successive portions of the objects and so obtains & perception of binocular perspective. The instantaneous perception of binocular relief is fatal to Briicke's theory. Le Conte's Theory. " All objects or points of objects, either beyond or nearer than the point of sight, are doubled, but differently the former homonymously, the latter heteronymously. The double images in the former case are united by less convergence, in the latter case by greater convergence, of the optic axes. Now, the observer knows instinctively and without trial, in any case of double images, whether they will be united by greater or less optic convergence, and therefore never makes a mistake, or attempts to unite by making a 130 PHYSIOLOGY OF VISION a b FIG. 13. c d B 2 B FIG. 14, BINOCULAR VISION 131 IT FIG. 15. FIG. 16. rr b FIG. 17. FIG. 19. FIG. 18. 132 PHYSIOLOGY OF VISION wrong movement of the optic axes. In other words, the eye (or the mind) instinctively distinguishes homonymous from heteronymous images, referring the former to objects beyond, and the latter to objects this side of the point of sight." Le Conte states that this theory is hinted at but not distinctly formulated by Helmholtz. Hering. Hering holds that retinal disparation is the physiological basis of our ideas of depth, either when the retinal disparation is so small that the images are combined into one or when double images are formed. My own view is that the perception of relief and binocular perspective is due to the projection outwards of the images falling upon disparate points in association with those falling upon corresponding points to a position in space capable of causing both images. Just as an object is localised monocularly in the visual field so is the appearance of a solid object projected outwards binocularly. When images of two lines fall upon disparate points which are closely together the images are fused and the mind projects outwards the appearance of a solid object which would give rise to the images which are falling upon the retinae. That this is the case is strikingly evident in some stereoscopic pictures. If the stereoscopic pictures of a wire basket which is made up of a number of circles of widely different diameter be examined, we notice that the separate pictures are very different. The small circle which corresponds to the opening of the basket occupies a very different BINOCULAR VISION 133 relative position on the right and left sides. When the picture is examined in the stereoscope a wire basket in high relief is obtained, and it will be noticed that the centre of the circles corresponds from the top to the bottom of the basket. It will be noticed that there is no trace of any double image when the pictures are viewed in the ordinary way, if, however, one portion be intently regarded, the stereoscopic effect will diminish and double images will appear. It has been stated that when the small circles are combined the large circles are seen double, but though this corresponds to the appearance on the retina it is not what is actually seen. It will be noted that the combination in many stereo- scopic pictures does not take place immediately even when double images are not perceived, then suddenly the figure appears to stand out more and more. On any future occasion the figure is seen at once. It will be noticed that this view differs from the others in that double images are not essential for binocular perspective. It accounts for the striking perspective which can be obtained with one eye, and the pictures in high relief by combining two halves of a stereoscopic photograph. CHAPTER XVI. SUMMARY. A brief recapitulation may be useful in order to show how the various phenomena of vision can be explained on the theory advanced. 1. Monocular Vision. The cones are the terminal perceptive visual organs. The rods are not perceptive elements, but are concerned with the formation and distribution of the visual-purple. Vision takes place by stimu- lation of the cones through the photo-chemical decomposition of the liquid surrounding them, which is sensitised by the visual- purple. (1) Visual acuity. This corresponds to the distribution of the cones. (2) The relation between the foveal and the para-foveal regions. There is no qualitative difference between the foveal and para- foveal regions. The Purkinje phenomenon, the variation in optical white equations by a state of light and dark adaptation, the colour- less interval for spectral lights of increasing intensity and the varying phases of the after-image have all been found in the fovea only gradually diminished. (3) The varying sensibility of the fovea. This is explained by the assumption that when there is visual purple in the fovea this is the most sensitive portion of the retina ; when there is none there it is blind. 134 SUMMARY 135 (4) Purkinje phenomenon. This is a photo-chemical phenomenon and is found with other photo-chemical substances. (5) Disappearance of lights falling upon thefovea. When the visual-purple in the fovea is used up and not renewed, the latter is blind. (6) Currents seen in the field of vision not due to the circulation. These are formed by the flow of sensitised liquid. (7) Movement of positive after-image. This is explained by the shifting of the photo-chemical stimulus. (8) Multiple after-images from a single light stimulus. These are caused by distribution of the photo-chemical stimuli. (9) The M ocular Star. This corresponds to the canals leading into the external fovea. (10) Entopic appearance of cone mosaic. This is a subjective appearance of the cones of the retina, the portions occupied by rods appearing as dark spaces. (11) The appearance of visual purple between the cones. This we should expect on the theory given. The fact is inexpli- cable on any other view. (12) Dark and light adaptation. Dark adaptation chiefly due to the accumulation of visual purple in the liquid surrounding the cones, making the liquid more sensitive. 2. Binocular Vision. In Binocular vision images on the two retinae are combined, the right eye dominating the right side of the field of vision and the left eye the left side. The mind projects outwards the image of an object which would be capable of producing both images, both in the case where different images are formed in the Monocular fields which take part in the combined field and when the images presented to each eye are distinct and there is no object in the combined field which is present in both fields. CHAPTER XVII. THE SENSATIONS CAUSED BY SIMPLE AND MIXED LIGHTS. The physical basis of the simple colours are the colours of the spectrum, and every portion of the spectrum differs in wave-length from that above and below it. The Limits of the Visible Spectrum. The limits of the spectrum are practically the lines A and H. A, wave-length, 764 for the red, and H, 396-8. These limits vary with different persons. Mixed Colours. All the colours of the spectrum and of nature can be imitated by mixtures of three selected spectral colours, so that in this sense colour vision is undoubtedly tri- chromatic. It is on this fact that certain methods of colour photography are founded. It does not at all follow, however, that because two simple lights when mixed give rise to the sensation of another simple colour that the receiving apparatus is similarly con- stituted. 136 SIMPLE AND MIXED LIGHTS 137 In mixing colours it is necessary to use pure spectral colours. When coloured pigments are used different results are obtained, which are due to the impurity of the light employed. A very good example of the results obtained when using impure colours can be shown with coloured glasses. If we look at a white cloud through a yellow and blue glass combined the cloud appears green, and this corresponds to the mixture of coloured pigments. Every artist knows that when a yellow and a blue pigment are mixed a green is obtained. But this green is obtained by subtraction as is the case with the coloured glasses. If the yellow glass be examined by placing it in front of the slit of a spectroscope, it will be found that in addition to yellow, orange, red and green rays are also transmitted. When N the blue glass is examined in a similar manner it will be found that in addition to the blue rays, green, violet and a band of red at the extreme end of the spectrum are transmitted. When the light transmitted by both glasses is examined in a similar manner it will be found that only the green rays are transmitted, and so the colour appears green, which is the only one which can pass through both glasses. When pure lights are used the results are different, and yellow and blue when mixed instead of making green make white. The following is a table by Helmholtz showing the result of mixing simple spectral colours : 138 PHYSIOLOGY OF VISION Violet. Indigo Blue. Cyan Blue. Blue Green. Green. Green Yellow. Yellow. Red Orange Purple Dark Dark rose Whitish Whitish rose White White Whitish Whitish yellow Yellow Gold yellow Yellow Orange Yellow rose Whitish rose White Whitish yellow Whitish Green Green rose White Whitish green Whitish green Green yellow yellow Green Whitish green Water green Blue Blue blue Water blue Water green green Cyan blue blue Indigo blue blue It will be noticed that a mixture of red and blue makes rose, in which a red element is perceptible. Two colours when mixed which make white are called complementary to each other. The following is a table of complementaries after Helmholtz : Colour. Wave Length. Colour. Wave Length. Batio of Wave Lengths. Red 656-2 Green blue 492-1 1-334 Orange 607-7 Blue 489-7 1-240 Gold yellow 585-3 Blue 485-4 1-206 Gold yellow 573-9 Blue 482-1 1-190 Yellow 567-1 Indigo blue 464-5 1-221 Yellow 564-4 Indigo blue 461-8 1-222 Green yellow 563-6 Violet 433 1-301 SIMPLE AND MIXED LIGHTS 139 It will be noticed that the complementary of green consists of two colours, red and violet, which, when mixed, make the series of purples. The number of sensations experienced will be con- sidered when dealing with colour perception. The number is very small. Most normal sighted persons say that they see six definite colours in the spectrum, red, orange, green, blue and violet. In addition to these there are the sensations of white and black ; white caused by a compound light, and black from the absence of light. It seems hardly fair to include black amongst the sensations, because it corresponds to the silence which is caused by the absence of sound. It is very difficult to obtain a really black object, nearly all reflect a certain amount of white light. The sensation of black is therefore experienced when a very small amount of light falls upon a portion of the retina which is sensitive to light, whilst the adjacent portions are stimulated by a comparatively much larger amount of light. Colours may differ in three different ways : (1) Difference of brightness or luminosity. (2) Difference of hue or colour-tone. (3) Difference of saturation. White Light. When we speak of white light it is necessary to mention the source, as when different sources of white light are examined by the aid of the spectrometer it will be found that their constitution is different. Even when 140 PHYSIOLOGY OF VISION daylight is employed it must be remembered that its constitution varies at different times of the day. The term white light used in a physical sense must not be confused with the sensation of white. Blue and yellow when mixed give rise to the sensation of white. CHAPTER XVIII. THE SIMPLE CHARACTER OF THE YELLOW SENSATION. A mixture of two lights may produce an effect which, is physiologically indistinguishable from that of a simple light without being reversible, the photochemical substances produced by red and green producing a substance which is similar to that evoked by the simple light, but that the substance produced by the simple light could no more be considered to be constituted of the sensations produced by the components of the compound light than a simple element produced in a non-reversible chemical reaction could be considered to be constituted of the substances which have given rise to it. It is, however, much more probable that the effects of a mixture of red and green are not actually the same as those of simple yellow, but are physiologically indis- tinguishable, the retino-cerebral apparatus not being sufficiently developed. The sensation yellow having replaced that of red-green of former ages the two ha\e become indistinguishable. The physical and mathe- matical aspects of this point of view have been shown 141 142 PHYSIOLOGY OF VISION in a very able paper by Houstoun. 1 A corollary to this point of \iew is that when the colour sense has been still further developed a mixture of red and green will no longer match a simple yellow but will appear as a distinct colour in the same way as a mixture of red and violet make a perfect match with green for the dichromic, but the normal sighted readily distinguish one as green and the other as purple. The following evidence supports the view of the simple character of the yellow sensation : 1. The impossibility of splitting the yellow sensation into its alleged hypothetical constituents when pure spectral yellow is used. 2. That in certain cases of defective colour-perception, pentachromic and tetrachromic, the yellow sensation is diminished and in other cases the trichromic lost altogether, whilst there are three definite colour sensa- tions, red, green, and violet, and the yellow region is seen as red-green. 3. In cases where there is shortening of the red end of the spectrum, and a yellow sensation is present, this yellow sensation corresponds to a portion of the spectrum nearer the violet end of the spectrum than with the normal sighted. If these cases were caused by diminution of a hypo- thetical red sensation the opposite should be the case, and the yellow should be found nearer the red end of the spectrum. 4. If the eye be fatigued with pure spectral yellow 1 Proceedings of Royal Society, A, vol. xcii. 1916, p. 424. THE YELLOW SENSATION 143 light the spectrum will appear to have lost its yellow, and though yellowish-red or yellowish-green will appear less yellow the terminal red of the spectrum will not be affected. If the terminal portion of the red end of the spectrum be isolated in my spectrometer it will appear as a faint red upon a black background. If the eye be fatigued with red light, even by looking through a red glass held against a light for one second, the red will not be visible for some considerable time, but the eye may be fatigued for twenty minutes with yellow light without interfering with the visibility of the red light. 5. It is known that if the intensity of a number of coloured lights be reduced in the same proportion all the colours do not disappear at the same moment. If, therefore, spectral yellow were a compound sensation it should change colour on being reduced in intensity. If, however, spectral yellow be isolated in my spectrometer, and the intensity be gradually reduced by moving the source of light away, the yellow becomes whiter and whiter until it becomes colourless, but does not change in hue. 6. The eye may be fatigued with red or green without altering the hue of spectral yellow. Spectacles glazed with red or green glass of a kind which is permeable to the yellow rays may be worn for a considerable time without altering the appearance of spectral yellow. If yellow were a compound sensation, a wearer of red spectacles should see the yellow through them as green, 144 PHYSIOLOGY OF VISION because the yellow would fall on a portion of the retina which had been fatigued for red. 7. In conditions which would appear to be particularly favourable for seeing yellow as red if it were a component sensation this does not occur. If black letters on a white ground be arranged in a dull light so that as little light as possible be reflected from the black letters, and a blue-green glass impervious to the red rays be suddenly interposed between the eye and the letters, the black letters appear for a fraction of a second a brilliant red. If spectral yellow be viewed in similar circumstances it still appears yellow. The explanation of the above phenomenon appears to be that the retina still contains substances caused by the action of white light, and the black letters appear red through simultaneous contrast. 8. Spectral yellow after colour adaptation to green still appears yellow and not red. 9. The after-image of pure spectral yellow is first seen as a bright yellow positive after-image ; this does not change to green on becoming negative. 10. The complementary of yellow is never strengthened by the after-image of yellow. 11. The stability of the after-image of spectral yellow is remarkable ; it does not change colour and is not influenced by subsequent light falling on the retina when this is not of too great intensity. 12. When the eye is fatigued by red light the after- image projected on yellow is grey or black ; no tinge THE YELLOW SENSATION 145 of colour is seen unless the fatiguing light is very bright, when the yellow image on screen becomes green. 13. When a sodium flame is viewed after fatigue with spectral red light it is very little affected in the region of the after-image, though the green blue after-image is very strongly marked on either side of the sodium flame. 14. A vivid blue-green after-image may be seen not only in the absence of all green light but whilst the eye is still being stimulated by a red light. 15. If yellow were a compound sensation it should be easier to obtain a green after-image after fatigue with red on a yellow than on white. It will be found that whilst the green after-image, is very easy to obtain on a white surface it is much less easy to se