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 XL THE SPECIAL SENSES. 
 
 A. VISION. 
 
 The" Physiology of Vision. The eye is the organ by means of which 
 certain vibrations of the luminiferous ether are enabled to affect our conscious- 
 ness, producing the sensation which we call " light." Hence the essential part 
 of an organ of vision is a substance or an apparatus which, on the one hand, 
 is of a nature to be stimulated by waves of light, and, on the other, is so con- 
 nected with a nerve that its activity causes nerve-impulses to be transmitted to 
 the nerve-centres. Any animal in which a portion of the ectoderm is thus 
 differentiated and connected may be said to possess an eye i. e. an organ 
 through which the animal may consciously or unconsciously react to the exist- 
 ence of light around it. 1 But the human eye, as well as that of all the higher 
 animals, not only informs us of the existence of light, but enables us to form 
 correct ideas of the direction from which the light conies and of the form, color, 
 and distance of the luminous body. To accomplish this result the substance 
 sensitive to light must form a part of a complicated piece of apparatus capable 
 of very varied adjustments. The eye is, in other words, an optical instrument, 
 and its description, like that of all optical instruments, includes a consideration 
 of its mechanical adjustments and of its refracting media. 
 
 Mechanical Movements. The first point to be observed in studying the 
 movements of the eye is that they are essentially those of a ball-and-socket 
 joint, the globe of the eye revolving freely in the socket formed by the capsule 
 of Tenon through a horizontal angle of almost 88 and a vertical angle of about 
 80. The centre of rotation of the eye (which is not, however, an absolutely 
 fixed point) does not coincide with the centre of the eyeball, but lies a little 
 behind it. It is rather farther forward in hypermetropic than in myopic eyes. 
 The movements of the eye, especially those in a horizontal direction, are sup- 
 plemented by the movements of the head upon the shoulders. The combined 
 eye and head movements are in most persons sufficiently extensive to enable 
 the individual, without any movement of the body, to receive upon the lateral 
 portion of the retina the image of an object directly behind his back. The 
 rotation of the eye in the socket is of course easiest and most extensive when 
 the eyeball has an approximately spherical shape, as in the normal or emme- 
 tropic eye. When the antero-posterior diameter is very much longer than those 
 
 1 In certain of the lower orders of animals no local differentiations seem to have occurred, 
 and the whole surface of the body appears to be obscurely sensitive to light. See Nagel : Der 
 Lichtxinn augerdoser Thiere, Jena, 1896. 
 744 
 
THE SENSE OF VISION. 745 
 
 at right angles to it, as in extremely myopic or short-sighted eyes, the rotation 
 of the eyeball may be considerably limited in its extent. In addition to the 
 movements of rotation round a centre situated in the axis of vision, the eye- 
 ball may be moved forward and backward in the socket to the-xtent of about 
 one millimeter. This movement may be observed whenever the eyelids are 
 widely opened, and is supposed to be effected by the simultaneous contraction of 
 both the oblique muscles. A slight lateral movement has also been described. 
 The movements of the eye will be best understood when considered as 
 referred to three axes at right angles to each other and passing through the 
 centre of rotation of the eye. The first of these axes, which may be called 
 the longitudinal axis, is best described as coinciding with the axis of vision 
 when, with head erect, we look straight forward to the distant horizon ; the 
 second, or transverse, axis is defined as a line passing through the centres of 
 rotation of the two eyes ; and the third, or vertical, axis is a vertical line nec- 
 essarily perpendicular to the other two and also passing through the centre of 
 rotation. When the axis of vision coincides with the longitudinal axis, the eye 
 is said to be in the primary position. When it moves from the primary posi- 
 tion by revolving around either the transverse or the vertical axis, it is said to 
 assume seeondary positions. All other positions are called tertiary positions, 
 and are reached from the primary position by rotation round an axis which 
 lies in the same plane as the vertical and horizontal axis i. e. in the " equato- 
 rial plane " of the eye. When the eye passes from a secondary to a tertiary 
 position, or from one tertiary position to another, the position assumed by the 
 eye is identical with that which it would have had if it had reached it from 
 the primary position by rotation round an axis in the equatorial plane. In 
 other words, every direction of the axis of vision is associated with a fixed 
 position of the whole eye a condition of the greatest importance for the easy 
 and correct use of the eyes. A rotation of the eye round its antero-posterior 
 axis takes place in connection with certain movements, but authorities diifer 
 with regard to the direction and amount of this rotation. 
 
 Muscles of the Eye. The muscles of the eye are six in number viz : 
 the superior, inferior, internal and external recti, and the superior and inferior 
 oblique. This apparent superfluity of muscles (for four muscles would suffice 
 to turn the eye in any desired direction) is probably of advantage in reducing 
 the amount of muscular exertion required to put the eye into any given posi- 
 tion, and thus facilitating the recognition of slight differences of direction, for, 
 according to Fechner's psycho-physic law the smallest perceptible difference in 
 a sensation is proportionate to the total amount of the sensation. Hence if the 
 eye can be brought into a given position by a slight muscular effort, a change 
 from that position will be more easily perceived than if a powerful effort were 
 necessary. 
 
 Each of the eye-muscles, acting singly, tends to rotate the eye round an axis 
 which may be called the axis of rotation of that muscle. Now, none of the 
 muscles have axes of rotation lying exactly in the equator of the eye i.e. 
 in a plane passing through the centre of rotation perpendicular to the axis 
 
746 ^4^ AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 of vision. 1 But all movements of the eye from the primary position take place, 
 as we have seen, round an axis lying in this plane. Hence all such movements 
 must be produced by more than one muscle, and this circumstance also is prob- 
 ably of advantage in estimating the extent and direction of the movement. In 
 this connection it is interesting to note that the eye-muscles have an exception- 
 ally abundant nerve-supply a fact which it is natural to associate with their 
 power of extremely delicate adjustment. It has been found by actual count 
 that in the muscles of the human eye each nerve-fibre supplies only two or three 
 muscle-fibres, while in the muscles of the limbs the ratio is as high as 1 to 
 40-1 25. 2 
 
 Although each eye has its own supply of muscles and nerves, yet the two 
 eyes are not independent of each other in their movements. The nature of 
 their connections with the nerve-centres is such that only those movements are, 
 as a rule, possible in which both axes of vision remain in the same plane. This 
 oondition being fulfilled, the eyes may be together directed to any desired point 
 above, below, or at either side of the observer. The axes may also be con- 
 verged, as is indeed necessary in looking at near objects, and to facilitate this 
 convergence the internal recti muscles are inserted nearer to the cornea than the 
 other muscles of the eye. Though in the ordinary use of the eyes there is never 
 any occasion ^to diverge the axes of vision, yet most persons are able to effect a 
 divergence of about four degrees, as shown by their power to overcome the ten- 
 dency to double vision produced by holding a prism in front of one of the eyes. 
 The nervous mechanism through which this remarkable co-ordination of the 
 muscles of the two eyes is effected, and their motions limited to those which 
 are useful in binocular vision, is not completely understood, but it is supposed 
 to have its seat in part in the tubercula quadrigemina, in connection with the 
 nuclei of origin of the third, fourth, and sixth cranial nerves. Its disturbance 
 by disease, alcoholic intoxication, etc. causes strabismus, confusion, dizziness, 
 and double vision. 
 
 A nerve termination sensitive to light, and so arranged that it can be turned 
 in different directions, is sufficient to give information of the direction from 
 which the light comes, for the contraction of the various eye-muscles indicates, 
 through the nerves of muscular sense, the position into which the eye is nor- 
 mally brought in order to best receive the luminous rays, or, in other words, 
 the direction of the luminous body. The eye, however, informs us not only of 
 the direction, but of the form of the object from which the light proceeds; and 
 to understand how this is effected it will be necessary to consider the refracting 
 media of the eye by means of which an optical image of the luminous object 
 is thrown upon the expanded termination of the optic nerve viz. the retina. 
 
 Dioptric Apparatus of the Eye. For the better comprehension of this 
 portion of the subject a few definitions in elementary optics mny be given. A 
 
 1 The axes of rotation of the internal and external recti, however, deviate l>ut slightly from 
 the equatorial plane. 
 
 2 P. Tergast : " Ueber das Verhiiltniss von Nerven und Muskelu," Arclu'r fur mikr. Anal.. 
 ix. 36-46. 
 
THE SENSE OF VISION. 
 
 '41 
 
 dioptric system in its simplest form consists of two adjacent media which have 
 different indices of refraction and whose surface of separation is the segment 
 of a sphere. A line joining the middle of the segment with the centre of the 
 sphere and prolonged in either direction is called the axis of the system. Let 
 the line APE in Figure 213 represent in section such a spherical surface the 
 
 B 
 
 FIG. 213. Diagram of simple optical system (after Foster). 
 
 centre of which is at N, the rarer medium being to the left and the denser me- 
 dium to the right of the line. Any ray of light which, in passing from the 
 rarer to the denser medium, is normal to the spherical surface will be unchanged 
 in its direction i. e. will undergo no refraction. Such rays are represented by 
 the lines P y MD, and M f E. If a pencil of rays having its origin in the rarer 
 medium at any point in the axis falls upon the spherical surface, there will be 
 one ray viz. the one which coincides with the axis of the system, which will 
 pass into the second medium unchanged in its direction. This ray is called 
 the principal ray (OP), and its point of intersection (P) with the spherical 
 surface is called the principal point. The centre of the sphere (N) through 
 which the principal ray necessarily passes is called the nodal point. All the 
 other rays in the pencil are refracted toward the principal ray by an amount 
 
 FIG. 214. Diagram to show method of finding principal foci (Neumann). 
 
 which depends, for a given radius of curvature, upon the difference in the 
 refractive power of the media, or, in other words, upon the retardation of light 
 in passing from one medium to the other. If the incident rays have their 
 origin at a point infinitely distant on the axis /. e. if they are parallel to each 
 other they will all be refracted to a point behind the spherical surface known 
 
748 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 as the principal focus , F. There is another principal focus (F f ) in front of the 
 spherical surface viz. the point from which diverging incident rays will be 
 refracted into parallelism on passing the spherical surface, or, in other words, 
 the point at which parallel rays coming from the opposite direction will be 
 brought to a focus. The position of these two principal foci may be deter- 
 mined by the construction shown in Figure 214. Let CA C f represent a sec- 
 tion of a spherical refracting surface with the axis A N 9 the nodal point N f and 
 the principal point A. The problem is to find the foci of rays parallel to the 
 axis. Erect perpendiculars at A and N. Set off on each perpendicular dis- 
 tances No, Np, A o f , Ap r proportionate to the rapidity of light in the two media 
 (e. g. 2 : 3). The points where the lines p' o and p o' prolonged will cut the 
 axis are the two principal foci F and F' i. e. the points at which parallel rays 
 coming from either direction are brought to a focus after passing the spherical 
 refracting surface. If the rays are not parallel, but diverging i. e. coming 
 from an object at a finite distance the point where the rays will be brought to 
 a focus, or, in other words, the point where the optical image of the luminous 
 object will be formed, may be determined by a construction which combines 
 any two of the three rays whose course is given in the manner above described. 
 Thus in Figure 215 let AN be the axis, and F and F f the principal foci of 
 
 \ 
 
 FIG. 215. Diagram to show method of finding conjugate foci. 
 
 the spherical refracting surface CA C f , with a nodal point at N. Let B be 
 the origin of a pencil of rays the focus of which is to be determined. Draw 
 the line B C representing the course of an incident ray parallel to the axis. 
 This ray will necessarily be refracted through the focus F, its course being 
 represented by the line CF and its prolongation. Similarly, the incident ray 
 passing through the focus F' and striking the spherical surface at C' will, after 
 refraction, be parallel to the axis i. e. it will have the direction C f b. The 
 principal ray of the pencil will of course pass through the spherical surface and 
 the nodal point N without change of direction. These three rays will come 
 together at the same point 6, the position of which may be determined by con- 
 structing the course of any two of the three. The points B and b are called 
 conjugate foci, and are related to each other in such a way that an optical image 
 is formed at one point of a luminous object situated at the other. When the 
 rays of light pass through several refracting surfaces in succession their course 
 may be determined by separate calculations for each surface, a process which 
 is much simplified when the surfaces are " centred " i. e. have their centres 
 of curvature lying in the same axis, as is approximately the case in the eye. 
 
 Refracting- Media of the Eye. Rays of light in passing through the eye 
 penetrate seven different media and are refracted at seven surfaces. The media 
 
THE SENSE OF VISION. 749 
 
 are as follows : layer of tears, cornea, aqueous humor, anterior capsule of lens 
 lens, posterior capsule of lens, vitreous humor. The surfaces are those which 
 separate the successive media from each other and that which separates the tear 
 layer from the air. For purposes of practical calculation thejmmber of 
 faces and media may be reduced to three. In the first place, the layer of tears 
 which moistens the surface of the cornea has the same index of refraction as 
 the aqueous humor. Hence the index of refraction of the cornea may be left 
 out of account, since, having practically parallel surfaces and being bounded 
 on both sides by substances having the same index of refraction, it does not 
 influence the direction of rays of light passing through it. 
 reason objects seen obliquely through a window a PI >ear in their true directi 
 the refraction of the rays of light on entering the glass being equal in amount 
 and opposite in direction to that which occurs in leaving it. For purpose 
 optical calculation we may, therefore, disregard the refraction of the cornea 
 (which moreover, does not differ materially from that of the aqueous humor), 
 and imagine the aqueous humor extending forward to the anterior surface of 
 the layer of tears which bathes the corneal epithelium. Furthermore, the cap- 
 sule of the lens has the same index of refraction as the outer layer of the lens 
 itself, and for optical purposes may be regarded as replaced by it. 
 the optical apparatus of the eye may be regarded as consisting of the : 
 lowing three refracting media: Aqueous humor, index of refraction 1.35 
 lens, average index of refraction 1.45; vitreous humor, index of refraction 
 1 33 The surfaces at which refraction occurs are also three in number : An- 
 terior surface of cornea, radius of curvature 8 millimeters; anterior surface 
 of lens radius of curvature 10 millimeters; posterior surface of lens, radius of 
 curvature 6 millimeters. It will thus be seen that the anterior surface < 
 lens is less and the posterior surface more convex than the cornea. 
 
 To the values of the optical constants of the eye as above given may I 
 added the following : Distance from the anterior surface of the cornea to the 
 anterior surface of the lens, 3.6 millimeters; distance from the posterior sur- 
 face of the lens to the retina, 15. millimeters ; thickness of lens, 3.6 millimeters. 
 The methods usually employed for determining these constants are the fol- 
 lowing: The indices of refraction of the aqueous and vitreous humor are 
 determined by filling the space between a glass lens and a glass plate with the 
 fresh humor/ The aqueous or vitreous humor thus forms a convex or concave 
 lens from the form and focal distance of which the index can be calculated. 
 Another method consists in placing a thin layer of the medium between the 
 hypothenuse surfaces of two right-angled prisms and determining the angle 
 which total internal reflection takes place. In the case of the crystalline le 
 the index is found by determining its focal distance as for an ordinary 1 ns 
 and solving the equation which expresses the value of the index in terms of 
 radius of curvature and focal distance, thickness, and focal length, 
 refractive index of the lens increases from the surface toward the centre, a 
 peculiarity which tends to correct the disturbances due to spherical aberration, 
 as well as to increase the refractive power of the lens as a whole. 
 
750 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 The curvature of the refracting surfaces of the eye is determined by an 
 instrument known as an ophthalmometer, which measures the size of the 
 reflected image of a known object in the various curved surfaces. The 
 radius of curvature of the surface is determined by the following formula : 
 
 r 2Ab 
 
 B :b = A:-; OT r -> i n which B the size of the object, b = the size of 
 
 the image, A = distance between the object and the reflecting surface, and 
 r = the radius of the reflecting surface. The distances between the various 
 surfaces of the eye are measured on frozen sections of the organ, or can be 
 determined upon the living eye by optical methods too complicated to be here 
 described. It should be borne in mind that the above values of the so-called 
 "optical constants" of the eye are subject to considerable individual variation, 
 and that the statements of authors concerning them are not always consistent. 
 The refracting surfaces of the eye may be regarded as still further sim- 
 plified, and a so-called " reduced eye " constructed which is very useful for 
 purposes of optical calculation. This reduced eye, which for optical purposes 
 is the equivalent of the actual eye, is regarded as consisting of a single refract- 
 ing medium having an index of 1.33, a radius of curvature of 5.017 milli- 
 meters, its principal point 2.148 millimeters behind the anterior surface of the 
 cornea, and its nodal point 0.04 millimeter in front of the posterior surface 
 of the lens. 1 The principal foci of the reduced eye are respectively 12.918 
 millimeters in front of and 22.231 millimeters behind the anterior surface of 
 the cornea. Its optical power is equal to 50.8 dioptrics. 2 The position of this 
 imaginary refracting surface is indicated by the dotted line in figure 216. The 
 
 PIG. 216. Diagram of the formation of a retinal image (after Foster). 
 
 nodal point, n, in this construction may be regarded as the crossing-point of all 
 the principal rays which enter the eye, and, as these rays are unchanged in their 
 direction by refraction, it is evident that the image of the point whence they 
 proceed will be formed at the point where they strike the retina. Hence to 
 determine the size and position of the retinal image of any external object 
 e. g. the arrow in Figure 216 it is only necessary to draw lines from various 
 
 1 Strictly speaking, there are in this imaginary refracting apparatus which is regarded as 
 equivalent to the actual eye two principal and two nodal points, each pair about 0.4 millimeter 
 apart. The distance is so small that the two points may, for all ordinary constructions, be 
 regarded as coincident. 
 
 2 The optical power of a lens is the reciprocal of its focal length. The dioptry or unit of 
 optical power is the power of a lens with a focal length of 1 meter. 
 
THE SENSE OF VISION. 
 
 751 
 
 points of the object through the above-mentioned nodal point and to prolong 
 them till they strike the retina. It is evident that the size of the retinal image 
 will be as much smaller than that of the object as the distance of the nodal 
 point from the retina is smaller than its distance from the object^ 
 
 According to the figures above given, the nodal point is about 7.2 milli- 
 meters behind the anterior surface of the cornea and about 15.0 millimeters in 
 front of the retina. Hence the size of the retinal image of an object of known 
 size and distance can be readily calculated a problem which has frequently to be 
 solved in the study of physiological optics. The construction given. in Figure 
 216 shows that from all external objects inverted images are projected upon the 
 retina, and such inverted images can actually be seen under favorable condi- 
 tions. If, for instance, the eye of a white rabbit, which contains no choroidal 
 pigment, be excised and held with the cornea directed toward a window or 
 other source of light, an inverted image of the luminous object will be seen 
 through the transparent sclerotic in the same way that one sees an inverted 
 image of a landscape on the ground-glass plate of a photographic camera. 
 The question is often asked, " Why, if the images are inverted in the retina, 
 do we not see objects upside down ?" The only answer to such a question is 
 that it is precisely because images are inverted on the retina that we do not see 
 objects upside down, for the eye has learned through lifelong practice to asso- 
 ciate an impression made upon any portion of the retina with light coming 
 from the opposite portion of the field of vision. Thus if an image falls upon 
 the lower portion of the retina, our experience, gained chiefly through mus- 
 cular movements and tactile sensations, has taught us that this image must cor- 
 respond to an object in the upper portion of our field of vision. In whatever 
 way the retina is stimulated the same effect is produced. If, for instance, 
 gentle pressure is made with the finger on the lateral portion of the eyeball 
 through the closed lids a circle of light known as a phosphene immediately 
 appears on the opposite side of the eye. Another good illustration of the 
 same general rule is found in the effect of throwing a shadow upon the retina 
 from an object as close as possible to the eye. For this purpose place a card 
 
 B p 
 
 FIG. 217. Diagram illustrating the projection of a shadow on the retina. 
 
 with a small pin-hole in it in front of a source of light, and three or four 
 centimeters distant from the eye. Then hold some object smaller than the 
 pupil e. g. the head of a pin as close as possible to the cornea. Under these 
 conditions neither the pin-hole nor the pin-head can be really seen i. e. they 
 
752 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 are both too near to have their image focussed upon the retina. The pin-hole 
 becomes itself a source of light, and appears as a luminous circle bounded by 
 the shadow thrown by the edge of the iris. Within this circle of light is seen 
 the shadow of the pin-head, but the pin-head appears inverted, for the obvious 
 reason that the eye, being accustomed to interpret all retinal impressions as 
 corresponding to objects in the opposite portion of the field of vision, regards 
 the upright shadow of the pin-head as the representation of an inverted object. 
 The course of the rays in this experiment is shown in Figure 217, in which 
 A R represents the card with a pin-hole in it, P the pin, and P' its upright 
 shadow thrown on the retina. 
 
 Accommodation. From what has been said of conjugate foci and their 
 relation to each other it is evident that any change in the distance of the object 
 from the refracting media will involve a corresponding change in the position 
 of the image, or, in other words, only objects at a given distance can be 
 focussed upon a plane which has a fixed position with regard to the refracting 
 surface or surfaces. Hence all optical instruments in which the principle of 
 conjugate foci finds its application have adjustments for distance. In the 
 telescope and opera-glass the adjustment is effected by changes in the distance 
 between the lenses, and in the photographic camera by a change in the posi- 
 tion of the ground-glass plate representing the focal plane. In the microscope 
 the adjustment is effected by changing the distance of the object to suit the 
 lenses, the higher powers having a shorter " working distance." 
 
 We must now consider in what way the eye adapts itself to see objects dis- 
 tinctly at different distances. That this power of adaptation, or " accommo- 
 dation," really exists we can easily convince ourselves by looking at different 
 objects through a network of fine wire held near the eyes. When with normal 
 vision the eyes are directed to the distant objects the network nearly disappears, 
 and if we attempt to see the network distinctly the outlines of the distant 
 objects become obscure. In other words, it is impossible to see both the 
 network and the distant objects distinctly at the same time. It is also evident 
 that in accommodation for distant objects the eyes are at rest, for when they 
 are suddenly opened after having been closed for a short time they are found 
 to be accommodated for distant objects, and we are conscious of a distinct 
 effort in directing them to any near object. 1 
 
 From the optical principles above described it is clear that the accommo- 
 dation of the eye for near objects may be conceived of as taking place in three 
 different ways : 1st, By an increase of the distance between the refracting sur- 
 faces of the eye and the retina ; 2d, By an increase of the index of refraction 
 of one or more of the media ; 3d, By a diminution of the radius of curvature 
 of one or more of the surfaces. The first of these methods was formerly sup- 
 posed to be the one actually in use, a lengthening of the eyeball under a pres- 
 
 1 It has been shown by Beer (Archivfur die gesammle Physiologie, Iviii. 523) that in fishes 
 the eyes when at rest are accommodated for near objects, and that accommodation for distant 
 objects is effected by the contraction of a muscle for which the name "retractor lentis" is pro- 
 posed. 
 
THE SENSE OF VISION. 753 
 
 sure produced by the eye-muscles being assumed to occur. This lengthening 
 would, in the case of a normal eye accommodating itself for an object at a 
 distance of 15 centimeters, amount to not less than 2 millimeters a change 
 which could hardly be brought about by the action of any muscles connected 
 with the eye. Moreover, accommodation changes can be observed upon elec- 
 trical stimulation of the excised eye. Its mechanism must, therefore, lie within 
 the eye itself. As for the second of these methods, there is no conceivable way 
 by which a change in the index of refraction of the media can be eifected, and 
 we are thus forced to the conclusion that accommodation is brought about by 
 a change in the curvature of the refracting surfaces i. e. by a method quite 
 different from any which is employed in optical instruments of human con- 
 struction. Now, by measuring the curvature of the cornea of a person who 
 looks alternately at near and distant objects it has been shown that the cornea 
 undergoes no change of form in the act of accommodation. By a process of 
 exclusion, therefore, the lens is indicated as the essential organ in this function 
 of the eye, and, in fact, the complicated structure and connections of the lens 
 at once suggest the thought that it is in the surfaces of this portion of the eye 
 that the necessary changes take place. Indeed, from a teleological point of 
 view the lens would seem somewhat superfluous if it were not important to 
 have a transparent refracting body of variable form in the eye, for the amount 
 of refraction which takes place in the lens could be produced by a slightly 
 increased curvature of the cornea. Now, the changes of curvature which occur 
 in the surfaces of the lens when the eye is directed to distant and near objects 
 alternately can be actually observed and measured with considerable accuracy. 
 For this purpose the changes in the form, size, and position of the images of 
 brilliant objects reflected in these two surfaces are studied. If a candle is held 
 in a dark room on a level with and about 50 centimeters away from the eye in 
 which the accommodation is to be studied, an observer, so placed that his own 
 axis of vision makes about the same angle (15-20) with that of the ob- 
 served eye that is made by a line joining the observed eye and the candle, will 
 readily see a small upright image of the candle reflected in the cornea of the 
 observed eye. Near this and within the outline of the pupil are two other 
 images of the candle, which, though much less easily seen than the corneal 
 image, can usually be made out by a proper adjustment of the light. The 
 first of these is a large faint upright image reflected from the anterior surface 
 of the lens, and the second is a small inverted image reflected from the pos- 
 terior surface of the lens. It will be observed that the size of these images 
 varies with the radius of curvature of the three reflecting surfaces as given on 
 p. 749. The relative size and position of these images having been recog- 
 nized while the eye is at rest i. e. is accommodated for distance let the 
 person who is under observation be now requested to direct his eye to a near 
 object lying in the same direction. When this is done the corneal image and 
 that reflected from the posterior surface of the lens will remain unchanged, 1 
 
 1 A very slight diminution in size may sometimes be observed in the image reflected from 
 the posterior surface of the lens. 
 48 
 
754 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 while that reflected from the anterior surface of the lens will become smaller 
 and move toward the corneal image. This change in the size and position of 
 the reflected image can only mean that the surface from which the reflection 
 takes place has become more convex and has moved forward. Coincident 
 with this change a contraction of the pupil will be observed. 
 
 An apparatus for making observations of this sort is known as the phako- 
 scope of Helmholtz (Fig. 218). The eye in which the changes due to accom- 
 modation are to be observed is placed at an opening 
 in the back of the instrument at C, and directed al- 
 ternately to a needle placed in the opening D and 
 to a distant object lying in the same direction. Two 
 prisms at B and B' serve to throw the light of a 
 candle on to the observed eye, and the eye of an 
 observer at A sees the three reflected images, each 
 as two small square spots of light. The movement 
 and the change of size of the image reflected from 
 the anterior surface of the lens can be thus much 
 better observed than when a candle-flame is used. 
 The course of the rays of light in this experi- 
 ment is shown diagrammatically in Figure 219. 
 The observed eye is directed to the point A, while 
 the candle and the eye of the observer are placed 
 symmetrically on either side. The images of the candle reflected from the various 
 surfaces of the eye will be seen projected on the dark background of the pupil 
 
 FIG. 218. Phakoscope of 
 Helmholtz. 
 
 FIG. 219. Diagram explaining the change in the position of the image reflected from the anterior surface 
 of the crystalline lens (Williams, after Bonders). 
 
 in the directions indicated by the dotted lines ending at a, 6, and c. When the 
 eye is accommodated for a near object the middle one of the three images moves 
 nearer the corneal image i. e. it changes in its direction from b to b', showing 
 that the anterior surface of the lens has bulged forward into the position indi- 
 
THE SENSE OF VISION. 
 
 755 
 
 cated by the dotted line. The change in the appearance of the images is 
 represented diagrammatically in Figure 220. On the left is shown the appear- 
 ance of the images as seen when the eye is at rest, a representing the corneal 
 image, b that reflected from the anterior, and c that from the posterior surface 
 of the lens when the observing eye and the candle are in the position repre- 
 
 FIG. 220. Reflected images of a candle-flame as seen in the pupil of an eye at rest and accommodated 
 
 for near objects (Williams). 
 
 sented in Figure 219. The images are represented as they appear in the dark 
 background of the pupil, though of course the corneal image may, in certain 
 positions of the light, appear outside of the pupillary region. When the eye 
 is accommodated for near objects the images appear as shown in the circle on 
 the right, the image 6 becoming smaller and brighter and moving toward the 
 corneal image, while the pupil contracts as indicated by the circle drawn round 
 the images. 
 
 The changes produced in the eye by an effort of accommodation are indi- 
 cated in Figure 221, the left-hand side of the diagram showing the condition 
 
 FIG. 221. Showing changes in the eye produced by the act of accommodation (Helmholtz). 
 
 of the eye at rest, and the right-hand side that in extreme accommodation for 
 near objects. 
 
 It will be observed that the iris is pushed forward by the bulging lens and 
 that its free border approaches the median line. In other words, the pupil is 
 contracted in accommodation for near objects. The following explanation of 
 the mechanism by which this change in the shape of the lens is effected has 
 been proposed by Helmholtz, and is still generally accepted. The structure 
 of the lens is such that by its own elasticity it tends constantly to assume a 
 more convex form than the pressure of the capsule and the tension of the sus- 
 pensory ligaments (s, s, Fig. 221) allow. This pressure and tension are dimin- 
 ished when the eye is accommodated for near vision by the contraction of the 
 ciliary muscles (c, c, Fig. 221), most of whose fibres, having their origin at the 
 
756 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 point of union of the cornea and sclerotic, extend radially outward in every 
 direction and are attached to the front part of the choroid. The contrac- 
 tion of the ciliary muscle, drawing forward the membranes of the eye, will 
 relax the tension of the suspensory ligament and allow the lens to take 
 the form determined by its own elastic structure. According to another 
 theory of accommodation proposed by Tscherning, 1 the suspensory liga- 
 ment is stretched and not relaxed by the contraction of the ciliary muscle. 
 In consequence of the pressure thus produced upon the 
 lens, the soft external portions are moulded upon the 
 harder nuclear portion in such a way as to give to the 
 anterior (and to some extent to the posterior) surface a 
 hyperboloid instead of a spherical form. A similar theory 
 has been recently brought forward by Schoen, 2 who com- 
 pares the action of the ciliary muscle upon the lens to that 
 of the fingers compressing a rubber ball, as shown in Fig- 
 ure 222. These theories have an advantage over that 
 offered by Helmholtz, inasmuch as they afford au expla- 
 nation of the presence in the ciliary muscle of circular 
 fibres, which, on the theory of Helmholtz, seem to be su- 
 perfluous. They also make the fact of so-called " astig- 
 FIG. 222. TO illustrate matic accommodation" comprehensible. This term is 
 
 Schoen's theory of ac- ,. , ,, i j. i > j n 
 
 commodation. applied to the power said to be sometimes gradually 
 
 acquired by persons with astigmatic 3 eyes of correcting 
 this defect of vision by accommodating the eye more strongly in one meridian 
 than another. 4 
 
 Whatever views may be entertained as to the exact mechanism by which its 
 change of shape is brought about, there can be no doubt that the lens is the 
 portion of the eye chiefly or wholly concerned in accommodation, and it is 
 accordingly found that the removal of the lens in the operation for cataract 
 destroys the power of accommodation, and the patient is compelled to use 
 convex lenses for distant and still stronger ones for near objects. 
 
 It is interesting to notice that the act of accommodation, though distinctly 
 voluntary, is performed by the agency of the unstriped fibres of the ciliary 
 muscles. It is evident, therefore, that the term " involuntary " sometimes 
 applied to muscular fibres of this sort may be misleading. The voluntary 
 character of the act of accommodation is not affected by the circumstance that 
 the will needs, as a rule, to be assisted by visual sensations. The fact that 
 most persons cannot affect the necessary change in the eye unless they direct 
 their attention to some near or far object is only an instance of the close rela- 
 tion between sensory impressions and motor impulses, which is further exem- 
 
 1 Archives de Physiologic, 1894, p. 40. 2 Archiv fur die gesammte Phys., lix. 427. 
 
 3 See p. 763. 
 
 * Recent observations by Hess (Archiv f. Ophthalmologie, xlii. 288) tend to confirm the Helm- 
 holtz theory by showing that the suspensory ligament is relaxed and not stretched in accommo- 
 dation for near objects. 
 
THE SENSE OF VISION. 757 
 
 plified by such phenomena as the paralysis of the lip of a horse caused by the 
 division of the trifacial nerve. It is found, moreover, that by practice the 
 power of accommodating the eye without directing it to near and distant 
 objects can be acquired. The nerve-channels through which_accommodation 
 is affected are the anterior part of the nucleus of the third pair of nerves 
 lying in the extreme hind part of the floor of the third ventricle, the most 
 anterior bundle of the nerve-root, the third nerve itself, the lenticular ganglion, 
 and the short ciliary nerves (see diagram p. 769). 
 
 The mechanism of accommodation is affected in a remarkable way by drugs, 
 the most important of which are atropia and physostigmin, the former para- 
 lyzing and the latter stimulating the ciliary muscle. As these drugs exert a 
 corresponding effect upon the iris, it will be convenient to discuss their action 
 in connection with the physiology of that organ. 
 
 The changes occurring in the eye during the act of accommodation are 
 indicated in the following table, which shows, both for the actual and the 
 reduced eye, the extent to which the refracting media change their form and 
 position, and the consequent changes in the position of the foci : 
 
 Accommodation for 
 Actual Eye. distant objects. near objects. 
 
 Radius of cornea 8 mm. 8 mm. 
 
 Radius of anterior surface of lens 10 " 6 " 
 
 Radius of posterior surface of lens 6 5.5 " 
 
 Distance from cornea to anterior surface of lens . . 3.6 " 3.2 " 
 
 Distance from cornea to posterior surface of lens . 7.2 " 7.2 " 
 
 Reduced Eye. 
 
 Radius of curvature 5.02 " 4.48 " 
 
 Distance from cornea to principal point 2.15 " 2.26 " 
 
 Distance from cornea to nodal point 7.16 " 6.74 " 
 
 Distance from cornea to anterior focus 12.918 " 11.241 " 
 
 Distance from cornea to posterior focus 22.231 " 20.248 " 
 
 It will be noticed that no change occurs in the curvature of the cornea, and 
 next to none in the posterior surface of the lens, while the anterior surface of 
 the lens undergoes material alterations both in its shape and position. 
 
 Associated with the accommodative movements above described, two other 
 changes take place in the eyes to adapt them for near vision. In the first 
 place, the axes of the eyes are converged upon the near object, so that the 
 images formed in the two eyes shall fall upon corresponding points of the 
 retinas, as will be more fully explained in connection with the subject of 
 binocular vision. In the second place, the pupil becomes contracted, thus 
 reducing the size of the pencil of rays that enters the eye. The importance 
 of this movement of the pupil will be better understood after the subject of 
 spherical aberration of light has been explained. These three adjustments, 
 focal, axial, and pupillary, are so habitually associated in looking at near objects 
 that the axial can only by an effort be dissociated from the other two, while 
 these two are quite inseparable from one another. This may be illustrated 
 by a simple experiment. On a sheet of paper about 40 centimeters distant 
 
758 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 from the eyes draw two letters or figures precisely alike and about 3 centimeters 
 apart. (Two letters cut from a newspaper and fastened to the sheet will answer 
 the same purpose.) Hold a small object like the head of a pin between the 
 eyes and the paper at the point of intersection of a line joining the right eye 
 and the left letter with a line joining the left eye and the right letter. If the 
 axes of vision are converged upon the pin-head, that object will be seen dis- 
 tinctly, and beyond it will be seen indistinctly three images of the letter, the 
 central one being formed by the blending of the inner one of each pair of 
 images formed on the two retinas. If now the attention be directed to the 
 middle image, it will gradually become perfectly distinct as the eye accommo- 
 dates itself for that distance. We have thus an axial adjustment for a very 
 near object and a focal adjustment for a more distant one. If the pupil of the 
 individual making this observation be watched by another person, it will be 
 found that at the moment when the middle image of the letter becomes distinct 
 the pupil, which had been contracted in viewing the pin-head, suddenly dilates. 
 It is thus seen that when the axial and focal adjustments are dissociated from 
 each other the pupillary adjustment allies itself with the latter. 
 
 The opposite form of dissociation viz. the axial adjustment for distance 
 and the focal adjustment for near vision is less easy to bring about. It may 
 perhaps be best accomplished by holding a pair of stereoscopic pictures before 
 the eyes and endeavoring to direct the right eye to the right and the left eye to 
 the left picture i. e. to keep the axes of vision parallel while the eyes are 
 accommodated for near objects. One who is successful in this species of ocular 
 gymnastics sees the two pictures blend into one having all the appearance of 
 a solid object. The power of thus studying stereoscopic pictures without a 
 stereoscope is often a great convenience to the possessor, but individuals differ 
 very much in their ability to acquire it. 
 
 Range of Accommodation. By means of the mechanism above described 
 it is possible for the eye to produce a distinct image upon the retina of objects 
 lying at various distances from the cornea. The point farthest from the eye 
 at which an object can be distinctly seen is called the far-point, and the nearest 
 point of distinct vision is called the near-point of the eye, and the distance 
 between the near-point and the far-point is called the range of distinct vision 
 or the range of accommodation. As the normal emmetropic eye is adapted, 
 when at rest, to bring parallel rays of light to a focus upon the retina, its far- 
 point may be regarded as at an infinite distance. Its near-point varies with age, 
 as will be described under Presbyopia. In early adult life it is from 10 to 
 13 centimeters from the eye. For every point within this range there will be 
 theoretically a corresponding condition of the lens adapted to bring rays pro- 
 ceeding from that point to a focus on the retina, but as rays reaching the eye 
 from a point 175 to 200 centimeters distant do not, owing to the small size of 
 the pupil, differ sensibly from parallel rays, there is no appreciable change in 
 the lens unless the object looked at lies within that distance. It is also evi- 
 dent that as an object approaches the eye a given change of distance will 
 cause a constantly increasing amount of divergence of the rays proceeding from 
 
THE SENSE OF VISION. 
 
 759 
 
 it, and will therefore necessitate a constantly increasing amount of change in 
 the lens to enable it to focus the rays on the retina. We find, accordingly, that 
 all objects more than two meters distant from the eye can be seen distinctly at 
 the same time i. e. without any change in the accommodative mechanism 
 but for objects within that distance we are conscious of a special effort of 
 accommodation which becomes more and more distinct the shorter the distance 
 between the eye aa<l the object. 
 
 Myopia and Hypermetropia. There are two conditions of the eye in 
 which the range of accommodation may differ from that which has just been 
 described as normal. These conditions, which are too frequent to be regarded 
 (except in extreme cases) as pathological, are generally dependent upon the 
 eyeball being unduly lengthened or 
 shortened. In Fig. 223 are shown 
 diagram matically the three conditions 
 known as emmetropia, myopia, and 
 hypermetropia. In the normal or 
 emmetropic eye, A, parallel rays are 
 represented as brought to a focus on 
 the retina; in the short-sighted, or 
 myopic, eye, B, similar rays are 
 focussed in front of the retina, since 
 the latter is abnormally distant; while 
 in the over-sighted, or hypermetropic, 
 eye, C, they are focussed behind the 
 retina, since it is abnormally near. 
 
 It is evident that when the eye is 
 at rest both the myopic and the hy- 
 permetropic eye will see distant ob- 
 jects indistinctly, but there is this 
 important difference : that in hyper- 
 metropia the difficulty can be cor- 
 rected by an effort of accommodation, 
 while in myopia this is impossible, 
 since there is no mechanism by which 
 the radius of the lenticular surfaces can be increased. Hence an individual 
 affected with myopia is always aware of the infirmity, while a person with 
 hypermetropic eyes often goes through life unconscious of the defect. In this 
 case the accomodation is constantly called into play even for distant objects, and 
 if the hypermetropia is excessive, any prolonged use of the eyes is apt to be 
 attended by a feeling of fatigue, headache, and a train of nervous symptoms 
 familiar to the ophthalmic surgeon. Hence it is important to discover this defect 
 where it exists and to apply the appropriate remedy viz. convex lenses placed 
 in front of the eyes in order to make the rays slightly convergent when they 
 enter the eye. Thus aided, the refractive power of the eye at rest is sufficient 
 to bring the rays to a focus upon the retina and thus relieve the accommoda- 
 
 FIG. 223. Diagram showing the difference between, 
 normal, myopic, and hypermetropic eyes. 
 
760 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 tion. This action of a convex lens in hyperraetropia is indicated by the dotted 
 lines in Fig. 222, C, and the corresponding use of a concave lens in myopia is 
 shown in Fig. 222, B. 
 
 The detection and quantitative determination of hypermetropia are best 
 made after the accommodation has been paralyzed by the use of atropia, by 
 ascertaining how strong a convex lens must be placed before the eye to pro- 
 duce distinct vision of distant objects. 
 
 The range of accommodation varies very much from the normal in myopic 
 and hypermetropic eyes. In myopia the near-point is often 5 or 6 centimeters 
 from the cornea, while the far-point, instead of being infinitely far off, is at a 
 variable but no very great distance from the eye. The range of accommoda- 
 tion is therefore very limited. In hypermetropia the near-point is slightly 
 farther than normal from the eye, and the far-point cannot be said to exist, 
 for the eye at rest is adapted to bring converging rays to a focus on the retina, 
 and such pencils of rays do not exist in nature. Mathematically, the far-point 
 may be said to be at more than an infinite distance from the eye. The range 
 of effective accommodation is therefore reduced, for a portion of the accommo- 
 dative power is used up in adapting the eye to receive parallel rays. 
 
 Presbyopia. The power of accommodation diminishes with age, owing 
 apparently to a loss of elasticity of the lens. The change is regularly pro- 
 gressive, and can be detected as early as the fifteenth year, though in normal 
 eyes it does not usually attract attention until the individual is between forty 
 and forty-five years of age. At this period of life a difficulty is commonly 
 experienced in reading ordinary type held at a convenient distance from the 
 eye, and the individual becomes old-sighted or presbyopia a condition which 
 can, of course, be remedied by the use of convex glasses. Cases are occasion- 
 ally reported of persons recovering their power of near vision in extreme old 
 age and discontinuing the use of the glasses previously employed for reading. 
 In these cases there is apparently not a restoration of the power of accommo- 
 dation, but an increase in the refractive power of the lens through local changes 
 in its tissue. A diminution in the size of the pupil, sometimes noticed in old 
 age, may also contribute to the distinctness of the retinal image, as will be 
 described in connection with spherical aberration. 
 
 Defects of the Dioptric Apparatus. The above-described imperfections 
 of the eye viz. myopia and hypermetropia being generally (though not 
 invariably) due to an abnormal length of the longitudinal axis, are to be 
 regarded as defects of construction affecting only a comparatively small 
 number of eyes. There are, however, a number of imperfections of the diop- 
 tric apparatus, many of which affect all eyes alike. Of these imperfections 
 some affect the eye in common with all optical instruments, while others are 
 peculiar to the eye and are not found in instruments of human construction. 
 The former class will be first considered. 
 
 Spherical Aberration. It has been stated that a pencil of rays falling 
 upon a .spherical refracting surface will be refracted to a common focus. 
 Strictly speaking, however, the outer rays of the pencil i. e. those which fall 
 
THE SENSE OF VISION. 
 
 761 
 
 near the periphery of the refracting surface will be refracted more than those 
 which lie near the axis and will come to a focus sooner. This phenomenon, 
 which is called spherical aberration, is more marked with diverging than with 
 parallel rays, and tends, of course, to produce an indistinctness of the image 
 which will increase with the extent of the surface through which the rays 
 pass. The effect of a diaphragm used in many optical instruments to reduce 
 the amount of spherical aberration by cutting off the side rays is shown dia- 
 grammatically in Fig. 224. 
 
 FIG. 224. Diagram showing the effect of a diaphragm in reducing the amount of spherical 
 
 aberration. 
 
 The role of the iris in the vision of near objects is now evident, for when 
 the eye is directed to a near object the spherical aberration is increased in con- 
 sequence of the rays becoming more divergent, but the contraction of the 
 pupil which accompanies accommodation tends, by cutting off the side rays, to 
 prevent a blurring of the image which otherwise would be produced. It must, 
 however, be remembered that the crystalline lens, unlike any lens of human 
 construction, has a greater index of refraction at the centre than at the periph- 
 ery. This, of course, tends to correct spherical aberration, and, in so far as it 
 does so, to render the cutting off of the side rays unnecessary. Indeed, the 
 total amount of possible spherical aberration in the eye is so small that its 
 effect on vision may be regarded as insignificant in comparison with that caused 
 by the other optical imperfections of the eye. 
 
 Chromatic Aberration. In the above account of the dioptric apparatus 
 of the eye the phenomena have been described as they would occur with mono- 
 chromatic light i. e. with light having but one degree of refrangibility. But 
 the light of the sun is composed of an infinite number of rays of different 
 degrees of refrangibility. Hence when an image is formed by a simple lens 
 the more refrangible rays ?'. e. the violet rays of the spectrum are brought 
 to a focus sooner than the less refrangible red rays. The image therefore 
 
762 AX AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 appears bordered by fringes of colored light. This phenomenon of chromatic 
 aberration can be well observed by looking at objects through the lateral por- 
 tion of a simple lens, or, still better, by observing them through two simple 
 lenses held at a distance apart equal to the sum of their focal distances. The 
 objects will appear inverted (as through an astronomical telescope) and sur- 
 rounded with borders of colored light. Now, the chromatic aberration of the 
 eve is so slight that it is not easily detected, and the physicists of the eighteenth 
 century, in their efforts to produce an achromatic lens, seem to have been 
 impressed by the fact that in the eye a combination of media of different 
 refractive powers is employed, and to have sought in this circumstance an 
 explanation of the supposed achromatism of the eye. Work directed on this 
 line was crowned with brilliant success, for by combining two sorts of glass of 
 different refractive and dispersive powers it was found possible to refract a ray 
 of light without dispersing it into its different colored rays, and the achromatic 
 lens, thus constructed, became at once an essential part of every first-class opti- 
 cal instrument. Now, as there is not only no evidence that the principle of 
 the achromatic lens is employed in the eye, but distinct evidence that the eye 
 is uncorrected for chromatic aberration, we have here a remarkable instance of 
 a misconception of a physical fact leading to an important discovery in physics. 
 The chromatic aberration of the eye, though so slight as not to interfere at all 
 with ordinary vision, can be readily shown to exist by the simple experiment 
 of covering up one half of the pupil and looking at a bright source of light 
 e. g. a window. If the lower half of the pupil be covered, the cross-bars of 
 
 FIG. 225. Diagram to illustrate chromatic aberration. 
 
 the window will appear bordered with a fringe of blue light on the lower and 
 reddish light on the upper side. The explanation usually given of the way in 
 which this result is produced is illustrated in Fig. 225. Owing to the chromatic 
 aberration of the eye all the rays emanating from an object at A are not 
 focussed accurately on the retina, but if the eye is accommodated for a ray of 
 medium refrangibility, the violet rays will be brought to a focus in front of 
 the retina at I 7 , while the red rays will be focussed behind the retina at R. 
 On the retina itself will be formed not an accurate optical image of the point 
 A, but a small circle of dispersion in which the various colored rays are mixed 
 together, the violet rays after crossing falling upon the same part of the retina 
 as the red rays before crossing. Thus by a sort of compensation, which, how- 
 ever, cannot be equivalent tc- the synthetic reproduction of white light by the 
 union of the spectral colors, the disturbing effect of chromatic aberration is 
 
THE SENSE OF VISION. 
 
 diminished. When the lower half of the pupil is covered by the edge of a 
 card held in front of the cornea at D y the aberration produced in the upper 
 half of the eye is not compensated by that of the lower half. Hence the 
 image of a point of white light at A will appear as a row of spectral colors 
 on the retina, and all objects will appear bordered by colored fringes. Another 
 good illustration of the chromatic aberration of the eye is obtained by cutting 
 two holes of any convenient shape in a piece of black cardboard and placing 
 behind one of them a piece of blue and behind the other a piece of red glass. 
 If the card is placed in a window some distance (10 meters) from the observer, 
 iu such a position that the white light of the sky may be seen through the col- 
 ored glasses, it will be found that the outlines of the two holes will generally 
 be seen with unequal distinctness. To most eyes the red outline will appear 
 quite distinct, while the blue figure will seem much blurred. To a few indi- 
 viduals the blue figure appears the more distinct, and these will generally be 
 found to be hypermetropic. 
 
 Astigmatism. The defect known as astigmatism is due to irregularities 
 of curvature of the refracting surfaces, in consequence of which all the rays 
 proceeding from a single point cannot be brought to a single focus on the 
 retina. 
 
 Astigmatism is said to be regular when one of the surfaces, generally the 
 cornea, is not spherical, but ellipsoidal i. e. having meridians of maximum 
 
 FIG. 226. Model to illustrate astigmatism. 
 
 and minimum curvature at right angles to each other, though in each meridians 
 the curvature is regular. When this is the case the rays proceeding from a 
 single luminous point are brought to a focus earliest when they lie in the 
 meridian in which the surface is most convex. Hence the pencil of rays will 
 
764 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 have two linear foci, at right angles to the meridians of greatest and least 
 curvature separated by a space in which a section of the cone of rays will be 
 first elliptical, then circular, and then again elliptical. This defect exists to a 
 certain extent in nearly all eyes, and is, in some cases, a serious obstacle to dis- 
 tinct vision. The course of the rays when thus refracted is illustrated in Fig. 226, 
 which represents the interior of a box through which black threads are drawn 
 to indicate the course of the rays of light. The threads start at one end of the 
 box from a circle representing the cornea, and converge with different degrees 
 of rapidity in different meridians, so that a section of the cone of rays will be 
 successively an ellipse, a straight line, an ellipse, a circle, etc., as shown by the 
 model represented in Fig. 227. It will be noticed that this and the preced- 
 
 FIG. 227. Model to illustrate astigmatism. 
 
 ing figure are drawn in duplicate, but that the lines are not precisely alike on 
 the two sides. In fact, the lines on the left represent the model as it would 
 be seen with the right eye, and those on the right as it would appear to 
 the left eye, which is just the opposite from an ordinary stereoscopic slide. 
 The figures are drawn in this way because they are intended to produce a 
 " pseudoscopic " effect in a way which will be explained in connection with 
 the subject of binocular vision. For this purpose it is only necessary to cross 
 the axes of vision in front of the page, as in the experiment described on page 
 758, for studying the relation between the focal, axial, and pupillary adjust- 
 ments of the eye. As soon as the middle image becomes distinct it assumes a 
 stereoscopic appearance, and the correct relations between the different parts of 
 the model are at once obvious. 
 
 This imperfection of the eye may be detected by looking at lines such as are 
 shown in Figure 228, and testing each eye separately. If the straight lines 
 
THE SENSE OF VISION. 765 
 
 drawn in various directions through a common point cannot be seen with equal 
 distinctness at the same time, it is evident that the eye is better adapted to focus 
 rays in one meridian than in another i. e. it is astigmatic. The concentric 
 
 FIG. 228. Lines for the detection of astigmatism. 
 
 circles are a still more delicate test. Few persons can look at this figure attentively 
 without noticing that the lines are not everywhere equally distinct, but that in 
 certain sectors the circles present a blurred appearance. Not infrequently it 
 will be found that the blurred sectors do not occupy a constant position, but 
 oscillate rapidly from one part of the series of circles to another. This phe- 
 nomenon seems to be due to slight involuntary contractions of the ciliary 
 muscle causing changes in accommodation. 
 
 The direction of the meridians of greatest and least curvature of the cornea 
 of a regularly astigmatic eye, and the difference in the amount of this curvature^ 
 can be very accurately measured by means of the ophthalmometer (see p. 750). 
 These points being determined, the defect of the eye can be perfectly corrected 
 by cylindrical glasses adapted to compensate for the excessive or deficient 
 refraction of the eye in certain meridians. 
 
 By another method known as " skiascopy," which consists in studying the 
 light reflected from the fundus of the eye when the ophthalmoscopic mirror is 
 moved in various directions, the amount and direction of the astigmatism of 
 the eye as a whole (and not that of the cornea alone) may be ascertained. 
 
 Astigmatism is said to be irregular when in certain meridians the curvatures 
 of the refracting surfaces are not arcs of circles or ellipses, or when there is a 
 lack of homogeneousness in the refracting media. This imperfection exists to 
 a greater or less extent in all eyes, and, unlike regular astigmatism, is incapable 
 of correction. It manifests itself by causing the outlines of all brilliant objects 
 to appear irregular. It is on this account that the fixed stars do not appear to 
 us like points of light, but as luminous bodies with irregular " star "-shaped 
 outlines. The phenomenon can be conveniently studied by looking at a pin- 
 hole in a large black card held at a convenient distance between the eye and a 
 strong light. The hole will appear to have an irregular outline, and to some 
 eyes will appear double or treble. 
 
 Intraocular Images. Light entering the eye makes visible, under certain 
 circumstances, a number of objects which lie within the eye itself. These 
 objects are usually opacities in the media of the eye which are ordinarily invisi- 
 
766 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 ble, because the retina is illuminated by light coming from all parts of the 
 pupil, and with such a broad source of light no object, unless it is a very large 
 one or one lying very near the back of the eye, can cast a shadow on the retina. 
 Such shadows can, however, be made apparent by allowing the media of the 
 eye to be traversed by parallel rays of light. This can be accomplished by 
 holding a small polished sphere e. g. the steel head of a shawl-pin illuminated 
 by sunlight or strong artificial light in the anterior focus of the eye i. e. 
 about 22 millimeters in front of the cornea, or by placing a dark screen with a 
 pin-hole in it in the same position between the eye and a source of uniform 
 diffused light, such as the sky or the porcelain shade of a student lamp. In 
 either case the rays of light diverging from the minute source will be refracted 
 into parallelism by the media of the eye, and will produce the sensation of a 
 circle of diffused light, the size of which will depend upon the amount of dila- 
 tation of the pupil. Within this circle of light will be seen the shadows of any 
 opaque substances that may be present in the media of the eye. These shadows, 
 being cast by parallel rays, will be of the same size as the objects themselves, 
 as is shown diagram matically in Figure 229, in which A represents a source 
 
 FIG. 229. Showing the method of studying intraocular images (Helmholtz). 
 
 of light at the anterior focus of the eye, and b an opacity in the vitreous humor 
 casting a shadow B of the same size as itself upon the retina. It is evident that 
 if the source of light A is moved from side to side the various opacities will be 
 displaced relatively to the circle of light surrounding them by an amount de- 
 pending upon the distance of the opacities from the retina. A study of these 
 displacements will therefore afford a means of determining the position of the 
 opacities within the media of the eye. 
 
 Muscae Volitantes. Among the objects to be seen in thus examining the 
 eye the most conspicuous are those known as the muscce volitantes. These pre- 
 sent themselves in the form of beads, either singly or in groups, or of streaks, 
 patches, and granules. They have an almost constant floating motion, which 
 is increased by the movements of the eye and head. They usually avoid the 
 line of vision, floating away when an attempt is made to fix the sight upon 
 them. When the eye is directed vertically, however, they sometimes place 
 themselves directly in line with the object looked at. If the intraocular object 
 is at the same time sufficiently near the back of the eye to cast a shadow which 
 is visible without the use of the focal illumination, some inconvenience may 
 thus be caused in using a vertical microscope. 
 
 A study of the motions of the muscce volitantes makes it evident that the 
 
THE SENSE OF VISION. 767 
 
 phenomenon is due to small bodies floating in a liquid medium of a little 
 greater specific gravity than themselves. Their movements are chiefly in 
 planes perpendicular to the axis of vision, for when the eye is directed verti- 
 cally upward they move as usual through the field of vision without increasing 
 the distance from the retina. They are generally supposed to be the remains 
 of the embyronic structure of the vitreous body i. e. portions of the cells and 
 fibres which have not undergone complete mucous transformation. 
 
 In addition to these floating opacities in the vitreous body various other 
 defects in the transparent media of the eye may be revealed by the method of 
 focal illumination. Among these may be mentioned spots and stripes due to 
 irregularities in the lens or its capsule, and radiating lines indicating the stel- 
 late structure of the lens. 
 
 Retinal Vessels. Owing to the fact that the blood-vessels ramify near the 
 anterior surface of the retina, while those structures which are sensitive to light 
 constitute the posterior layer of that organ, it is evident that light entering the 
 eye will cast a shadow of the vessels on the light-perceiving elements of the 
 retina. Since, however, the diameter of the largest blood-vessels is not more 
 than one-sixth of the thickness of the retina, and the diameter of the pupil is 
 one-fourth or one-fifth of the distance from the iris to the retina, it is evident 
 that when the eye is directed to the sky or other broad illuminated surfaces it 
 is only the penumbra of the vessels that will reach the rods and cones, the umbra 
 terminating conically somewhere in the thickness of the retina. But if light 
 is allowed to enter the eye through a pin-hole in a card held a short distance 
 from the cornea, as in the above-described method of focal illumination, a 
 sharply defined shadow of the vessels will be thrown on the rods and cones. 
 Yet under these conditions the retinal vessels are not rendered visible unless 
 the perforated card is moved rapidly to and fro, so as to throw the shadow 
 continually on to fresh portions of the retinal surface. When this is done the 
 vessels appear, ramifying usually as dark lines on a lighter background, but 
 the dark lines are sometimes bordered by bright edges. It will be observed 
 that those vessels appear most distinctly the course of which is at right angles 
 to the direction in which the card is moved. Hence in order to see all the 
 vessels with equal distinctness it is best to move the card rapidly in a circle 
 the diameter of which should not exceed that of the pupil. In this manner 
 the distribution of the vessels in one's own retina may be accurately observed, 
 and in many cases the position of the fovea centralis may be determined by the 
 absence of vessels from that portion of the macula lutea. 
 
 The retinal vessels may also be made visible in several other ways e. g., 
 1. By directing the eye toward a dark background and moving a candle to and 
 fro in front of the eye, but below or to one side of .the line of vision. 2. By 
 concentrating a strong light by means of a lens of short focus upon a point 
 of the sclerotic as distant as possible from the cornea. By either of these 
 methods a small image of the external source of light is formed upon the 
 lateral portion of the eye, and this image is the source of light which throws 
 shadows of the retinal vessels on to the rods and cones. 
 
768 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 Circulation of Blood in the Retina. When the eye is directed toward a 
 surface which is uniformly and brightly illuminated e. g. the sky or a sheet 
 of white paper on which the sun is shining the field of vision is soon seen to 
 be filled with small bright bodies moving with considerable rapidity in irregu- 
 lar curved lines, but with a certain uniformity which suggests that their 
 movements are confined to definite channels. They are usually better seen 
 when one or more sheets of cobalt glass are held before the face, so that the 
 eyes are bathed in blue light. That the phenomenon depends upon the circu- 
 lation of the blood globules in the retina is evident from the fact that the 
 moving bodies follow paths which correspond with the form of the retinal 
 capillaries as seen by the methods above described, and also from the corre- 
 spondence between the rate of movement of the intraocular image and the 
 rapidity of the capillary circulation in those organs in which it can be di- 
 rectly measured under the microscope. The exact way in which the moving 
 
 \ globules stimulate the retina so as to produce the observed phenomenon must 
 
 j be regarded as an unsettled question. 
 
 We have thus seen that the eye, regarded from the optician's point of view, 
 has not only all the faults inherent in optical instruments generally, but many 
 others which would not be tolerated in an instrument of human construction. 
 Yet with all its imperfections the eye is perhaps the most wonderful instance 
 in nature of the development of a highly specialized organ to fulfil a definite 
 
 \ purpose. In the accomplishment of this object the various parts of the eye 
 
 *have been perfected to a degree sufficient to enable it to meet the requirements 
 of the nervous system with which it is connected, and no farther. In the 
 ordinary use of the eye we are unconscious of its various irregularities, shadows, 
 opacities, etc., for these imperfections are all so slight that the resulting inac- 
 curacy of the image does not much exceed the limit which the size of the 
 light-perceiving elements of the retina imposes upon the delicacy of our visual 
 perceptions, and it is only by illuminating the eye in some unusual way that 
 the existence of these imperfections can be detected. In other words, the eye 
 is as good an optical instrument as the nervous system can appreciate and 
 make use of. Moreover, when we reflect upon the difficulty of the problem 
 which nature has solved, of constructing an optical instrument out of living 
 and growing animal tissue, we cannot fail to be struck by the perfection of the 
 dioptric apparatus of the eye as well as by its adaptation to the needs of the 
 organism of which it forms a part. 
 
 Iris. The importance of the iris as an adjustable diaphragm for cutting 
 off side rays and thus securing good definition in near vision has been described 
 in connection with the act of accommodation. Its other function of protecting 
 the retina from an excess of light is no less important, and we must now con- 
 sider how this pupillary adjustment may be studied and by what mechanism 
 it is effected. The changes in the size of the pupil may be conveniently ob- 
 served in man and animals by holding a millimeter scale in front of the eye 
 and noticing the variations in the diameter of the pupil. It should be borne 
 in mind that the iris, seen in this way, does not appear in its natural size and 
 
THE SENSE OF VISION. 
 
 769 
 
 position, but somewhat enlarged and bulged forward by the magnifying effect 
 of the cornea and the aqueous humor. The changes in one's own pupil may 
 be readily observed by noticing the varying size of the circle of light thrown 
 upon the retina when the eye is illuminated by a point of light held at the 
 anterior focus, as in the method above described for the study of intraocular 
 images. 
 
 The muscles of the iris are, except in birds, of the unstriped variety, and 
 are arranged concentrically around the pupil. Radiating fibres are also recog- 
 nized by many observers, though their existence has been called in question 
 by others. The circular or constricting muscles of the iris are under the con- 
 trol of the third pair of cranial nerves, 
 and are normally brought into activity 
 in consequence of light falling upon 
 the retina. This is a reflex phenom- 
 enon, the optic nerve being the affer- 
 ent, and the third pair, the ciliary 
 ganglion, and the short ciliary nerves 
 the efferent, channel, as indicated in 
 Figure 230. This reflex is in man 
 and many of the higher animals bi- 
 lateral i. e. light falling upon one 
 retina will cause a contraction of both 
 pupils. This may readily be observed 
 in one's own eye when focally illumi- 
 nated in the manner above described. 
 Opening the other eye will, under 
 these conditions, cause a diminution, 
 and closing it an increase, in the size 
 of the circle of light. This bilateral 
 character is found to be dependent 
 upon the nature of the decussation of 
 the optic nerves, for in animals in 
 which the crossing is complete the 
 
 reflex is confined to the illuminated nerves governing the pupil (after Foster) : II, optic 
 rpi / i ft i nerve ; 1. g, ciliary ganglion ; r. b, its short root from 
 
 eye. Ine arrangement ot the nbres ///, mot or-ocuii nerve -.sym, its sympathetic root -,r.i, 
 
 in the Optic commissure is in general its long root from F,ophthalmo-nasal branch of oph- 
 T . , , . . ft t thalmic division of fifth nerve ; 8. c. short ciliary 
 
 associated With the position of the nerve s ; 1. c, long ciliary nerves. 
 
 eyes in the head. When the eyes 
 
 are so placed that they can both be directed to the same object, as in man 
 and many of the higher animals, the fibres of each optic nerve are usually 
 found to be distributed to both optic tracts, while in animals whose eyes 
 are in opposite sides of the head there is complete crossing of the optic nerves. 
 Hence it may be said that animals having binocular vision have in general 
 a bilateral pupillary reflex. The rule is, however, not without exceptions, 
 for owls, though their visual axes are parallel, have, like other birds, a corn- 
 
 course of constrictor nerve-fibres - 
 " dilator " 
 
 FIG. 230. Diagrammatic representation of the 
 
770 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 plete crossing of the optic nerves, and consequently a unilateral pupillary 
 reflex. 1 
 
 A direct as well as a reflex constriction of the pupil under the influence of 
 light has been observed in the excised eyes of eels, frogs, and some other ani- 
 mals. As the phenomenon can be seen in preparations consisting of the iris 
 alone or of the iris and cornea together, it is evident that the light exerts its 
 influence directly upon the tissues of the iris and not through an intraocular 
 connection with the retina. The maximum effect is produced by the yellowish- 
 green portion of the spectrum. 
 
 Antagonizing the motor oculi nerve in its constricting influence on the 
 pupil is a set of nerve-fibres the function of which is to increase the size of 
 the pupil. Most of these fibres seem to run their course from a centre which 
 lies in the floor of the third ventricle not far from the origin of the third pair, 
 through the bulb, the cervical cord, the anterior roots of the upper dorsal 
 nerves, the upper thoracic ganglion, the cervical sympathetic nerve as far as 
 the upper cervical ganglion ; then through a branch which accompanies the 
 internal carotid artery, passes over the Gasserian ganglion and joins the oph- 
 thalmic branch of the fifth pair ; then through the nasal branch of the latter 
 nerve and the long ciliary nerves to the eye 2 (see diagram, p. 769). These 
 fibres appear to be in a state of tonic activity, for section of them in any part 
 of their course (most conveniently in the cervical sympathetic) causes a con- 
 traction of the pupil which, on stimulation of the peripheral end of the divided 
 nerve, gives place to a marked dilatation. Their activity can be increased in 
 various ways. Thus dilatation of the pupil may be caused by dyspnea, vio- 
 lent muscular efforts, etc. Stimulation of various sensory nerves may also* 
 cause reflex dilatation of the pupil, and this phenomenon may be observed, 
 though greatly diminished in intensity, after extirpation of the superior cervi- 
 cal sympathetic ganglion. It is therefore evident that the dilator nerves of the 
 pupil do not have their course exclusively in the cervical sympathetic nerve. 
 
 Since the cervical sympathetic nerve contains vaso-constrictor fibres for the 
 head and neck, it has been thought that its dilating effect upon the pupil might 
 be explained by its power of causing changes in the amount of blood in the 
 vessels of the iris. There is no doubt that a condition of vascular turgescence 
 or depletion will tend to produce contraction or dilatation of the pupil, but it is 
 impossible to explain the observed phenomena in this way, since the pupillary 
 are more prompt than the vascular changes, and may be observed on a bloodless 
 eye. Moreover, the nerve-fibres producing them are said to have a somewhat 
 different course. Another explanation of the influence of the sympathetic on 
 the pupil is that it acts by inhibiting the contraction of the sphincter muscles,, 
 and that the dilatation is simply an elastic reaction. But since it is posssible to 
 produce local dilatation of the pupil by circumscribed stimulation at or near 
 
 1 Steinach : Archiv fur die gesammte Physiologic, xlvii. 313. 
 
 ' 2 Langley : Journal of Physiology, xiii. p. 575. For the evidence of the existence of a 
 "cilio-spinal" centre in the cord, see Steil and Langendorff: Archiv fur die gesammte Phys- 
 ioiogif, Iviii. p. 155 ; also Schenck : Ibid., Ixii. p. 494. 
 
THE SENSE OF VISION. Ill 
 
 the outer border of the iris, it seems more reasonable to conclude that the 
 dilator nerves of the pupil act upon radial muscular fibres in the substance of 
 the iris, in spite of the fact that the existence of such fibres has not been uni- 
 versally admitted. 
 
 Whatever view may be taken of the mechanism by which the sympathetic 
 nerves influence the pupil, there is no doubt that the iris is under the control 
 of two antagonistic sets of nerve-fibres, both of which are, under normal cir- 
 cumstances, in a state of tonic activity. Therefore, when the sympathetic 
 nerve is divided the pupil contracts under the influence of the motor oculi, and 
 section of the motor oculi causes dilatation through the unopposed influence of 
 the sympathetic. 
 
 The movements of the iris, though performed by smooth muscles, are more 
 rapid than those of smooth muscles found elsewhere e. g. in 'the intestines 
 and the arteries. The contraction of the pupil when the retina of the oppo- 
 site eye is illuminated occupies about 0.3' f ; the dilatation when the light is cut 
 off from the eye, about 3" or 4". The latter determination is, however, diffi- 
 cult to make with precision, since dilatation of the pupil takes place at first 
 rapidly and then more slowly, so that the moment when the process is at an 
 end is not easily determined. After remaining a considerable time in absolute 
 darkness the pupils become enormously dilated, as has been shown by flash- 
 light photographs taken under these conditions. In sleep, though the eyes are 
 protected from the light, the pupils are strongly contracted, but dilate on 
 stimulation of sensory nerves, even though the stimulation may be insufficient 
 to rouse the sleeper. 
 
 Many drugs when introduced into the system or applied locally to the con- 
 junctiva produce effects upon the pupil. Those which dilate it are known as 
 mydriatics, those which contract it as myotics. Of the former class the most 
 important is atropin, the alkaloid of the Atropa belladonna, and of the latter 
 physostigmin, the alkaloid of the Calabar bean. In addition to their action 
 upon the pupil, mydriatics paralyze the accommodation, thus focussing the eye 
 for distant objects, while myotics, by producing a cramp of the ciliary muscle, 
 adjust the eye for near vision. The effect on the accommodation usually 
 begins later and passes off sooner than the affection of the pupil. Atropin 
 seems to act by producing local paralysis of the terminations of the third pair 
 of cranial nerves in the sphincter iridis and the ciliary muscle. In large 
 doses it may also paralyze the muscle-fibres of the sphincter. With this para- 
 lyzing action there appears to be combined a stimulating effect upon the dilator 
 muscles of the iris. The myotic action of physostigmin seems to be due to a 
 local stimulation of the fibres of the sphincter of the iris. 
 
 Although in going from a dark room to a lighter one the pupil at first con- 
 tracts, this contraction soon gives place to a dilatation, and in about three or 
 four minutes the pupil usually regains its former size. In a similar manner 
 the primary dilatation of the pupil caused by entering a dark room from a 
 lighter one is followed by a contraction which usually restores the pupil to its 
 original size within fifteen or twenty minutes. It is thus evident that the 
 
772 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 amount of light falling upon the retina is not the only factor in determining 
 the size of the pupil. In fact, if the light acts for a sufficient length of time 
 the pupil may have the same size under the influence of widely different 
 degrees of illumination. 1 
 
 This so-called " adaptation " of the eye to various amounts of light seems 
 to be connected with the movements of the retinal pigment-granules and with 
 the chemical changes of the visual purple, to be more fully described in con- 
 nection with the physiology of the retina. 
 
 The Ophthalmoscope. Under normal conditions the pupil of the eye 
 appears as a black spot in the middle of the colored iris. The cause of this 
 dark appearance of the pupil is to be found in the fact that a source of light 
 and the retina lie in the conjugate foci of the dioptric apparatus of the eye. 
 Hence any light entering the eye that escapes absorption by the retinal pig- 
 ment and is reflected from the fundus must be refracted back to the source 
 from which it came. The eye of an observer who looks at the pupil from 
 another direction will see no light coming from it, and it will therefore appear 
 to him black. It is therefore evident that the essential condition for perceiving 
 light coming from the fundus of the eye is that the line of vision of the 
 observing eye shall be in the line of illumination. This condition is fulfilled 
 by means of instruments known as ophthalmoscopes. The principles involved 
 in the construction of the most common form of ophthalmoscope are illustrated 
 diagrammatically in Figure 231. 
 
 FIG. 231. Diagram to illustrate the principles of a simple ophthalmoscope (after Foster). 
 
 The rays from a source of light Z, after being brought to a focus at a by 
 the concave perforated mirror M M, pass on and are rendered parallel by the 
 lens I. Then, entering the observed eye 5, they are brought to a focus on the 
 retina at a'. Any rays which are reflected back from the part of the retina 
 thus illuminated will follow the course of the entering rays and be brought to 
 a focus at a. The eye of an observer at A, looking through the hole in the 
 mirror, will therefore see at a an inverted image of the retina, the observation 
 of which may be facilitated by a convex lens placed immediately in front of 
 the observer's eye. 
 
 1 Schirmer : Archivfur Ophthalmologie, xi. 5. 
 
THE SENSE OF VISION. 773 
 
 The fund us of the eye thus observed presents a reddish background on 
 which the retinal vessels are distinctly visible. 
 
 Retina. Having considered the mechanism by which optical images of 
 objects at various distances from the eye are formed upon the retina, we must 
 next inquire what part of the retina is affected by the rays of light, and in 
 what this affection consists. To the former of these questions it will be found 
 possible to give a fairly satisfactory answer, With regard to the latter nothing 
 positive is known. 
 
 The structure of the retina is exceedingly complicated, but, as very little 
 is known of the functions of the ganglion cells and of the molecular and 
 nuclear layers, it will suffice for the present purpose of physiological descrip- 
 tion to regard the retina as consisting of fibres of the optic nerve which are 
 connected through various intermediate structures with the layer of rods and 
 cones. 
 
 A 
 FIG. 232. Diagrammatic representation of the retina. 
 
 Figure 232 is intended to show, diagram matically, the mutual relation of 
 these various portions of the retina in different parts of the eye, and is not 
 drawn to scale. It will be observed that the optic nerve 0, where it enters the 
 eye, interrupts the continuity of the layer of rods and cones R and of the 
 intermediate structures /. Its fibres spread themselves out in all directions, 
 forming the internal layer of the retina N. The central artery of the retina 
 A accompanying the optic nerve ramifies in the layer of nerve-fibres and in 
 the immediately adjacent layers of the retina, forming a vascular layer V. In 
 the fovea centralis F of the macula lutea (the centre of distinct vision) the 
 layer of rods and cones becomes more highly developed, while the other layers 
 of the retina are much reduced in thickness and the blood-vessels entirely dis- 
 appear. This histological observation points strongly to the conclusion that 
 the rods and cones are the structures which are essential to vision, and that in 
 them are found the conditions for the conversion of the vibrations of the 
 luminiferous ether into a stimulus for a nerve-fibre. This view derives con- 
 firmation from the observations on the retinal blood-vessels, for it is found 
 that the distance between the vascular layer of the retina and the layer 
 of rods and cones determined by histological methods corresponds with that 
 which must exist between the vessels and the light-perceiving elements of the 
 retina, as calculated from the apparent displacement of the shadow caused by 
 given movements of the source of light used in studying intraocular images l as 
 
 1 " Dimmer Verh. d. phys. Clubs zu Wien, 24 April, 1894," Ceniralbl.fur Physiologic, 1894, 159. 
 
774 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 described on p. 767. Another argument in favor of this view is found in the 
 correspondence between the size of the smallest visible images on the retina and 
 the diameter of the rods and cones. A double star can be recognized as double 
 by the normal eye when the distance between the components corresponds to 
 a visual angle of 60". Two white lines on a black ground are seen to be dis- 
 tinct when the distance between them subtends a visual angle of 64 r/ -73 r/ . 
 These angles correspond to a retinal image of 0.0044, 0.0046, and 0.0053 mil- 
 limeter. Now, the diameter of the cones in the macula lutea, as determined 
 by Kolliker, is 0.0045-0.0055 millimeter, a size which agrees well with the 
 hypothesis that each cone when stimulated can produce a special sensation of 
 light distinguishable from those caused by the stimulation of the neighboring 
 cones. The existence of the so-called blind spot in the retina at the point of 
 entrance of the optic nerve is sometimes regarded as evidence of the light- 
 perceiving function of the rods and cones, but as the other layers of the retina, 
 as well as the rods and cones, are absent at this point, and the retina here 
 consists solely of nerve-fibres, it is evident that the presence of the blind spot 
 
 FIG. 233. To demonstrate the blind spot. 
 
 only proves that the optic nerve-fibres are insensible to light. Figure 233 is 
 intended to demonstrate this insensibility. For this purpose it should be held 
 at a distance of about 23 centimeters from the eyes (i. e. about 3.5 times the dis- 
 tance between the cross and the round spot). If the left eye be closed and the 
 right eye fixed upon the cross, the round spot will disappear from view, though 
 it will become visible if the eye be directed either to the right or to the left of 
 the cross, or if the figure be held either a greater or a less distance from the 
 eye. The size and shape of the blind spot may readily be determined as 
 follows : Fix the eye upon a definite point marked upon a sheet of white 
 paper. Bring the black point of a lead pencil (which, except the point, has 
 been painted white or covered with white paper) into the invisible portion of 
 the field of vision and carry it outward in any direction until it becomes vis- 
 ible. Mark upon the paper the point 
 at which it just begins to be seen, and 
 by repeating the process in as many 
 different directions as possible the out- 
 line of the blind spot may be marked 
 out. Figure 234 shows the shape of 
 the blind spot determined by Helm- 
 holtz in his own right eye, a being 
 
 FIG. 234.-Form of the blind spot (Helmholtz). the P int f fixati n f the e 7 e and 
 
 the line AB being one-third of the 
 distance between the eye and the paper. The irregularities of outline, as at 
 
THE SENSE OF VISION. 
 
 775 
 
 d, are due to shadows of the large retinal vessels. During this determination 
 it is of course necessary that the head should occupy a fixed position with 
 regard to the paper. This condition can be secured by holding firmly between 
 the teeth a piece of wood that is clamped in a suitable position to the edge of 
 the table. The diameter of the blind spot, as thus determined, has been found 
 to correspond to a visual angle varying from 3 39' to 9 47', the average 
 measurement being 6 10'. This is about the angle that is subtended by the 
 human face seen at a distance of two meters. Although a considerable por- 
 tion of the retina is thus insensible to light, we are, in the ordinary use of the 
 eyes, conscious of no corresponding blank in the field of vision. By what 
 psychical operation we " fill up " the gap in our subjective field of vision 
 caused by the blind spot of the retina is a question that has been much dis- 
 cussed without being definitely settled. 
 
 The above-mentioned reasons for regarding the rods and cones as the light- 
 perceiving elements of the retina seem sufficiently conclusive. Whether there 
 is any difference between the rods and the cones with regard to their light- 
 perceiving function is a question which may be best considered in connection 
 with a description of the qualitative modifications of light. 
 
 The histological relation between the various layers of the retina is still 
 under discussion. According to recent observations of Cajal, 1 the connection 
 between the rods and cones on the one 
 side and the fibres of the optic nerve 
 on the other is established in a man- 
 ner which is represented diagram- 
 matically in Figure 235. The pro- 
 longations of the bipolar cells of the 
 internal nuclear layer E break up into 
 fine fibres in the external molecular 
 (or plexiform) layer C. Here they are 
 brought into contact, though not into 
 anatomical continuity, with the termi- 
 nal fibres of the rods and cones. The 
 inner prolongations of the same bipolar 
 cells penetrate into the internal molec- 
 ular (or plexiform) layer F, and there 
 come into contact with the dendrites 
 coming from the layer of ganglion-cells 
 G. These cells are, in their turn, con- 
 nected by their axis-cylinder processes 
 
 Rods. 
 
 Cones. 
 
 FIG. 235. Diagrammatic representation of the 
 
 .., ! /i /> ,1 .. rrii r Hi. &. uiagrammauc represeuiauou ui MIC 
 
 with the fibres of the optic nerve. The structure of the retina (Ca jai): A, layer of rods 
 
 bipolar Cells which Serve as Connective and cones; B, external nuclear layer ; C, external 
 
 molecular (or plexiform) layer; E, internal nu- 
 
 1 i 1 1 i i| . iiiuit-t/LiJLWi \\.n picAm-niii./ Lnyui. , AJ IIILCI iic*x ut 
 
 links between the rods and the OptlC clear layer . Ff internal molecular (or plexiform) 
 
 nerve-fibres are anatomically distin- la y er ' <*. lft y er of gangiion-ceiis : H, layer of 
 
 nerve-fibres. 
 
 .guishable (as indicated in the diagram) 
 
 1 Die Retina der Wirbeltkiere, Wiesbaden, 1894. 
 
776 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 from those which perform the same function for the cones. Whatever be the 
 precise mode of connection between the rods and cones and the fibres of the 
 optic nerve, it is evident that each retinal element cannot be connected with 
 the nerve-centres by a separate independent nerve-channel, since the retina 
 contains many millions of rods and cones, while the optic nerve has only 
 about 438,000 nerve-fibres, 1 though of course such a connection may exist in 
 the fovea centralis, as Cajal has shown is probably the case in reptiles and birds. 
 Changes Produced in the Retina by Light. We must now inquire 
 what changes can be supposed to occur in the rods and cones under the influ- 
 ence of light by means of which they are able to transform the energy of the 
 ether vibrations into a stimulus for the fibres of the optic nerve. Though in 
 the present state of our knowledge no satisfactory answer can be given to this 
 question, yet certain direct effects of light upon the retina have been observed 
 which are doubtless associated in some way with the transformation in 
 question. 
 
 The retina of an eye which has been protected from light for a considerable 
 length of time has a purplish-red color, which upon exposure to light changes 
 to yellow and then fades away. This bleaching occurs also in monochromatic 
 light, the most powerful rays being those of the greenish-yellow portion of 
 the spectrum i. e. those rays which are most completely absorbed by the pur- 
 plish-red coloring matter. A microscopic examination of the retina shows 
 that this coloring matter, which has been termed visual purple, is entirely con- 
 fined to the outer portion of the retinal rods and does not occur at all in the 
 cones. After being bleached by light it is, during life, restored through the 
 agency of the pigment epithelium, the cells of which, under the influence of 
 light, send their prolongations inward to envelop the outer limbs of the rods 
 and cones with pigment. If an eye, either excised or in its natural position, 
 is protected from light for a time, and then placed in such a position that the 
 image of a lamp or a window is thrown upon the retina for a time which may 
 vary with the amount of light from seven seconds to ten minutes, it will be 
 found that the retina, if removed and examined under red light, will show the 
 image of the luminous object impressed upon it by the 
 bleaching of the visual purple. 
 
 If the retina be treated with a 4 per cent, solution of 
 alum, the restoration of the visual purple will be pre- 
 vented, and the so-called " optogram " will be, as pho- 
 tographers say, " fixed." 2 
 
 Fi g lll> e 236 shows the appearance of a rabbit's retina 
 on which the optogram of a window has been impressed. 
 Although the chemical changes in the visual purple under the influence of 
 light seem, at first sight, to afford an explanation of the transformation of the 
 vibrations of the luminiferous ether into a stimulation for the optic nerve, yet 
 the fact that vision is most distinct in the fovea centralis of the retina, which,, 
 
 1 Salzer: Wiener Sitzungsberichte, 1880, Bd. Ixxxi. S. 3. 
 
 2 Kiihne: Unlersuchungen a. d. phys. InsL d. Universitdt Heidelberg, i. 1. 
 
THE SENSE OF VISION. 777 
 
 as it contains no rods, is destitute of visual purple, makes it impossible to 
 regard this coloring matter as essential to vision. The most probable theory 
 of its function is perhaps that which connects it with the adaptation of the 
 eye to varying amounts of light, as described on p. 772. 
 
 In addition to the above-mentioned movements of the pigment epithelium 
 cells under the influence of light, certain changes in the retinal cones of frogs 
 and fishes have been observed. 1 The change consists in a shortening and thick- 
 ening of the inner portion of the cones when illuminated, but the relation of 
 the phenomenon to vision has not been explained. 
 
 Like most of the living tissues of the body, the retina is the seat of electri- 
 cal currents. In repose the fibres of the optic nerve are said to be positive in 
 relation to the layer of rods and cones. When light falls upon the retina this 
 current is at first increased and then diminished in intensity. 
 
 Sensation of Light. Whatever view may be adopted with regard to the 
 mechanism by which light is enabled to become a stimulus for the optic nerve, 
 the fundamental fact remains that the retina (and in all probability the layer 
 of rods and cones in the retina) alone supplies the conditions under which this 
 transformation of energy is possible. But in accordance with the " law of 
 specific energy " a sensation of light may be produced in whatever way the 
 optic nerve be stimulated, for a stimulus reaching the visual centres through 
 the optic nerve is interpreted as a visual sensation, in the same way that 
 pressure on a nerve caused by the contracting cicatrix of an amputated leg 
 often causes a painful sensation which is referred to the lost toes to which the 
 nerve was formerly distributed. Thus local pressure on the eyeball by stimu- 
 lating the underlying retina causes luminous sensations, already described as 
 " phosphenes," and electrical stimulation of the eye as a whole or of the stump 
 of the optic nerve after the removal of the eye is found to give rise to sensa- 
 tions of light. 
 
 Vibrations of the luminiferous ether constitute, however, the normal stim- 
 ulus of the retina, and we must now endeavor to analyze the sensation thus 
 produced. In the first place, it must be borne in mind that the so-called ether 
 waves differ among themselves very widely in regard to their rate of oscilla- 
 tion. The slowest known vibrations of the ether molecules have a frequency 
 of about 107,000,000,000,000 in a second, and the fastest a rate of about 
 40,000,000,000,000,000 in a second a range, expressed in musical terms, of 
 about eight and one-half octaves. All these ether waves are capable of warm- 
 ing bodies upon which they strike and of breaking up certain chemical com- 
 binations, the slowly vibrating waves being especially adapted to produce the 
 former and the rapidly vibrating ones the latter effect. Certain waves of 
 intermediate rates of oscillation viz. those ranging between 392,000,000,- 
 000,000 and 757,000,000,000,000 in a second not only produce thermic and 
 chemical effects, but have the power, when they strike the retina, of causing 
 changes in the layer of rods and cones, which, in their turn, act as a stimulus 
 to the optic nerve. The ether waves which produce these various phenomena 
 
 1 Engelmann : Archivfiir die gesammte Physiologic, xxxv. 498. 
 
778 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 are often spoken of as heat rays, light rays, and actinic or chemical rays, but 
 it must be remembered that the same wave may produce all three classes of 
 phenomena, the effect depending upon the nature of the substance upon which 
 it strikes. It will be observed that the range of vibrations capable of affecting 
 the retina is rather less than one octave, a limitation which obviously tends to 
 reduce the amount of chromatic aberration. 
 
 In this connection it is interesting to notice that the highest audible note is 
 produced by about 40,000 sonorous impulses in a second. Between the high- 
 est audible note and the lowest visible color there is a gap of nearly thirty-four 
 octaves in which neither the vibrations of the air nor those of the luminifer- 
 ous ether affect our senses. Even if the slowly vibrating heat-rays which 
 affect our cutaneous nerves are taken into account, there still remain over 
 thirty-one octaves of vibrations, either of the air or of the luminiferous ether, 
 which may be, and very likely are, filling the universe around us without in 
 any way impressing themselves upon our consciousness. 
 
 Qualitative Modifications of Light. All the ethereal vibrations which 
 are capable of affecting the retina are transmitted with very nearly the same 
 rapidity through air, but when they enter a denser medium the waves having 
 a rapid vibration are retarded more than those vibrating more slowly. Hence 
 when a ray of sunlight composed of all the visible ether waves strikes upon a 
 
 plane surface of glass, the greater 
 retardation of the waves of rapid 
 vibration causes them to be more 
 refracted than those of slower vibra- 
 tion, and if the glass has the form 
 of a prism, as shown in Figure 237, 
 this so-called " dispersion " of the 
 rays is still further increased when 
 the rays leave the glass, so that the 
 emerging beam, if received upon a 
 
 FIG. 237.-Diagram illustra^mgthe dispersion of light wh j te sur f ace> instead of f orming a 
 
 spot of white light, produces a band 
 
 of color known as the solar spectrum. The colors of the spectrum, though 
 commonly spoken of as seven in number, really form a continuous series from 
 the extreme red to the extreme violet, these colors corresponding to ether vibra- 
 tions have rates of 392,000,000,000,000 and 757,000,000,000,000 in 1 second, 
 and wave lengths of 0.7667 and 0.3970 micromillimeters * respectively. 
 
 Colors, therefore, are sensations caused by the impact upon the retina of 
 certain ether waves having definite frequencies and wave-lengths, but these 
 are not the only peculiarities of the ether vibration which influence the retinal 
 sensation. The energy of the vibration, or the vis viva of the vibrating mole- 
 cule, determines the " intensity " of the sensation or the brilliancy of the light. 2 
 
 ^ne micromillimeter = 0.001 millimeter = one //. 
 
 2 The energy of vibration capable of producing a given subjective sensation of intensity 
 varies with the color of the light, as will be later explained (see p. 786). 
 
THE SENSE OF VISION. 779 
 
 Furthermore, the sensation produced by the impact of ether waves of a definite 
 length will vary according as the eye is simultaneously affected by a greater or 
 less amount of white light. This modification of the sensation is termed its 
 degree of " saturation/ 7 light being said to be completely saturated when it is 
 " monochromatic" or produced by ether vibrations of a single wave-length. 
 
 The modifications of light which taken together determine completely the 
 character of the sensation are, then, three in number viz. : 1. Color, depend- 
 ent upon rate of vibration or length of the ether wave ; 2. Intensity, dependent 
 upon the energy of the vibration ; 3. Saturation, dependent upon the amount 
 of white light mingled with the monochromatic light. These three qualitative 
 modifications of light must now be considered in detail. 
 
 Color. In our profound ignorance of the nature of the process by which, 
 in the rods and cones, the movements of the ether waves are converted into a 
 stimulus for the optic nerve-fibres, all that can be reasonably demanded of a 
 color theory is that it shall present a logically consistent hypothesis to account 
 for the sensations actually produced by the impact of ether waves of varying 
 rates, either singly or combined, upon different parts of the retina. Some of 
 the important phenomena of color sensation of which every color theory must 
 take account may be enumerated as follows : 
 
 1. Luminosity is more readily recognized than color. This is shown by 
 the fact that a colored object appears colorless when it is too feebly illuminated, 
 and that a spectrum produced by a very feeble light shows variations of inten- 
 sity with a maximum nearer than normal to the blue end, but no gradations 
 of color. A similar lack of color is noticed when a colored object is observed 
 for too short a time or when it is of insufficient size. In all these respects the 
 various colors present important individual differences which will be considered 
 later, 
 
 2. Colored objects seen with increasing intensity of illumination appear 
 more and more colorless, and finally present the appearance of pure white. 
 Yellow passes into white more readily than the other colors. 
 
 3. The power of the retina to distinguish colors diminishes from the centre 
 toward the periphery, the various colors, in this respect also, differing mate- 
 rially from each other. Sensibility to red is lost at a short distance from the 
 macula lutea, while the sensation of blue is lost only on the extreme lateral 
 portions of the retina. The relation of this phenomenon to the distribution 
 of the rods and cones in the retina will be considered in connection with the 
 perception of the intensity of light. 
 
 Color-mixture. Since the various spectral colors are produced by the dis- 
 persion of the white light of the sun, it is evident that white light may be 
 reproduced by the reunion of the rays corresponding to the different colors, and 
 it is accordingly found that if the colored rays emerging from a prism, as in 
 Fig. 237, are reunited by suitable refracting surfaces, a spot of white light will be 
 produced similar to that which would have been caused by the original beam 
 of sunlight. But white light may be produced not only by the union of all 
 the spectral colors, but by the union of certain selected colors in twos, threes, 
 
780 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 fours, etc. Any two spectral colors which by their union produce white are 
 said to be " complementary " colors. The relation of these pairs of comple- 
 mentary colors to each other may be best understood by reference to Figure 238. 
 
 p 
 
 FIG. 238. Color diagram. 
 
 Here the spectral colors are supposed to be disposed around a curved line, 
 as indicated by their initial letters, and the two ends of the curve are united 
 by a straight line, thus enclosing a surface having somewhat the form of a tri- 
 angle with a rounded apex. If the curved edge of this surface be supposed to 
 be loaded with weights proportionate to the luminosity of the different colors, 
 the centre of gravity of the surface will be near the point W. Now, if a 
 straight line be drawn from any point on the curved line through the point 
 JFand prolonged till it cuts the curve again, the colors corresponding to the 
 two ends of this straight line will be complementary colors. Thus in Figure 
 238 it will be seen that the complementary color of red is bluish-green, and 
 that of yellow lies near the indigo. It is also evident that the complementary 
 color of green is purple, which is not a spectral color at all, but a color 
 obtained by the union of violet and red. The union of a pair of colors 
 lying nearer together than complementary colors produces an intermediate color 
 mixed with an amount of white which is proportionate to the nearness of the 
 colors to the complementary. Thus the union of red and yellow produces 
 orange, but a less saturated orange than the spectral color. The union of two 
 colors lying farther apart than complementary colors produces a color which 
 borders more or less upon purple. 
 
 The mixing of colors to demonstrate the above-mentioned effects may be 
 accomplished in three different ways : 
 
 1. By employing two prisms to produce two independent spectra, and then 
 directing the colored rays which are to be united so that they will illuminate 
 the same white surface. 
 
 2. By looking obliquely through a glass plate at a colored object placed 
 behind it, while at the same time light from another colored object, placed in 
 front of the glass, is reflected into the eye of the observer, as shown in Figure 
 239. Here the transmitted light from the colored object A and the reflected 
 light from the colored object B enter the eye at C from the same direction, 
 and are therefore united upon the retina. 
 
 3. By rotating before the eye a disk on which the colors to be united are 
 
THE SENSE OF VISION. 781 
 
 painted upon different sectors. This is most readily accomplished by using 
 a number of disks, each painted with one of the colors to be experimented 
 with, and each divided radially by a cut running from the centre to the circum- 
 ference. The disks can then be lapped over each other and rotated together, and 
 in this way two or more colors can be mixed in any desired proportions. This 
 method of mixing colors depends upon 
 the property of the retina to retain an 
 impression after the stimulus causing [V 
 
 it has ceased to act a phenomenon of / " \ 
 
 great importance in physiological optics, / 
 
 and one which will be further discussed / 
 
 
 in connection with the subject of " after- / \ 
 
 images." 
 
 _ 
 
 The physiological mixing of Colors FIG. 239. Diagram to illustrate color mixture by 
 
 cannot be accomplished by the mixture 
 
 of pigments or by allowing sunlight to pass successively through glasses of 
 different colors, for in these cases rays corresponding to certain colors are 
 absorbed by the medium through which the white light passes, and the phe- 
 nomenon is the result of a process of subtraction and not addition. Light 
 reaching the eye through red glass, for instance, looks red because all the rays 
 except the red rays are absorbed, and light coming through green glass appears 
 green for a similar reason. Now, when light is allowed to pass successively 
 through red and green glass the only rays which pass through the red glass 
 will be absorbed by the green. Hence no light will pass through the combi- 
 nation of red and green glass, and darkness results. But when red and green 
 rays are mixed by any of the three methods above described the result of this 
 process of addition is not darkness, but a yellow color, as will be understood 
 by reference to the color diagram on p. 780. In the case of colored pigments 
 similar phenomena occur, for here too light reaches the eye after rays of cer- 
 tain wave-lengths have been absorbed by the medium. This subject will be 
 further considered in connection with color-theories. 
 
 Color-theories. From what has been said of color-mixtures it is evident 
 that every color sensation may be produced by the mixture of a number of 
 other color sensations, and that certain color sensations viz. the purples can 
 be produced only by the mixture of other sensations, since there is no single 
 wave-length corresponding to them. Hence the hypothesis is a natural one 
 that all colors are produced by the mixture in varying proportions of a certain 
 number of fundamental colors, each of which depends for its production upon 
 the presence in the retina of a certain substance capable of being affected 
 (probably through some sort of a photo-chemical process) by light of a certain 
 definite wave-length. A hypothesis of this sort lies at the basis of both the 
 Young-Helmholtz and the Hering theories of color sensation. 
 
 The former theory postulates the existence in the retina of three substances 
 capable of being affected by red, green, and violet rays, respectively i. e. by 
 the three colors lying at the three angles of the color diagram given on p. 780 
 
782 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 and regards all other color sensations as produced by the simultaneous affec- 
 tion of two of these substances in varying proportions. Thus when a ray of 
 blue light falls on the retina it stimulates the violet- and green-perceiving sub- 
 stances, and produces a sensation intermediate between the two, while simul- 
 taneous stimulation of the red- and green-perceiving substances produces the 
 sensations corresponding to yellow and orange ; and when the violet- and red- 
 perceiving substances are affected at the same time, the various shades of 
 purple are produced. Each of these three substances is, however, supposed to 
 be affected to a slight extent by all the rays of the visible spectrum, a suppo- 
 sition which is rendered necessary by the fact that even the pure spectral 
 colors do not appear to be perfectly saturated, as will be explained in connec- 
 tion with the subject of saturation. Furthermore, the disappearance of color 
 when objects are very feebly or very brightly illuminated or when they are 
 seen with the lateral portions of the retina (as described on p. 779) necessitates 
 the additional hypotheses that these three substances are all equally affected by 
 all kinds of rays when the light is of either very small or very great intensity 
 or when it falls on the extreme lateral portions of the retina, and that they 
 manifest their specific irritability for red, green, and violet rays respectively 
 only in light of moderate intensity falling not too far from the fovea centralis 
 of the retina. 
 
 The modifications of the Young- Hemholtz theory introduced by these sub- 
 sidiary hypotheses greatly diminish the simplicity which was its chief claim to 
 acceptance when originally proposed. Moreover, there will always remain a 
 psychological difficulty in supposing that three sensations so different from each 
 other as those of red, green, and violet can by their union produce a fourth 
 sensation absolutely distinct from any of them viz. white. 
 
 The fact that in the Hering theory this difficulty is obviated has contributed 
 greatly to its acceptance by physiologists. In this theory the retina is supposed 
 to contain three substances in which chemical changes may be produced by ether 
 vibrations, but each of these substances is supposed to be affected in two oppo- 
 site ways by rays of light which correspond to complementary color sensa- 
 tions. Thus in one substance viz. the white-black visual substance kata- 
 bolic or destructive changes are supposed to be produced by all the rays of the 
 visible spectrum, the maximum effect being caused by the yellow rays, while 
 anabolic or constructive changes occur when no light at all falls upon the 
 retina. The chemical changes of this substance correspond, therefore, to the 
 sensation of luminosity as distinguished from color. In a second substance red 
 rays are supposed to produce katabolic, and green rays anabolic changes, while 
 a third substance is similarly affected by yellow and blue rays. These two 
 substances are therefore spoken of as red-green and yellow-blue visual sub- 
 stances respectively. 
 
 It has been sometimes urged as an objection to this theory that the effect of 
 a stimulus is usually katabolic and not anabolic. This is true with regard to 
 muscular contraction, from the study of which phenomenon most of our know- 
 ledge of the effect of stimulation has been obtained, but it should be remem- 
 
THE SENSE OF VISION. 783 
 
 bered that observations on the augmentor and inhibitory cardiac nerves have 
 shown us that nerve-stimulation may produce very contrary effects. There 
 seems to be, therefore, no serious theoretical difficulty in supposing that light 
 rays of different wave-lengths may produce opposite metabolic effects upon the 
 substances in which changes are associated with visual sensations. 
 
 A more serious objection lies in the difficulty of distinguishing between the 
 sensation of blackness, which, on Bering's hypothesis, must correspond to active 
 anabolism of the white-black substance, and the sensation of darkness (such as 
 we experience when the eyes have been withdrawn for some time from the 
 influence of light), which must correspond to a condition of equilibrium of 
 the white-black substance in which neither anabolism nor katabolism is 
 occurring. 
 
 Another objection to the Hering theory is to be found in the results of 
 experiments in comparing grays or whites produced by mixing different colored 
 rays under varying intensities of light. The explanation given by Hering of 
 the production of white through the mixture of blue and yellow or of red and 
 green is that when either of these pairs of complementary colors is mixed 
 the anabolic and the katabolic processes balance each other, leaving the corre- 
 sponding visual substance in a condition of equilibrium. Hence, the white- 
 black substance being alone stimulated, the result will be a sensation of white 
 corresponding to the intensity of the katabolic process caused by the mixed 
 rays. Now, it is found that when blue and yellow are mixed in certain pro- 
 portions on a revolving disk a white can be produced which will, with a certain 
 intensity of illumination, be undistinguishable from a white produced by mix- 
 ing red and green. If, however, the intensity of the illumination is changed, 
 it will be found necessary to add a certain amount of white to one of the mix- 
 tures in order to bring them to equality. On the theory that complementary 
 colors produce antagonistic processes in the retina it is difficult to understand 
 why this should be the case. 
 
 A color theory which is in some respects more in harmony with recent 
 observations in the physiology of vision has been proposed by Mrs. C. L. 
 Franklin. In this theory it is supposed that, in its earlier periods of de- 
 velopment, the eye is sensitive only to luminosity and not to color i. e. it 
 possesses only a white-black or (to use a single word) a gray-perceiving sub- 
 stance which is affected by all visible light rays, but most powerfully by those 
 lying near the middle of the spectrum. The sensation of gray is supposed to 
 be dependent upon the chemical stimulation of the optic nerve-terminations by 
 some product of decomposition of this substance. 
 
 In the course of development a portion of this gray visual substance becomes 
 differentiated into three different substances, each of which is affected by rays 
 of light corresponding to one of the three fundamental colors of the spectrum 
 viz. red, green, and blue. When a ray of light intermediate between two 
 of the fundamental colors falls upon the retina, the visual substances corre- 
 sponding to these two colors will be affected to a degree proportionate to the 
 proximity of these two colors to that of the incident ray. Since this effect is 
 

 784 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 exactly the same as that which is produced when the retina is acted upon simul- 
 taneously by light of two fundamental colors, we are incapable of distinguish- 
 ing in sensation between an intermediate wave-length and a mixture in proper 
 amounts of two fundamental wave-lengths. 
 
 When the retina is affected by two or more rays of such wave-lengths that 
 all three of the color visual substances are equally affected, the resulting decom- 
 position will be the same as that produced by the stimulation of the gray visual 
 substance out of which the color visual substances were differentiated, and the 
 corresponding sensation will therefore be that of gray or white. 
 
 It will be noticed that the important feature of this theory is that it pro- 
 vides for the independent existence of the gray visual substance, while at the 
 same time the stimulation of this substance is made a necessary result of the 
 mixture of certain color sensations. 
 
 Color-blindness. The fact that many individuals are incapable of distin- 
 guishing between certain colors i. e. are more or less " color-blind " is one 
 of fundamental importance in the discussion of theories of color vision. By 
 far the most common kind of color-blindness is that in which certain shades 
 of red and green are not recognized as different colors. The advocates of the 
 Young-Helmholtz theory explain such cases by supposing that either the red 
 or the green perceiving elements of the retina are deficient, or, if present, are 
 irritable, not by rays of a particular wave-length, but by all the rays of the 
 visible spectrum. In accordance with this view these cases of color-blindness 
 are divided into two classes viz. the red-blind and the green-blind the basis 
 for the classification being furnished by more or less characteristic curves repre- 
 senting the variations in the luminosity of the visible spectrum as it appears 
 to the different eyes. There are, however, cases which cannot easily be brought 
 under either of these two classes. Moreover, it has been proved in cases of 
 monocular color-blindness, and is admitted even by the defenders of the Helm- 
 holtz theory, that such persons see really only two colors viz. blue and yellow. 
 To such persons the red end of the spectrum appears a dark yellow, and the 
 green portion of the spectrum has luminosity without color. 
 
 A better explanation of this sort of color-blindness is given in the Hering 
 theory by simply supposing that in such eyes the red-green visual substance is 
 deficient or wholly wanting, but the theory of Mrs. Franklin accounts for the 
 phenomena in a still more satisfactory way ; for, by supposing that the differ- 
 entiation of the primary gray visual substance has first led to the formation 
 of a blue and a yellow visual substance, and that the latter has subsequently 
 been differentiated into a red and a green visual substance, color-blindness is 
 readily explained by supposing that this second differentiation has either not 
 occurred at all or has taken place in an imperfect manner. It is, in other 
 words, an arrest of development. 
 
 Cases of absolute color-blindness are said to occasionally occur. To such 
 persons nature is colorless, all objects presenting simply differences of light 
 and shade. 
 
 In whatever way color-blindness is to be explained, the defect is one of 
 
THE SENSE OF VISION. 785 
 
 considerable practical importance, since it renders those affected by it incapable 
 of distinguishing the red and green lights ordinarily used for signals. Such 
 persons are, therefore, unsuitable for employment as pilots, railway engineers, 
 etc., and it is now customary to test the vision of all candidates for employment 
 in such situations. It has been found that no satisfactory results can be 
 reached by requiring persons to name colors which are shown them, and the 
 chromatic sense is now commonly tested by what is known as the " Holmgren 
 method," which consists in requiring the individual examined to select from a 
 pile of worsteds of various colors those shades which seem to him to resemble 
 standard skeins of green and pink. When examined in this way about 4 per \ v 1 
 cent, of the male and one-quarter of 1 per cent, of the female sex are found to^ 
 be more or less color-blind. The defect may be inherited, and the relatives 
 of a color-blind person are therefore to be tested with special care. Since 
 females are less liable to be affected than males, it often happens that the 
 daughters of a color-blind person, themselves with normal vision, have sons 
 who inherit their grandfather's infirmity. 
 
 Although in all theories of color vision the different sensations are supposed 
 to depend upon changes produced by the ether vibrations of varying rates 
 acting upon different substances in the retina, yet it should be borne in mind 
 that we have at present no proof of the existence of any such substances. The 
 visual purple or, to adopt Mrs. Franklin's more appropriate term, " the rod 
 pigment" was at one time thought to be such a substance, but for the reasons 
 above given cannot be regarded as essential to vision. 1 
 
 That a centre for color vision, distinct from the visual centre, exists in the 
 cerebral cortex is rendered probable by the occurrence of cases of hemianopsia 
 for colors, and also by the experiments of Heidenhain and Cohn on the influ- 
 ence of the hypnotic trance upon color-blindness. 
 
 Intensity. The second of the above-mentioned qualitative modifications of 
 light is its intensity, which is dependent upon the energy of vibrations of the 
 molecules of the luminiferous ether. The sensation of luminosity is not, how- 
 ever, proportionate to the intensity of the stimulus, but varies in such a way 
 that a given increment of intensity causes a greater difference in sensation with 
 feeble than with strong illuminations. This phenomenon is illustrated by the 
 disappearance of a shadow thrown by a candle in a darkened room on a sheet 
 of white paper when sunlight is allowed to fall on the paper from the opposite 
 direction. In this case the absolute difference in luminosity between the 
 shadowed and unshadowed portions of the paper remains the same, but it 
 becomes imperceptible in consequence of the increased total illumination. 
 
 Although our power of distinguishing absolute differences in luminosity 
 diminishes as the intensity of the illumination increases, yet with regard to 
 relative differences no such dependence exists. On the contrary, it is found 
 within pretty wide limits that, whatever be the intensity of the illumination, 
 
 1 In a recently developed theory by Ebbinghaus (Zeitschrtft fur Psychologie und Physiologic 
 der Sinnesorgane, v. 145) a physiological importance in relation to vision is attached to this 
 substance in connection with other substances of a hypothetical character. 
 50 
 
786 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 it must be increased by a certain constant fraction of its total amount in order 
 to produce a perceptible difference in sensation. This is only a special case of 
 a general law of sensation known as Weber's law, which has been formulated 
 by Foster as follows : " The smallest change in the magnitude of a stimulus 
 which we can appreciate through a change in our sensation always bears the 
 same proportion to the whole magnitude of the stimulus." 
 
 Luminosity of Different Colors. When two sources of light having the 
 same color are compared, it is possible to estimate their relative luminosity 
 with considerable accuracy, a difference of about 1 per cent, of the total 
 luminosity being appreciated by the eye. When the sources of light have 
 different colors, much less accuracy is attainable, but there is still a great differ- 
 ence in the intensity with which rays of light of different wave-lengths affect 
 the retina. We do not hesitate to say, for instance, that the maximum 
 intensity of the solar spectrum is found in the yellow portion, but it is import- 
 ant to observe that the position of this maximum varies with the illumina- 
 tion. In a very brilliant spectrum the maximum shifts toward the orange, 
 and in a feeble spectrum (such as may be obtained by narrowing the slit of 
 the spectroscope) it moves toward the green. The curves in Figure 240 illus- 
 
 :;.*- 
 :',6- 
 :$.4- 
 
 Li- 
 ft, 
 
 2.H. 
 
 2.6 
 
 2.4 
 
 2.2 
 
 1. 
 
 1.8 
 
 !.> 
 
 1.4 
 
 1.2 
 
 1 
 
 0.8 
 
 0.6 
 
 0.4 
 
 0.2 
 
 Intensity H 
 G 
 F 
 E 
 D 
 C 
 B 
 A 
 
 450 
 
 430 
 O 
 
 670 660 625 605 590 575 555 535 520 505 
 BCD E F 
 
 FIG. 240. Diagram showing the distribution of the intensity of the spectrum as dependent upon the 
 
 degree of illumination (Konig). 
 
 trate this shifting of the maximum of luminosity of the spectrum with vary- 
 ing intensities of illumination. The abscissas represent wave-lengths in 
 millionths of a millimeter, and the ordinates the luminosity of the different 
 colors as expressed by the reciprocal values of the width of the slit necessary 
 to give to the color under observation a luminosity equal to that of an arbi- 
 
THE SENSE OF VISION. 787 
 
 trarily chosen standard. The curves from A to H represent the distribution 
 of the intensity of light in the spectrum with eight different grades of illumi- 
 nation. This shifting of the maximum of luminosity in the spectrum 
 explains the so-called " Purkinje's phenomenon " viz. the changing rela- 
 tive values of colors in varying illumination. This can be best observed 
 at nightfall, the attention being directed to a carpet or a wall-paper 
 the pattern of which is made up of a number of different colors. As 
 the daylight fades away the red colors, which in full illumination are 
 the most intense, become gradually darker, and are scarcely to be distin- 
 guished from black at a time when the blue colors are still very readily 
 distinguished. 
 
 Function of Rods and Cones. The layer of rods and cones has thus far 
 been spoken of as if all its elements had one and the same function. There 
 is, however, some reason to suppose that the rods and cones have different 
 functions. That color sensation and accuracy of definition are most perfect 
 in the central portion of the retina is shown by the fact that when we desire 
 to obtain the best possible idea of the form and color of an object we direct 
 our eyes in such a way that the image falls upon the fovea centralis of the 
 retina. The luminosity of a faint object, however, seems greatest when we 
 look not directly at it, but a little to one side of it. This can be readily 
 observed when we look at a group of stars, as, for example, the Pleiades. 
 When the eyes are accurately directed to the stars so as to enable us to count 
 them, the total luminosity of the constellation appears much less than when 
 the eyes are directed to a point a few degrees to one side of the object. Now, 
 an examination of the retina shows only cones in the fovea centralis. In the 
 immediately adjacent parts a small number of rods are found mingled with 
 the cones. In the lateral portions of the retina the rods are relatively more 
 numerous than the cones, and in the extreme peripheral portions the rods alone 
 exist. Hence this phenomenon is readily explained on the supposition that 
 the rods are a comparatively rudimentary form of visual apparatus taking 
 cognizance of the existence of light with special reference to its varying 
 intensity, and that the cones are organs specially modified for the localization 
 of stimuli and for the perception of differences of wave-lengths. The view 
 that the rods are specially adapted for the perception of luminosity and the 
 <x>nes for that of color derives support from the fact that in the retina of cer- 
 tain nocturnal animals e. g. bats and owls rods alone are present. This 
 theory has been further developed by Von Kries, 1 who in a recent article 
 describes the rods as differing from the cones in the following respects : (1) 
 They are color-blind i. e. they produce a sensation of simple luminosity 
 whatever be the wave-length of the light-ray falling on them ; (2) they are 
 more easily stimulated than the cones, and are particularly responsive to light- 
 waves of short wave-lengths ; (3) they have the power of adapting themselves \ 
 to light of varying intensity. 
 
 On this theory it is evident that we must get the sensation of white or 
 
 1 Zeiischrift fiir Psychologic und Physiologic der Sinneswgane, ix. 81. 
 
788 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 colorless light in two different ways : (1) In consequence of the stimulation 
 of the rods by any sort of light-rays, and (2) in consequence of the stimula- 
 tion of the cones by certain combinations of light-rays i. e. complementary 
 colors. In this double mode of white perception lies perhaps the explanation 
 of the effect of varying intensity of illumination upon the results of color- 
 mixtures which has been above alluded to (see p. 783) as an objection to the 
 Hering theory. The so-called " Purkinje's phenomenon," described on p. 787, 
 is readily explained in accordance with this theory, for, owing to the greater 
 irritability of the rods, the importance of these organs, as compared with the 
 cones, in the production of the total visual sensation is greater with feeble 
 than with strong illumination of the field of vision. At the same time, the 
 power of the rods to respond particularly to light-rays of short wave-length 
 will cause a greater apparent intensity of the colors at the blue than at the red 
 end of the spectrum. In this connection it is interesting to note that the phe- 
 nomenon is said not to occur when the observation is limited to the fovea 
 centralis, where cones alone are found. 1 
 
 Saturation. The degree of saturation of light of a given color depends, as 
 above stated, upon the amount of white light mixed with it. The quality of 
 light thus designated is best studied and appreciated by means of experiments 
 with rotating disks. If, for instance, a disk consisting of a large white and a 
 small red sector be rapidly rotated, the effect produced is that of a pale pink 
 color. By gradually increasing the relative size of the red sector the pink 
 color becomes more and more saturated, and finally when the white sector is 
 reduced to zero the maximum of saturation is produced. It must be borne 
 in mind, however, that no pigments represent completely saturated colors. 
 Even the colors of the spectrum do not produce a sensation of absolute 
 saturation, for, whatever theory of color vision be adopted, it is evident that 
 all the color-perceiving elements of the retina are affected more or less by all 
 the rays of light. Thus when rays of red light fall upon the retina they will 
 stimulate not only the red-perceiving elements, but to a slight extent also (to 
 use the language of the Helmholtz theory) the green- and violet-perceiving 
 elements of the retina. The effect of this will be that of mixing a small 
 amount of white with a large amount of red light i. e. it will produce the 
 sensation of incompletely saturated red light. This dilution of the sensation 
 can be avoided only by previously exhausting the blue- and green-perceiving 
 elements of the retina in a manner which will be explained in connection with 
 the phenomena of after-images. 
 
 Retinal Stimulation. Whenever by a stimulus applied to an irritable 
 substance the potential energy there stored up is liberated the following phe- 
 nomena may be observed : 1. A so-called latent period of variable duration 
 during which no effects of stimulation are manifest ; 2. A very brief period 
 during which the effect of the stimulation reaches a maximum ; 3. A period 
 of continued stimulation during which the effect diminishes in consequence of 
 the using up of the substance containing the potential energy i. e. a period 
 
 1 Von Kries : Centralblatt fur Physiologic, 1896, i. 
 
THE SENSE OF VISION. 
 
 789 
 
 of fatigue ; 4. A period after the stimulation has ceased in which the effect 
 slowly passes away. 
 
 FIG. 241. Diagram showing the effect of stimulation of an irritable substance. 
 
 The curve drawn by a muscle in tetanic contraction, as shown in Figure 
 
 241, illustrates this phenomenon. Thus, if A D represents the duration of the 
 stimulation, A B indicates the latent period, B C the period of contraction, 
 C D the period of fatigue under stimulation, and D E the after-effect of 
 stimulation showing itself as a slow relaxation. When light falls upon the 
 retina corresponding phenomena are to be observed. 
 
 Latent Period. That there is a period of latent sensation in the retina 
 (i. e. an interval between the falling of light on the retina and the beginning 
 of the sensation) is, judging from the analogy of other parts of the nervous 
 system, quite probable, though its existence has not been demonstrated. 
 
 Rise to Maximum of Sensation. The rapidity with which the sensation of 
 light reaches its maximum increases with the intensity of the light and varies 
 with its color, red light producing its maximum sensation sooner than green 
 and blue. Consequently, when the image of a white object is moved across 
 the retina it will appear bordered by colored fringes, since the various con- 
 stituents of white light do not produce their maximum effects at the same 
 time. This phenomena can be readily observed when a disk on which a 
 black and a white spiral band alternate with each other (as shown in Figure 
 
 242, A) is rotated before the eyes. The white band as its image moves out- 
 
 FIG. 242. Disks to illustrate the varying rate at which colors rise to their maximum of sensation. 
 
 ward or inward over the retinal surface appears bordered with colors which 
 vary with the rate of rotation of the disk and with the amount of exhaustion 
 of the retina. Chromatic effects due to a similar cause are also to be seen 
 when a disk, such as is shown in Figure 242, B (known as Benham's spectrum 
 
790 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 top), is rotated with moderate rapidity. The concentric bands of color appear 
 in reverse order when the direction of rotation is reversed. The apparent 
 movement of colored figures on a background of a different color when the 
 eye moves rapidly over the object or the object is moved rapidly before the 
 eye seems to depend upon this same retinal peculiarity. The phenomenon 
 may be best observed when small pieces of bright-red paper are fastened upon 
 a bright-blue sheet and the sheet gently shaken before the eyes. The red 
 figures will appear to move upon the blue background. The effect may be 
 best observed in a dimly-lighted room. 
 
 In this connection should be mentioned the phenomenon of " recurrent 
 images " or " oscillatory activity of the retina." l This may be best observed 
 when a black disk containing a white sector is rotated at a rate of about one 
 revolution in two seconds. If the disk is brightly illuminated, as by sunlight, 
 
 and the eye fixed steadily upon the axis of rota- 
 tion, the moving white sector seems to have a 
 shadow upon it a short distance behind its ad- 
 vancing border, and this shadow may be followed 
 by a second fainter, and even by a third still 
 fainter shadow, as shown in Figure 243. The 
 distance of the shadows from each other and 
 from the edge of the sector increases with the rate 
 of rotation of the disk and corresponds to a time 
 FIG. 243,-To illustrate the oscillatory interval of about 0.01 5". It thus appears that 
 
 activity of the retina (Charpentier). , 
 
 wnen light is suddenly thrown upon the retina 
 
 the sensation does not at once rise to its maximum, but reaches this point by 
 a sort of vibratory movement. The apparent duplication of a single very 
 brief retinal stimulation, as that caused by a flash of lightning, may perhaps 
 be a phenomenon of the same sort. 
 
 Fatigue of Retina. When the eye rests steadily upon a uniformly illu- 
 minated white surface (e. g. a sheet of white paper), we are usually unconscious 
 of any diminution in the intensity of the sensation, but it can be shown that 
 the longer we look at the paper the less brilliant it appears, or, in other words, 
 that the retina really becomes fatigued. To do this it is only necessary to place 
 a disk of black paper on the white surface and to keep the eyes steadily fixed 
 for about half a minute upon the centre of the disk. Upon removing the disk 
 without changing the direction of the eyes a round spot will be seen on the 
 white paper in the place previously occupied by the disk. On this spot the 
 whiteness of the paper will appear much more intense than on the neighboring 
 portion of the sheet, because we are able in this experiment to bring into direct 
 contrast the sensations produced by a given amount of light upon a fresh and 
 a fatigued portion of the retina. 2 
 
 1 Charpentier: Archives de Physiologic, 1892, pp. 541, 629; and 1896, p. 677. 
 
 2 Although the retina is here spoken of as the portion of the visual apparatus subject to 
 fatigue, it should be borne in mind that we cannot, in the present state of our knowledge, dis- 
 criminate between retinal fatigue and exhaustion of the visual nerve-centres. 
 
THE SENSE OF VISION. 791 
 
 The rapidity with which the retina becomes fatigued varies with the color 
 of the light. Hence when intense white light falls upon the retina, as when 
 we look at the setting sun, its disk seems to undergo changes of color as one 
 or another of the constituents of its light becomes, through fatigue, less and 
 less conspicuous in the combination of rays which produces the sensation of 
 white. 
 
 The After-effect of Stimulation. The persistence of the sensation after the 
 stimulus has ceased causes very brief illuminations (e. g. by an electric spark) to 
 produce distinct effects. On this phenomenon depends also the above-described 
 method of mixing colors on a revolving disk, since a second color is thrown 
 upon the retina before the impression produced by the first color has had time 
 enough to become sensibly diminished. The interval at which successive stim- 
 ulations must follow each other in order to pro- 
 duce a uniform sensation (a process analogous 
 to the tetanic stimulation of a muscle) may be 
 determined by rotating a disk, such as repre- 
 sented in Figure 244, and ascertaining at what 
 speed the various rings produce a uniform sen- 
 sation of gray. The interval varies with the 
 intensity of the illumination from 0.1 " to 
 0.033". The duration of the after-effect de- 
 pends also upon the length of the stimulation 
 and upon the color of the light producing it, 
 the most persistent effect being produced by the FIG. 244,-Disk to illustrate the persistence 
 
 of retinal sensation (Helmholtz). 
 
 red rays. In this connection it is interesting to 
 
 note that while with the rapidly vibrating blue rays a less intense illumination 
 suffices to stimulate the eye, the slowly vibrating red rays produce the more 
 permanent impression. 
 
 After-images. When the object looked at is very brightly illuminated the 
 impression upon the retina may be so persistent that the form and color of the 
 object are distinctly visible for a considerable time after the stimulus has ceased 
 to act. This appearance is known as a " positive after-image," and can be best 
 observed when we close the eyes after looking at the sun or other bright source 
 of light. Under these circumstances we perceive a brilliant spot of light which, 
 owing to the above-mentioned difference in the persistence of the impressions 
 produced by the various colored rays, rapidly changes its color, passing gen- 
 erally through bluish green, blue, violet, purple, and red, and then disappear- 
 ing. This phenomenon is apt to be associated with or followed by another 
 effect known as a " negative after-image." This form of after-image is much 
 more readily observed than the positive variety, and seems to depend upon the 
 fatigue of the retina. It is distinguished from the positive after-image by the 
 fact that its color is always complementary to that of the object causing it. In 
 the experiment to demonstrate the fatigue of the retina, described on p. 790, 
 the white spot which appears after the black disk is withdrawn is the " nega- 
 tive after-image " of the disk, white being complementary to black. If a 
 
792 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 colored disk be placed upon a sheet of white paper, looked at attentively for a 
 few seconds, and then withdrawn, the eye will perceive in its place a spot of 
 light of a color complementary to that of the disk. If, for example, the disk 
 be yellow, the yellow-perceiving elements of the retina become fatigued in 
 looking at it. Therefore when the mixed rays constituting white light are 
 thrown upon the portion of the retina which is thus fatigued, those rays which 
 produce the sensation of yellow will produce less effect than the other rays for 
 which the eye has not been fatigued. Hence white light to an eye fatigued for 
 yellow will appear blue. 
 
 If the experiment be made with a yellow disk resting on a sheet of blue 
 paper, the negative after-image will be a spot on which the blue color will 
 appear (1) more intense than on the neighboring portions of the sheet, owing 
 to the blue-perceiving elements of that portion of the retina not being fatigued ; 
 (2) more saturated, owing to the yellow-perceiving elements being so far 
 exhausted that they no longer respond to the slight stimulation which is pro- 
 duced when light of a complementary color is thrown upon them, as has been 
 explained in connection with the subject of saturation. 
 
 Contrast. As the eye wanders from one part of the field of vision to 
 another it is evident that the sensation produced by a given portion of the 
 field will be modified by the amount of fatigue produced by that portion on 
 which the eye has last rested, or, other words, the sensation will be the result 
 
 FIG. 245. To illustrate the phenomenon of contrast. 
 
 of the stimulation by the object looked at combined with the negative after- 
 image of the object previously observed. The effect of this combination is to 
 produce the phenomenon of successive contrast, the principle of which may be 
 thus stated : Every part of the field of vision appears lighter near a darker 
 
THE SENSE OF VISION. 793 
 
 part and darker near a lighter part, and its color seen near another color 
 approaches the complementary color of the latter. A contrast phenomenon 
 similar in its effects to that above described may be produced under conditions 
 in which negative after-images can play no part. This kind of contrast is 
 known as simultaneous contrast, and may perhaps be explained on the theory 
 that a stimulation of a given portion of the retina produces in the neighboring 
 portions an effect to some extent antagonistic to that caused by direct stimulation. 
 
 A good illustration of the phenomenon of contrast is given in Figure 245, 
 in which black squares are separated by white bands which at their points of 
 intersection appear darker than where they are bordered on either side by the 
 black squares. 
 
 A black disk on a yellow background seen through white tissue-paper 
 appears blue, since the white paper makes the black disk look gray and the 
 yellow background pale yellow. The gray disk in contrast to the pale yellow 
 around it appears blue. 
 
 The phenomenon of colored shadows also illustrates the principle of con- 
 trast. These may be observed whenever an object of suitable size and shape 
 is placed upon a sheet of white paper and illuminated from one direction by 
 daylight and from another by gaslight. Two shadows will be produced, one 
 of which will appear yellow, since it is illuminated only by the yellowish gas- 
 light, while the other, though illuminated by the white light of day, will 
 appear blue in contrast to the yellowish light around it. 
 
 Space-perception. Rays of light proceeding from every point in the 
 field of vision are refracted to and stimulate a definite point on the sur- 
 face of the retina, thus furnishing us with a local sign by which we can 
 recognize the position of the point from which the light proceeds. 
 Hence the size and shape of an optical image upon the retina enable us to 
 judge of the size of the corresponding object in the same way that the cutane- 
 ous terminations of the nerves of touch enable us to judge of the size and 
 shape of an object brought in contact with the skin. This spatial perception 
 is materially aided by the muscular sense of the muscles moving the eyeball, 
 for we can obtain a much more accurate idea of the size of an object if 
 we let the eye rest in succession upon its different parts than if we gaze fixedly 
 at a given point upon its surface. The conscious effort associated with a given 
 amount of muscular motion gives, in the case of the eye, a measure of distance 
 similar to that secured by the hand when we move the fingers over the surface 
 of an object to obtain an idea of its size and shape. 
 
 The perception of space by the retina is limited to space in two dimensions 
 i. e. in a plane perpendicular to the axis of vision. Of the third dimension 
 in space i. e. of distance from the eye the retinal image gives us no know- 
 ledge, as may be proved by the study of after-images. If an after-image of 
 any bright object e. g. a window be produced upon the retina in the man- 
 ner above described and the eye be then directed to a sheet of paper held in 
 the hand, the object will appear outlined in miniature upon the surface of the 
 paper. If, however, the eye be directed to the ceiling of the room, the object 
 
794 AN AMERICAN 1EXT-BOOK OF PHYSIOLOGY. 
 
 will appear enlarged and at a distance corresponding to that of the surface 
 looked at. Hence one and the same retinal image may, under different cir- 
 cumstances, give rise to the impression of objects at different distances. We 
 must therefore regard the perception of distance not as a direct datum of vision, 
 but, as will be later explained, a matter of visual judgment. 
 
 When objects are of such a shape that their images may be thrown suc- 
 cessively upon the same part of the retina, it is possible to judge of their rela- 
 tive size with considerable accuracy, the retinal surface serving as a scale to 
 which the images are successively applied. When this is not the case, the 
 error of judgment is much greater. We can compare, for instance, the relative 
 length of two vertical or of two horizontal lines with a good deal of precision,, 
 but in comparing a vertical with a horizontal line we are liable to make a con- 
 siderable error. Thus it is difficult to realize that the vertical and the hori- 
 zontal lines in Figure 246 are of the same length. The error consists in an 
 
 over-estimation of the length of the vertical 
 lines relatively to horizontal ones, and appears to 
 depend, in part at any rate, upon the small size 
 of the superior rectus muscle relatively to the 
 other muscles of tKeTeye. The difference amounts 
 to 30-45 per cent, in weight and 40-53 per cent, 
 in area of cross section. It is evident, therefore, 
 that a given motion of the eye in the upward 
 direction will require a more powerful contraction 
 of the weaker muscle concerned in the movement 
 
 FIG. 246.-To illustrate the over-esti- than will be demanded of the stronger muscles 
 
 mation of vertical lines. 
 
 moving the eye laterally to an equal amount. 
 
 Hence we judge the upward motion of the eye to be greater because to accom- 
 plish it we make a greater effort than is required 
 for a horizontal movement of equal extent. 
 
 The position of the vertical line bisecting the 
 horizontal one (in Fig. 246) aids the illusion, as 
 may be seen by turning the page through 90, so 
 as to bring the bisected line into a vertical posi- 
 tion, or by looking at the lines in Figure 247, in 
 which the illusion is much less marked than in 
 Figure 246. 
 
 The tendency to over-estimate the length of 
 vertical lines is also illustrated by the error 
 commonly made in supposing the height of the 
 crown of an ordinary silk hat to be greater 
 
 ,1 ., i 1,1 FIG. 247. To illustrate the over-estima- 
 
 than its breadth. tion of vertical lines . 
 
 Irradiation. Many other circumstances 
 
 affect the accuracy of the spatial perception of the retina. One of the most 
 important of these is the intensity of the illumination. All brilliantly illumi- 
 nated objects appear larger than feebly illuminated ones of the same size, as is 
 
THE SENSE OF VISION. 795 
 
 well shown by the ordinary incandescent electric lamp, the delicate filament of 
 which is scarcely visible when cold, but when intensely heated by the electric 
 current glows as a broad band of light. The phenomenon is known as " irra- 
 diation/ 7 and seems to depend chiefly upon the above-described imperfections 
 in the dioptric apparatus of the eye, in consequence of which points of light 
 produce small circles of dispersion on the retina and bright objects produce 
 
 FIG. 248. To illustrate the phenomenon of irradiation. 
 
 images with imperfectly defined outlines. The white square surrounded by 
 black and the black square surrounded by white (Figure 248), being of the 
 same size, would in an ideally perfect eye produce images of the same size on 
 the retina, but owing to the imperfections of the eye the images are not sharply 
 
 FIG. 249. To illustrate the phenomenon of irradiation. 
 
 defined, and the white surfaces consequently appear to encroach upon the darker 
 portions of the field of vision. Hence the white square looks larger than the 
 black one, the difference in the apparent size depending upon the intensity of 
 the illumination and upon the accuracy with which the eye can be accommo- 
 
796 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 dated for the distance at which the objects are viewed. The effect of irradi- 
 ation is most manifest when the dark portion of the field of vision over which 
 the irradiation takes place has a considerable breadth. Thus the circular white 
 spots in Figure 249, when viewed from a distance of three or four meters, 
 appear hexagonal, since the irradiation is most marked into the triangular dark 
 space between three adjacent circles. A familiar example of the effect of irra- 
 diation is afforded by the appearance of the new moon, whose sun-illuminated 
 crescent seems to be part of a much larger circle than the remainder of the 
 disk, which shines only by the light reflected upon it from the surface of the 
 
 Subdivided Space. A space subdivided into smaller portions by inter- 
 mediate objects seems more extensive than a space of the same size not so sub- 
 divided. Thus the distance from A to B (Fig. 250) seems longer than that from 
 
 D E 
 
 FIG. 250. To illustrate the illusion of subdivided space. 
 
 B to C, though both are of the same length, and for the same reason the square 
 D seems higher than it is broad, and the square E broader than it is high, the 
 illusion being more marked in the case of D than in the case of E, because, as 
 above explained, vertical distances are, as a rule, over-estimated. 
 
 The explanation of this illusion seems to be that the eye in passing over a 
 subdivided line or area recognizes the number and size of the subdivisions, 
 and thus gets an impression of greater total size than when no subdivisions 
 are present. 
 
 A good example of this phenomenon is afforded by the apparently increased 
 extent of a meadow when the grass growing on it is cut and arranged in hay- 
 cocks. 1 
 
 The relations of lines to each other gives rise to numerous illusions of 
 spatial perception, among the most striking of which are those afforded by the 
 so-called " Zollner's lines," an example of which is given in Figure 251. Here 
 
 1 It is interesting to note that a similar illusion has been observed when an interval of time 
 subdivided by audible signals is compared with an equal interval not so subdivided (Hall and 
 Jastrow : Mind, xi. 62). 
 
THE SENSE OF VISION. 797 
 
 the horizontal lines, though strictly parallel to each other, seem to diverge and 
 converge alternately, their apparent direction being changed toward greater per- 
 
 xxx xx xx xx xx 
 
 X X X XX XX X X X X 
 
 \\\\\ \\x\\ 
 
 \\ \ \ \ \ \ \ \ \ \ 
 XXXXXXXXXXX 
 
 XX X X X XX XX XX 
 
 xwwwwx v 
 
 \\N\\N\\\\\ 
 
 FIG. 251. Zollner's lines. 
 
 pendicularity to the short oblique lines crossing them. This illusion is to be ex- 
 
 plained in part by the tendency of the eye to over-estimate the size of acute and fo- 
 
 under-estimate that of obtuse angles a tendency which 
 
 also affords a partial explanation of the illusion in 
 
 Figure 252, where the line d is the real and the line/ 
 
 the apparent continuation of the line a. The illusion 
 
 in Zollner's figures is more marked when the figure is 
 
 so held that the long parallel lines make an angle of 
 
 about 45 with the horizon, since in this position the 
 
 eye appreciates their real position less accurately than 
 
 when they are vertical or horizontal. It is dimin- 
 
 ished, but does not disappear, when the eye, instead 
 
 of being allowed to wander over the figure, is fixed 
 
 upon any one point of the field of vision. Hence the 
 
 J . . ' , , . . FIG. 252,-To illustrate illusion 
 
 motions of the eye must be regarded as a factor in, but O f space-perception. 
 not the sole cause of, the illusion. 
 
 Our estimate of the size of given lines, angles, and areas is influenced by 
 neighboring lines, angles, and areas with which they are compared. This 
 influence is sometimes exerted in accordance with the principle of contrast, 
 and tends to make a given extension appear larger in presence of a smaller, 
 
 FIG. 253. To illustrate contrast in space-perception (Muller-Lyer). 
 
 and smaller in presence of a larger extension. This effect is illustrated in 
 Figure 253, in which the middle portion of the shorter line appears larger 
 than the corresponding portion of the longer line, in Figure 254, in which a 
 similar effect is observed in the case of angles, and in Figure 255, in which 
 
798 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 the space between the two squares seems smaller than that between the two 
 oblong figures. 
 
 In some case, however, an influence of the opposite sort l seems to be 
 
 FIG. 254. To illustrate contrast in space-perception (Muller-Lyer). 
 
 exerted, as is shown in Figure 256, in which the middle one of three parallel 
 lines seems longer when the outside lines are longer, and shorter when they 
 are shorter than it is itself, and in Figure 257, where a circle appears larger 
 
 FIG. 255. To illustrate contrast in space-perception (Muller-Lyer). 
 
 if surrounded by a circle larger than itself, and smaller if a smaller circle is 
 shown concentrically within it. 
 
 Lines meeting at an angle appear longer when the included angle is large 
 
 FIG. 256. To illustrate so-called " confluxion " in space-perception (Muller-Lyer). 
 
 than when it is small, as is shown in Figure 258. This influence of the 
 included angle affords a partial explanation of the illusion shown in Figure 
 259, where the horizontal line at B seems longer than at A ; but the distance 
 
 1 For this influence the name "confluxion" has been proposed by Muller-Lyer, from whose 
 article in the Archiv fur Physiologic, 1889, Sup. Bd., the above examples are taken. 
 
THE SENSE OF VISION. 
 
 799 
 
 between the extremities of the oblique lines seems also to affect our estimate 
 of the horizontal line in the same way as the outside lines in Figure 256 
 influence our judgment of the length of the line between them. 
 
 Perception of Distance. The retinal image gives us, as we have seen, 
 no direct information as to the distance of the object from the eye. This 
 
 FIG. 257. To illustrate so-called " confluxion " in space-perception (Miiller-Lyer). 
 
 knowledge is, however, quite as important as that of position in a plane per- 
 pendicular to the line of vision, and we must now consider in what way it is 
 obtained. The first fact to be noted is that there is a close connection between 
 the judgments of distance and of actual size. A retinal image of a given 
 size may be produced by a small object near the eye or by a large one at a 
 
 FIG. 258. To illustrate the influence of angles upon the apparent length of lines (Mviller-Lyer). 
 
 distance from it. Hence when we know the actual size of any object (as, for 
 example, a, human figure) we judge of its distance by the size of its image on 
 the retina. Conversely, our estimate of the actual size of an object will 
 depend upon our judgment of its distance. The fact that children constantly 
 misjudge both the size and distance of objects shows that the knowledge of 
 
 FIG. 259. Illusion of space-perception. 
 
 this relation is acquired only by experience. If circumstances mislead us 
 with regard to the distance of an object, we necessarily make a corresponding 
 error with regard to its size. Thus, objects seen indistinctly, as through a fog, 
 are judged to be larger, because we suppose them to be farther off, than they 
 really are. The familiar fact that the moon seems to be larger when near the 
 horizon than when near the zenith is also an illustration of this form of illu- 
 
800 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 sion. When the raoon is high above our heads we have no means of esti- 
 mating its distance from us, since there are no intervening objects with which 
 we can compare it. Hence we judge it to be nearer than when, seen on the 
 horizon, it is obviously farther off than all terrestrial objects. Since the size 
 of the retinal image of the moon is the same in the two cases, we reconcile 
 the sensation with its apparent greater distance when seen on the horizon by 
 attributing to the moon in this position a greater actual size. 
 
 If the retinal image have the form of a familiar object of regular shape 
 e. g. a house or a table we interpret its outlines in the light of experience 
 and distinguish without difficulty between the nearer and more remote parts of 
 the object. Even the projection of the outlines of such an object on to a plane 
 surface (i. e. a perspective drawing) suggests the real relations of the different 
 parts of the picture so strongly that we recognize at once the relative distances 
 of the various portions of the object represented. How powerfully a familiar 
 outline can suggest the form and relief usually associated with it is well illus- 
 trated by the experiment of looking into a mask painted on its interior to 
 resemble a human face. In this case the familiar outlines of a human face 
 are brought into unfamiliar association with a receding instead of a projecting 
 form, but the ordinary association of these outlines is strong enough to force 
 the eye to see the hollow mask as a projecting face. 1 rThe fact that the pro- 
 jecting portions of an object are usually more brightly illuminated than the 
 receding or depressed portions is of great assistance in determining their rela- 
 tive distance. This use of shadows as an aid to the perception of relief pre- 
 supposes a knowledge of the direction from which the light falls on an object, 
 and if we are deceived on the latter we draw erroneous conclusions with 
 regard to the former point. Thus, if we look at an embossed letter or figure 
 through a lens which makes it appear inverted the accompanying reversal of 
 the shadows will cause the letter to appear depressed. The influence of 
 shadows on our judgment of relief is, however, not so strong as that of the 
 outline of a familiar object. In a case of conflicting testimony the latter 
 usually prevails, as, for example, in the above-mentioned experiment with the 
 mask. 
 
 Aided by these peculiarities of the retinal picture, the mind interprets it as 
 corresponding in its different parts to points at different distances from the eye, 
 and it is interesting to notice that painters, whose work, being on a plane sur- 
 face, is necessarily in all its parts at the same distance from the eye, use similar 
 devices in order to give depth to their pictures. Distant hills are painted with 
 indistinct outlines to secure what is called " aerial perspective." Figures of 
 men and animals are introduced in appropriate dimensions to suggest the dis- 
 tance between the foreground and the background of the picture. Landscapes 
 are painted preferably by morning and evening light, since at these hours the 
 marked shadows aid materially in the suggestion of distance. 
 
 1 In the experiment the mask should be placed at a distance of about two meters and one 
 eye closed. Even with both eyes open the illusion often persists if the distance is increased to 
 five or six meters. 
 
THE SENSE OF VISION. 801 
 
 The eye, however, can aid itself in the perception of depth in ways which 
 the painter has not at his disposal. By the sense of effort associated with the 
 act of accommodation we are able to estimate roughly the relative distance of 
 objects before us. This aid to our judgment can, of course, be employed only 
 in the case of objects comparatively near the eye. Its effectiveness is greater 
 for objects not far from the near-point of vision, and diminishes rapidly as the 
 distance is increased, and disappears for distances more than two or three meters 
 from the eye. 
 
 When the head is moved from side to side an apparent change in the rela- 
 tive position of objects at different distances is produced, and, as the extent of 
 this change is inversely proportional to the distance of the objects, it serves as 
 a measure of distance. This method of obtaining the " parallax " of objects 
 by a motion of the head is often noticeable in persons whose vision in one 
 eye is absent or defective. 
 
 Binocular Vision. The same result which is secured by the comparison 
 of retinal images seen successively from slightly different points of view is 
 obtained by the comparison of the images formed simultaneously by any object 
 in the two eyes. In binocular vision we obtain a much more accurate idea of 
 the shape and distance of objects around us than is possible with monocular 
 vision, as may be proved by trying to touch objects in our neighborhood with 
 a crooked stick, first with both eyes open and then with one eye shut. When- 
 ever we look at a near solid object with two eyes, the right eye sees farther 
 round the object on the right side and the left eye farther round on the left. 
 The mental comparison of these two slightly different images produces the 
 perception of solidity or depth, since experience has taught us that those objects 
 only which have depth or solidity can affect the eyes in this way. Conversely, 
 if two drawings or photographs differing from each other in the same way that 
 the two retinal images of a solid object differ from each other are presented, 
 one to the right and the other to the left eye, the two images will become 
 blended in the mind and the perception of solidity will result. Upon this fact 
 depends the effect of the instrument known as the ( stereoscope^ the slides of 
 which are generally pairs of photographs of natural objects taken simultaneous- 
 
 FIG. 260. To illustrate stereoscopic vision. 
 
 ly with a double camera, of which the lenses are at a distance from each other 
 equal to or slightly exceeding that between the two axes of vision. The prin- 
 ciple of the stereoscope can be illustrated in a very simple manner by drawing 
 circles such as are represented in Figure 260 on thin paper, and fastening each 
 
 51 
 
802 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 pair across the end of a piece of brass tube about one inch or more in diameter 
 and ton inches long. Let the tubes be held one in front of each eye with the 
 distant ends nearly in contact with each other, as shown in Figure 261. If 
 the tubes are in such a position that the small circles are brought as near to 
 Jeach other as possible, as shown in Figure 260, th retinal images will blend, 
 
 the smaller circle will seem to be much 
 nearer than the larger one, and the eyes will 
 appear to be looking down upon a truncated 
 cone, such as is shown in Figure 262, since 
 a solid body of this form is the only one 
 
 
 Fio. 261. To illustrate stereoscopic vision. 
 
 FIG. 262. To illustrate stereoscopic vision. 
 
 bounded by circles related to each other as those shown in this experiment. 
 
 Stereoscopic slides often serve well to illustrate the superiority of binocular 
 over monocular vision. If the slide represents an irregular mass of rocks or 
 ice, it is often very difficult by looking at either of the pictures by itself to 
 determine the relative distance of the various objects represented, but if the 
 slide is placed in the stereoscope the true relation of the different parts of the 
 picture becomes at once apparent. 
 
 Since the comparison of two slightly dissimilar images received on the two 
 retinas is the essential condition of stereoscopic vision, it is evident that if the 
 two pictures are identical no sensation of relief can be produced. Thus, when 
 two pages printed from the same type or two engravings printed from the same 
 plate are united in a stereoscope, the combined picture appears as flat as either 
 of its components. If, however, one of the pictures is copied from the other, 
 ven if the copy be carefully executed, there will be slight differences in the 
 distances between the lines or in the spacing of the letters which will cause 
 apparent irregularities of level in the different portions of the combined pic- 
 ture. Thus, a suspected banknote may be proved to be a counterfeit if, when 
 placed in a stereoscope by the side of a genuine note, the resulting combined 
 picture shows certain letters lying apparently on different planes from the rest. 
 
 Pseudoscopic Vision. If the pictures of an ordinary stereoscopic slide be 
 
 reversed^ so that the picture bplong''"g 1>n 
 
 the left eye, and 
 
 presented to 
 
 Drives place to what is called a pseudo- 
 
 right 
 
 scopic effect /. c. we perceive not a solid but a hallow body. The effect is best 
 
THE SENSE OF VISION. 803 
 
 vl 
 
 obtained with the outlines of geometrical solids, photographs of corns or medals 
 or of objects 'which may readily exist ir^ an inyerted fo^m. ..IKfafifff the photo- 
 graphs represent objects which cannot be thus inverted, such as buildings and 
 landscapes, the pseudoscopic effect is not readily produced another example 
 of the power (see p. 800) of the outline of a familiar object to outweigh other 
 snrfo of 
 
 A pseudoscopic effect may be readily obtained without the use of a stereo- 
 scope by simply converging the visual axes so that the right eye looks at the 
 left and the left eye at the right picture of- a stereoscopic slide. The eyes may 
 hfi_aided in. assuming the right degree of convergence, by looking at a small 
 object like the head of a pin held between the eyes and the slide in the manner 
 described on p. 758. Figure 260, viewed in this way, will present the appear- 
 ance of a hollow truncated cone with the base turned toward the observer. A 
 stereoscopic slide with its pictures reversed will, of course r when viewed in this 
 way, present not a pseudoscopic, but a true stereoscopic, appearance, as shown 
 by Figures 226 and 227. 
 
 Binocular Combination of Colors. The effect of binocnlarly combin- 
 ing two different colors varies with the difference in wave-length of the colors. 
 Colors lying near each other in the spectrum will generally blend together 
 and produce the sensation of a mixed color, such as would result from the 
 union of colors by means of the revolving disk or by the method of reflected 
 and transmitted light, as above described. Thus a red and a yellow disk 
 placed in a stereoscope may be generally combined to produce the sensation 
 of orange. If, however, the colors are complementary to each other, as blue 
 and yellow, no such mixing occurs, but the field of vision seems to be occupied 
 alternately by a blue and by a yellow color. This so-called " rivalry of the 
 fields of vision " seems to depend, to a certain extent, upon the fact that in 
 order to see the different colors with equal distinctness the eyes must be dif- 
 ferently accommodated, for it is found that if the colors are placed at different 
 distances from the eyes (the colors with the less refrangible rays being at the 
 greater distance), the rivalry tends to disappear and the mixed color is more 
 easily produced. 
 
 An interesting effect of the stereoscopic combination of a black and a 
 white object is the production of the appearance of a metallic lustre or polish. 
 If, for instance, the two pictures of a stereoscopic slide represent the slightly 
 different outlines of a geometrical solid, one in black upon white ground and 
 the other in white upon black ground, their combination in the stereoscope 
 will produce the effect of a solid body having a smooth lustrous surface. 
 The explanation of this effect is to be found in the fact that a polished surface 
 reflects the light differently to the two eyes, a given point appearing bril- 
 liantly illuminated to one eye and dark to the other. Hence the stereoscopic 
 combination of black and white is interpreted as indicating a polished surface, 
 since it is by means of a polished surface that this effect is usually produced. 
 
 Corresponding 1 Points. When the visual axes of both eyes are directed 
 to the same object two distinct images of that object are formed upon widely 
 
804 ^Vr AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 separated parts of the nervous system. Yet but a single object is perceived. 
 The phenomenon is the same as that which occurs when a grain of sand is 
 held between the thumb and finger. In both cases we have learned (chiefly 
 through the agency of muscular movements and the nerves of muscular sense) 
 to interpret the double sensation as produced by a single object. 
 
 Any two points, lying one in each retina, the stimulation of which by rays 
 of light gives rise to the sensation of light proceeding from a single object are 
 said to be " corresponding points." Now, it is evident that thefovece centrales 
 of the two eyes must be corresponding points, for an object always appears 
 single when both eyes are fixed upon it. That double vision results when the 
 images are formed on points which are not corresponding may be best illus- 
 trated by looking at three pins stuck in a straight rod at distances of 35, 45, 
 and 55 centimeters from the end. If the end of the rod is held against the 
 nose and the eyes directed to each of the three pins in succession, it will be 
 found that, while the pin looked at appears single, each of the others appears 
 double, and that the three pins therefore look like five. 
 
 The two fovece centrales are not, of course, the only corresponding points. 
 In fact, it may be said that the two retinas correspond to each other, point for 
 point, almost as if they were superposed one upon the other with the fovese 
 together. The exact position of the points in space which are projected on to 
 corresponding points of the two retinas varies with the position of the eyes. 
 The line or surface in which such points lie is known as the " horopter." A 
 full discussion of the horopter would be out of place in this connection, but 
 one interesting result of its study may be pointed out viz. the demonstration 
 that when, standing upright, we direct our eyes to the horizon the horopter is 
 approximately a plane coinciding with the ground on which we stand. It is 
 of course important for security in walking that all objects on the ground 
 should appear single, and, as they are known by experience to be single, the 
 eye has apparently learned to see them so. 
 
 Since the vertical meridians of the two eyes represent approximately rows 
 of corresponding points, it is evident that when two lines are so situated that 
 their images are formed each upon a vertical meridian of one of the eyes, the 
 impression of a single vertical line will be produced, for such a line seen bin- 
 ocularly is the most frequent cause of this sort of retinal stimulation. This 
 is the explanation commonly given of the singular optical illusion which is 
 produced when lines drawn as in Figure 263 are looked at with both eyes fixed 
 upon the point of intersection of the lines and with the plane in which the 
 visual axes lie forming an angle of about 20 with that of the paper, the dis- 
 tance of the lines from the eyes being such that each line will lie approximately 
 in the same vertical plane with one of the visual axes. Under these circum- 
 stances each line will form its image on a vertical meridian of one of the eyes, 
 and the combination of these images results in the perception of a third line, 
 not lying in the plane of the paper, but apparently passing through it more or 
 less vertically, and swinging round its middle point with every movement of 
 the head or the paper. In this experiment it will be found that the illusion 
 
THE SENSE OF VISION. 
 
 805 
 
 of a line placed vertically to the plane of the paper does not entirely dis- 
 appear when one eye is closed. Hence it is evident that there is, as Mrs. 
 
 FIG. 264. Monocular illusion of vertical lines. 
 
 C. L. Franklin has pointed out, 1 a strong tendency to regard 
 lines which form their images approximately on the vertical 
 meridian of the eye as themselves vertical. This tendency 
 is well shown when a number of short lines converging 
 toward a point outside of the paper on which they are 
 drawn, as in Figure 264, are looked at with one eye held 
 a short distance above the point of convergence. Even 
 when the lines are not convergent, but parallel, so that their 
 images cannot fall upon the vertical meridian of the eye, the 
 illusion is not entirely lost. It will be found, for instance, 
 that when the Zollner lines, as given in Figure 251, are 
 looked at obliquely with one eye from one corner of the 
 FIG. 263. Binocu- figure, the short lines which lie nearly in a plane with the 
 ticanine visual axis appear to stand vertically to the plane of the 
 
 paper. 
 
 In this connection it may be well to allude to the optical illusion in conse- 
 quence of which certain portraits seem to follow the beholder with the eyes. 
 This depends upon the fact that the face is painted looking straight out from 
 the canvas i. e. with the pupil in the middle of the eye. The painting being 
 upon a flat surface, it is evident that, from whatever direction the picture is 
 viewed, the pupil will always seem to be in the middle of the eye, and the 
 eye will consequently appear to be directed upon the observer. The phenom- 
 enon is still more striking in the case of pictures of which the one repre- 
 sented in Figure 265 may be taken as an example. Here the soldier's rifle 
 
 1 Am. Journal of Psychology, vol. i. p. 99. 
 
806 
 
 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 
 
 FIG. 265. Illusion of lines always pointing 
 toward observer. 
 
 is drawn as it appears to an eye looking straight down the barrel, and, as this 
 foreshortening is the same in all positions of the observer, it is evident that 
 
 when such a picture is hung upon the wall 
 of a room the soldier will appear to be 
 aiming directly at the head of every person 
 present. 
 
 In concluding this brief survey of some 
 of the most important subjects connected 
 with the physiology of vision it is well to 
 utter a word of caution with regard to a 
 danger connected with the study of the sub- 
 ject. This danger arises in part from the 
 fact that in the scientific study of vision it 
 is often necessary to use the eyes in a way 
 quite different from that in which they are 
 habitually employed, and more likely, there- 
 fore, to cause nervous and muscular fatigue. 
 We have seen that in any given position of 
 the eye distinct definition is limited to an 
 area which bears a very small proportion to 
 
 the whole field of vision. Hence in order to obtain an accurate idea of the 
 appearance of any large object our eyes must wander rapidly over its whole 
 surface, and we use our eyes so instinctively and unconsciously in this way 
 that, unless our attention is specially directed to the subject, we find it diffi- 
 cult to believe that the power of distinct vision is limited to such a small 
 portion of the retina. In most of the experiments in physiological optics, 
 however, this rapid change of direction of the axis of vision must be carefully 
 avoided, and the eye-muscles held immovable in tonic contraction. 
 
 Our eyes, moreover, like most of our organs, serve us best when we do not 
 pay too much attention to the mechanism by which their results are brought 
 about. In the ordinary use of the eyes we are accustomed to neglect after- 
 images, intraocular images, and all the other imperfections of our visual appa- 
 ratus, and the usefulness of our eyes depends very much upon our ability thus 
 to neglect their defects. Now, the habit of observing and examining these 
 defects that is involved in the scientific study of the eye is found to interfere 
 with our ability to disregard them. A student of the physiology of vision 
 who devotes too much attention to the study of after-images, for instance, may 
 render his eyes so sensitive to these phenomena that they become a decided 
 obstacle to ordinary vision.