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Vision

VISION (from Lat. videre, to see), or SIGHT, the function, in physiology, of the organ known as the eye (<?..). The sense of vision is excited by the influence of light on the retina, the special terminal organ connected with the optic nerve. By excitation of the retina, a change is induced in the optic nerve fibres, and is conveyed by these to the brain, the result being a luminous perception, or what we call a sensation of light or colour. If light were to act uniformly over the retina, there would be no image of the source of the light formed on that structure, and consequently there would be only a general consciousness of light, without reference to any particular object. One of the first conditions, therefore, of vision for useful purposes is the formation of an image on the retina. To effect this, just as in a photographic camera, refractive structures must be placed in front of the retina which will so bend luminous rays as to bring them to a focus on the retina, and thus produce an image. Throughout the animal kingdom various arrangements are found for this purpose; but they may be all referred to three types, namely (i) eye-specks or eye-dots, met with in Medusae, Annelidae, etc.; (2) the compound eye, as found in insects and crustaceans; and (3) the simple eye, common to all vertebrates. The eye-specks may be regarded simply as expansions of optic nerve filaments, covered by a transparent membrane, but having no refractive media, so that the creature would have the consciousness of light only, or a simple luminous impression, by which it might distinguish light from darkness. The compound eye consists essentially of a series of transparent cone-like bodies, arranged in a radiate manner against the inner surface of the cornea, with which their bases are united, while their apices are connected with the ends of the optic filaments. As each cone is separated from its neighbours, it admits only a ray of light parallel with its axis, and its apex represents only a portion of the image, which must be made up, like a mosaic-work, of as many parts as there are cones in the eye. When the cones are of considerable length, it is evident, from their form and direction, their apices being directed inwards, that the oblique rays emanating from a luminous surface will be cut off, and that only those rays proceeding along the axis of the cone will produce an effect. Thus distinctness or sharpness of definition will be secured. The size of the visual field will depend on the form of the eye, the outermost cones marking its limits. Consequently the size of the visual field will depend on the size of the segment of the Sphere forming its surface. The eyes of many insects have a field of about half a Sphere, so that the creature will see objects before and behind it as well as those at the side. On the other hand, in many the eyes have scarcely any convexity, so that they must have a narrow field of vision. For anatomical details, and diseases of the eye, see EYE; the pathological aspects of vision itself are treated at the end of this article.

i. PHYSICAL CAUSES OF VISION A luminous sensation may be excited by various modes of irritation of the retina or of the optic nerve. Pressure, cutting or electrical shocks may act as stimuli, but the normal excitation is the influence of !ight on the retina. From a physical point of view, light is a mode of movement occurring in a medium, termed the aether, which pervades ah 1 space; but the physiologist studies the operation of these movements on the sentient organism as resulting in consciousness of the particular kind which we term a luminous impression. Outside of the body, such movements have been studied with great accuracy; but the physiological effects depend upon such complex conditions as to make it impossible to state them in the same precise way. Thus, when we look at the spectrum, we are conscious of the sensations of red and violet, referable to its two extremities: the physicist states that red is produced by 392 billions of impulses on the retina per second, and that violet corresponds to 757 billions per second; but he has arrived at this information by inductive reasoning from facts which have not at present any physiological explanation. We cannot at present trace any connexion, as cause and effect, between 392 billions of impulses on the retina per second and a sensation of red. Below the red and above the violet ends of the spectrum there are vibrations which do not excite luminous sensations. In the first case, below the red, the effect as a sensation is heat; and above the violet the result is that of chemical activity. Thus the method of dispersion of light, as is followed in passing a ray through a prism, enables us to recognize these general facts: (i) rays below the red excite thermal impressions; (2) from the lower red up to the middle of the violet, the thermal rays become gradually weaker until they have no effect; (3) from the lower red to the extreme violet, they cause luminous impressions, which reach their greatest intensity in the yellow; and (4) from about the end of the yellow to far beyond the extreme violet, the rays have gradually a less and less luminous effect, but they have the power of exciting such chemical changes as are produced in photography. In general terms, therefore, the lower end of the spectrum may be called thermal, the middle luminous, and the upper actinic or chemical; but the three merge into and overlap one another. It may be observed that the number of vibrations in the extreme violet is not double that of the low red, so that the sensibility of the eye to vibrations of light does not range through an octave. The ultra-violet rays may act on the retina in certain conditions, as when they are reflected by a solution of sulphate of quinine, constituting the phenomenon of fluorescence. Far above the violet are the Rontgen radiations and probably others.

2. OPTICAL ARRANGEMENTS OP THE EYE i. General. When light traverses any homogeneous transparent medium, such as the air, it passes on in a straight course with a certain velocity; but if it meet with any other transparent body of a different density, part of it is reflected or returned to the first medium, whilst the remainder is propagated through the second medium in a different direction and with a different velocity. Thus we may account for the phenomena of reflection of light (q.v.) and of refraction (<?..). Let ab, in fig. i, be a plane surface of some transparent substance, say a sheet *. of glass; a ray, cd, perpendicular to the surface, will pass through without refraction; but an oblique ray, ef, will be sent in the direction eh. If the ray eh had passed from a dense into a rarer medium, then the direction would have been eg. It might also be shown that the FIG. i. Refraction of Light.

sine of the angle of incidence always bears a certain ratio to the sine of the angle of refraction; this ratio is termed the index of refraction. Thus, if a ray pass from air into water, the sine of the angle of incidence will have to the sine of the angle of the refraction the ratio of 4:3, or f .

Before a ray of light can reach the retina, it must pass through a number of transparent and refractive surfaces. The eye is a nearly spherical organ, formed of transparent parts situated behind each other, and surrounded by various membranous structures, the anterior part of which is also transparent. The transparent parts are (i) the cornea; (2) the aqueous humour, found in the anterior chamber of the eye; (3) the crystalline lens, formed by a transparent convex body, the anterior surface of which is less convex than the posterior; and (4) the vitreous humour, filling the posterior chamber of the eye. The ray must therefore traverse the cornea, aqueous humour, lens and vitreous humour. As the two surfaces of the cornea are parallel, the rays practically suffer no deviation in passing through that structure, but they are bent or refracted during their transmission through the other media.

From the optical point of view, the eye may be regarded as a dioptric system consisting of various refractive media. In such a system, as shown by K. F. Gauss, there are six cardinal points, which have a certain relation to each other. These are (i) Two focal points-.every ray passing through theirs* focal point becomes, after its refraction, parallel to the axis, and every ray which before refraction is parallel to the axis passes after its refraction to the second focal point; (2) two principal points: every ray which passes through the first point before refraction passes after refraction through the second, and every ray which passes through any point of a plane elevated on a perpendicular axis from the first principal point (the first principal plane) _passes through the corresponding point of an analogous plane raised upon the axis at the second principal point (the second principal plane); and (3) too nodal points, which correspond to the optical centres of the two principal planes just alluded to. The distance of the first principal point from the first focal point is called the anterior focal length, and the term posterior focal length is applied to the distance of the posterior focal point from the second principal point. Listing has given the following measurements in millimetres from the centre of the cornea for the cardinal points in an ideal eye :

Anterior focal point . 12-8326 Posterior focal point . 22-6470 First principal point . 2-1746 Second principal point. 2-5724 First nodal point . 7-2420 Second nodal point . 7-6398 Anterior focal length . 15-0072 Posterior focal length . 20-0746 A view of such an ideal eye is shown in fig. 2.

The remaining measurements of such an eye are as follows :

Radii of Curvature Of anterior face of cornea = 8 millimetres. Of anterior face of lens = 10 Of posterior face of lens =6 Indices of Refraction Aqueous humour .... SV = ' '3379 Crystalline lens .... It = 1-4545 Vitreous humour .... W= 1-3379 I 3 2 The optical constants of the human eye may be still further simplified by assuming that the two principal points and the two FIG. 2. Transverse Section of an Ideal or Schematique Eye.

A, summit of cornea; SC, sclerotic; S, Schlemm's canal; CH, choroid; I, iris; M, ciliary muscle; R, retina; N, optic nerve; HA, aqueous humour; L, crystalline lens, the anterior of the double lines on its face showing its form during accommodation; HV, vitreous humour; DN, internal rectus muscle; DE, external rectus; YY', principal optical axis; **, visual axis, making an angle of 5 with the optical axis; C, centre of the ocular globe. The cardinal points of Listtne: HiH 2 , principal points; KiKs, nodal points; ViFi, principal focal points. The dioptric constants according to Giraud-Teulon: H, principal points united ; *i*2, principal foci during the repose of accommodation ; *'i*'z, principal foci during the maximum of accommodation ; O, fused nodal points.

nodal points respectively are identical. Thus we may construct a reduced eye, in which the principal point is 2-3448 mm. behind the cornea and the single nodal point is 1-4764 mm. in front of the posterior surface of the, lens. The refracting surface, or lens, has a radius of 5 mm. and is 3 mm. behind the cornea; and the index of refraction is that of the aqueous humour, or \f, or 1-3379- 2. The Formation of an Image on the Retina. This may be well illustrated with the aid of a photographic camera. If properly focused, an inverted image will be seen on the glass plate at the back of the camera. It may also be observed by bringing the eyeball of a rabbit near a candle flame. The action of a lens in forming an inverted image is illustrated by fig. 3, where the pencil of rays proceeding from a is brought to a focus at a', and those from b at b'; consequently the image , of 06 is inverted as at b'a'. The three characteristic features of the retinal image are: (i) it is reversed; (2) it is sharp and well defined if it be accurately focused on the retina; and (3) its size depends on the visual angle. If we look at a distant object, say a star, the rays' reaching the eye are parallel, and in passing through the refractive media they are focused at the posterior focal point that is, on the retina. A line from the luminous point on the retina passing through the nodal point is called the line of direction. If the luminous object be not nearer than, say, 60 yds. the image is still brought to a focus on the retina without any effort on the part of the eye. Within this distance, supposing the condition of the eye to be the same as in looking at a star, the image would be formed somewhat behind the posterior focal point, and the effect would be an indistinct impression on the retina. To obviate this, for near distances, accommodation, so as to adapt the eye, is effected by a mechanism to be afterwards described.

When rays, reflected from an object or coming from a luminous point, are not brought to an accurate focus on the retina, the image is not distinct in consequence of the formation of circles of diffusion, the production of which will be rendered evident by fig. 4. From the point A luminous rays enter the eye in the form of a cone, the kind of which will depend FIG. 3. Inversion bv Action of a Lens.

on the pupil. Thus it may be circular, or oval, or even tri-' angular. If the pencil is focused in front of the retina, as at FIG. 4. Formation of Circles of Diffusion.

d, or behind it as at /, or, in other words, if the retina, in place of being at F, be in the positions G or H, there will be a luminous circle or a luminous triangular space, and many elements of the retina will be affected. The size of these diffusion circles depends on the distance from the retina of the point where the rays are focused: the greater the distance, the more extended will be the diffusion circle. Its size will also be affected by the greater or less diameter of the pupil. Circles of diffusion may be studied by the following experiment, called the experiment of Scheiner:

D E FIG. 5. Diagram illustrating the Experiment of Scheiner.

Let C be a lens, and DEF be screens placed behind it. Hold in front of the lens a card perf orated by two holes A and B, and allow rays from a luminous point a to pass through these holes. The point o on the screen E will be the focus of the rays emanating from a; if a were removed farther from the lens, the focus would be on F, and if it were brought near to C, the focus would then be on D. The screens F and D show two images on the point a. If, then, we close the upper opening in AB, the upper image m on F and the lower image n on D disappear. Suppose now that the retina be substituted for the screens D and F, the contrary will take place, in consequence of the reversal of the retinal _image. If the eye be placed at o, only one image will be seen; but if it be placed either in the plane of F or D, then two images will be seen, as at mm, or nn; consequently, in either of these planes there will be circles of diffusion and indistinctness, and only in the plane E will there be sharp definition of the image.

To understand the formation of an image on the retina, suppose a line drawn from each of its two extremities to the nodal point and continued onwards to the retina, as in fig. 6, where the visual angle is x. It is evident that its size will depend on the size of the object and the distance of the object from the eye. Thus, also, objects of different sizes, c, d, e in fig. 6, may be included in the same visual angle, as they are at different distances from the eye. The size of the retinal image may be calculated if we know the size of the object, its distance from the nodal point o, and the distance of the nodal point from the posterior focus. Let A be the size of the object, B its distance from the nodal point, and C the distance of o from the retina, or 15 mm.; then the size of the retinal image oe=(A+is)/B. The smallest visual angle in which two distinct points may be observed is 60 seconds; below this, the two sensations fuse into one; and the size of the retinal image FIG. 6. The Visual Angle.

FIG. 7. Spherical Aberration, being focused at N. Thus on the corresponding to this angle is -004 mm., nearly the diameter of a single retinal rod or cone. Two objects, therefore, included in a visual angle of less than 60 seconds, appear as one point. A small visual angle is in most eyes a condition of sharpness of definition. With a large angle, objects appear less sharply marked. Acuteness is determined by a few retinal elements, or even only one, being affected. A very minute image, if thrown on a single retinal element, is apparently sufficient to excite it. Thus it is possible to see a brilliant point in an angle even so small as J of a second, and a sharp eye can see a body the ^th of a line in diameter that is, about the a-foth part of an inch.

3. The Optical Defects oj the ,ye. As an optical instrument, the eye is defective; but from habit, and want of attention, its defects are not appreciated, and consequently they have little or no influence on our sensations. These defects are chiefly of two kinds (i) those due to the curvature of the refractive surfaces, and (2) those due to the dispersion of light by the refractive media.

(a) Aberration of Sphericity. Suppose, as in fig. 7, M A K to be a refractive surface on which parallel rays from L to S impinge, it will be seen that [ those rays passing near the circumference are brought to a focus at F 1 , and those passing near the centre at F* intermediate rays at N. Thus on the portion of the axis between F 1 and P there will be a series of focal points, and the effect will be a blurred and bent image. In the eye this defect is to a large extent corrected by the following arrangements: (i) the iris cuts off the outer and more strongly refracted rays; (2) the curvature of the cornea is more ellipsoidal than spherical, and consequently those farthest from the axis are least deviated; (3) the anterior and posterior curvatures of the lens are such that the one corrects, to a certain extent, the action of the other; and (4) the structure of the lens is such that its power of refraction diminishes from the centre to the circumference, and consequently the rays farthest from the axis are less refracted.

(b) Astigmatism. Another defect of the eye is due to different meridians having different degrees of curvature. This defect is known as astigmatism. It may be thus detected. Draw on a sheet of white paper a vertical and a horizontal line with ink, crossing at a right angle; at the point of distinct vision, it will be found impossible to see the lines with equal distinctness at the same time; to see the horizontal line distinctly the paper must be brought near the eye, and removed from it to see the vertical. In the cornea the vertical meridian has generally a shorter radius of curvature, and is consequently more refractive than the horizontal. The meridians of the lens may also vary; but, as a rule, the asymmetry of the cornea is greater than that of the lens. The optical explanation of the defect will be understood with the aid of fig. 8. Thus, suppose the vertical meridian C A D to be more strongly curved than the horizontal F A E, the rays which fall on C A D will be brought to a focus G, and those falling on F A E at B. If we divide the pencil of rays at successive points, G, H, I, K, B, by a section perpendicular to A B, the various forms it would present at these points are seen in the figures underneath, so that if the eye were placed at G, it would see a horizontal line a a'; if at H, an ellipse with the long axis a a' parallel to A B; if at I, a circle; if at K, an ellipse, with the long axis, b c, at right angles to A B; and if at B, a vertical line b c. The degree of astigmatism is ascertained by measuring the difference of refraction in the two chief meridians; and the defect is corrected by the use of cylindrical glasses, the curvature of which, added to that of the minimum meridian, makes its focal length equal to that of the maximum meridian.

f" IG -9- Diagram illustrating the Dispersion of Light by a Lens.

FIG. 8. Diagram illustrating Astigmatism.

(c) Aberration of Refrangibilily. When a ray of white light traverses on a lens, the different rays composing it, being unequally refrangible, are dispersed: the violet rays (see fig. 9), the most refran- A gible, are brought to a focus at e, and the red rays, less refrangible, - at d. If a screen were placed at e, a series of concentnc coloured circles would be formed, the central being of a violet, and the circumference of a red colour. The reverse effect would be produced if the screen were placed at d. Imagine the retina in place of the screen in the two positions, the sensational effects would be those just mentioned. Under ordinary circumstances, the error of refrangibility due to the optical construction of the eye is not observed, as for vision at near distances the interval between the focal point of the red and violet rays is very small. If, however, we look at a candle flame through a bit of cobalt blue glass, which transmits only the red and blue rays, the flame may appear violet surrounded by blue, or blue surrounded by violet, according as we have accommodated the eye for different distances. Red surfaces always appear nearer than violet surfaces situated in the same plane, because the eye has to be accommodated more for the red than for the violet, and consequently we imagine them to be nearer. Again, if we contemplate red letters or designs on a violet ground the eye soon becomes fatigued, and the designs may appear to move.

(d) Defects due to Opacities, etc., in the Transparent Media. When small opaque particles exist in the transparent media, they may cast their shadow on the retina so as to give rise to images which are projected outwards by the mind into space, and thus appear to exist outside of the body. Such phenomena are termed entoptic. They may be of two kinds: (i) extraretinal, that is, due to opaque or semi-transparent bodies in any of the refractive structures anterior to the retina, and presenting the appearance of drops, striae, lines, twisted bodies, forms of grotesque shape, or minute black dots dancing before the eye; and (2) intra-retinal, due to opacities, etc., in the layers of the retina, in front of Jacob's membrane. The intra-retinal may be produced in a normal eye in various ways, (i) Throw a strong beam of light on the edge of the sclerotic, and a curious branched figure will be seen, which is an image of the retinal vessels. The construction of these images, usually called Purkinje's figures, will be understood from fig. 10. Thus, in the figure to the left, the rays passing through the sclerotic at b", in the direction b" c, will throw a shadow of a vessel at c on the retina at b', and this will appear as a dark line at B. If the light move from b* to a", the retinal shadow will move from 4' to a', and the line in the field of vision will pass from B to A.

It may be shown that the distance c b' corresponds to the distance of the retinal vessels from the layer of rods and cones.

S If the light enter the cornea, as in the figure to the right, and if the light be moved, the image * will be displaced in the same direction as the light, if the movement does not extend beyond the middle of the cornea, but in the FIG. to Purkinje's Figures. opposite direction In the eye to the right the illumination is to the light through the sclerotic, and in the one to the when the latter left through the cornea. ; s moved up and down. Thus, if a be moved to a', d will be moved to d', the shadow on the retina from c to c', and the image b to b'. If, on the other hand, a be moved above the plane of the paper, d will move below, consequently c will move above, and b' will appear to sink. (2) The retinal vessels may also be seen by looking at a strong light through a minute aperture, in front of which a rapid to-and-fro movement is made. Such experiments prove that the sensitive part of the retina is its deepest and most external layer (Jacob's membrane).

4. Accommodation, or the Mechanism of Adjustment for Different Distances. When a camera is placed in front of an object, it is necessary to focus accurately in order to obtain a clear and distinct image on the sensitive plate. This may be done by moving either the lens or the sensitive plate backwards or forwards so as to have the posterior focal point of the lens corresponding with the sensitive plate. For similar reasons, a mechanism of adjustment, or accommodation for different distances, is necessary in the human eye. In the normal eye, any number of parallel rays, coming from a great distance, are focused on the retina. Such an eye is termed emmetropic (fig. ii, A). Another form of eye (B) may be such that parallel rays are brought to a focus in front of the retina. This form of eye is myopic or shortsighted, inasmuch as, for distinct vision, the object must be brought near the eye, so as to catch the divergent rays, which are then focused on the retina. A third form is seen in C, where the focal point, for ordinary distances, is behind the retina, and consequently the object must be held far off, so as to allow only the less divergent or parallel rays to reach the eye. kind of eye is called hypermetropic, or far-sighted. For ordinary distances, at which objects must be seen distinctly in everyday life, the fault of the myopic eye may be corrected by the use of concave and of the hypermetropic by convex glasses. In the first case, the concave glass will move the posterior focal point a little farther back, and in the second the convex glass will bring it farther forwards; in both cases, however, the glasses may be so adjusted, both as regards refractive index and radius of curvature, as to bring the rays to a focus on the retina, and consequently secure distinct vision.

From any point 65 metres distant, rays may be regarded FIG. ii.

A, Emmetropic or normal eye; B, Myopic or short-sighted eye; C, Hypermetropic or long-sighted eye.

as almost parallel, and the point will be seen without any effort of accommodation. This point, either at this distance or in infinity, is called the punctum remotum, or the most distant point seen without accommodation. In the myopic eye it is much nearer, and for the hypermetropic there is really no such point, and accommodation is always necessary. If an object were brought too close to the eye for the refractive media to focus it on the retina, circles of diffusion would be formed, with the result of causing indistinctness of vision, unless the eye possessed some power of adapting itself to different distances. That the eye has some such power of accommodation is proved by the fact that, if we attempt to look through the meshes of a net at a distant object, we cannot see both the meshes and the object with equal distinctness at the same time. Again, if we look continuously at very near objects, the eye speedily becomes fatigued. Beyond a distance of 65 metres, no accommodation is necessary; but within it, the condition of the eye must be adapted to the diminished distance until we reach a point near the eye which may be regarded as the limit of visibility for near objects. This point, called the punctum proximum, is usually 12 centimetres (or 4-8 inches) from the eye. The range of accommodation is thus from the punctum remotum to the punctum proximum.

The mechanism of accommodation has been much disputed, but there can be no doubt it is chiefly effected by a change in the curvature of the anterior surface of the crystalline lens. If we hold a lighted candle in front and a little to the side of an eye to be examined, three reflections may be seen in the eye, as represented in fig. 12. The first, a, is erect, large and bright, from the anterior surface of the cornea; the second, b, also erect, but dim, from the anterior surface of the crystalline lens; and the third, c, inverted, and very dim, from the posterior surface of the lens, or perhaps the concave surface of the vitreous humour to which the convex surface of the lens is adapted. Suppose the three images to be in the position shown in the figure for FIG. 12. Reflected distant vision, it will be found that the middle Images in the Eye. image 6 moves towards a, on looking at a near object. The change is due to an alteration of the curvature of the lens, as shown in fig. 13. The changes occurring during accommodation are:

FIG. 13. Mechanism of Accommodation.

A, The lens during accommodation, showing its anterior surface advanced; B, The lens as for distant vision; C, Position of the ciliary muscle.

(i) the curvature of the anterior surface of the crystalline lens increases, and may pass from 10 to 6 mm.; (2) the pupil contracts; and (3) the intraocular pressure increases in the posterior part of the eye. An explanation of the increased curvature of the anterior surface of the lens during accommodation has been thus given by H. von Helmholtz. In the normal condition, that is, for the emmetropic eye, the crystalline lens is flattened anteriorly by the pressure of the anterior layer of the capsule; during accommodation, the radiating fibres of the ciliary muscles pull the ciliary processes forward, thus relieving the tension of the anterior layer of the capsule, and the lens at once bulges forward by its elasticity.

By this mechanism the radius of curvature of the anterior surface of the lens, as the eye accommodates from the far to the near point, may shorten from 10 mm. to 6 mm. The ciliary muscle, however, contains two sets of fibres, the longitudinal or meridional, which run from before backwards, and the circular or equatorial (MUller's muscle), which run, as their name indicates, around the band of longitudinal fibres forming the muscle. Direct observation on the eye of an animal immediately after death shows that stimulation of the ciliary nerves actually causes a forward movement of the ciliary processes, and there can be little doubt that the explanation above given applies to man, probably most mammals, and to birds and most reptiles. In birds, which are remarkable for acuteness of vision, the mechanism is somewhat peculiar. In them the fibres of the ciliary muscle have a strong attachment posteriorly, and when these contract they pull back the inner posterior layers of the cornea, and thus relax that part of the ciliary zone called the ligamentum pectinatum. In a state of rest this structure in the bird's eye is tense, but in accommodation it becomes relaxed. Thus by a somewhat different mechanism in the bird, accommodation consists in allowing the anterior surface of the lens to become more and more convex. In reptiles generally the mechanism resembles that of the bird; but it is said that in snakes and amphibia there is a movement forwards of the lens as a whole, so as to catch rays at a less divergent angle. When the eye is directed to a distant object, such as a star, the mechanism of accommodation is at rest in mammals, birds, reptiles and amphibia, but in fishes and cephalopods the eye at rest is normally adjusted for near vision. Consequently accommodation in the latter is brought about by a mechanism that carries the lens as a whole backwards. There is still some difficulty in explaining the action of the equatorial (circular) fibres. Some have found that the increased convexity of the anterior surface of the lens takes place only in the central portions of the lens, and that the circumferential part of the lens is actually flattened, presumably by the contraction of the equatorial fibres. Seeing, however, that the central part of the lens is the portion used in vision, as the pupil contracts during accommodation, a flattening of the margins of the lens can have no optical effect. Further, another explanation can be offered of the flattening. As just stated, during accommodation the pupil contracts, and the pupillary edge of the iris, thinned out, spreads over the anterior surface of the capsule of the lens, which it actually touches, and this part of the iris, along with the more convex central part of the lens, bulges into the anterior chamber, and must thus displace some of the aqueous humour. To make room for this, however, the circumferential part of the iris, related to the ligamentum pectinatum, moves backwards very slightly, while the flattening of the circumferential part of the lens facilitates this movement.

Helmholtz succeeded in measuring with accuracy the sizes of these reflected images by means of an instrument termed an ophthalmometer, the construction of which is based on the following optical principles: When a luminous ray traverses a plate of glass having parallel sides, if it fall perpendicular to the plane of the plate, it will pass through without deviation ; but if it fall obliquely on the plate (as shown in the left-hand diagram in fig. 14) it undergoes a lateral deviation, but in a direction parallel to that of the incident ray, so that to an eye placed behind the glass plate, at the lower A, the luminous point, upper A, would be in the direction of the prolonged emergent ray, and thus there would be an apparent lateral displacement of the point, the amount of which would increase with the obliquity of the incident ray. If, instead of one plate, we take two plates of equal thickness, one placed above the other, two images will be seen, and by turning the one plate with reference to the other, each image may be displaced a little to one side. The instrument consists of a small telescope (fig. 14) T, the axis of which coincides with the plane separating the two glass plates C C and B B. When we look at an object X Y, and turn the plates till we see two objects xy, xy touching each other, the size of the image X Y will be equal to the distance the one object is displaced to the one side and the other object to the other side. Having thus measured the size of the reflection, it is not difficult, if we know the size of the object reflecting the light and its distance from the eye, to calculate the radius of the curved surface (Appendix to M' Kendricks's Outlines of Physiology, 1878). The general result is that, in accommodation for near objects, the middle reflected image FIG. 14. Diagrammatic Viewofthe Ophthalmometer of Helmholtz.

becomes smaller, and the radius of curvature of the anterior surface of the lens becomes shorter.

5. Absorption and Reflection of Luminous Rays from the Eye. When light enters the eye, it is partly absorbed by the black pigment of the choroid and partly reflected. The reflected rays are returned through the pupil, not only following the same direction as the rays entering the eye, but uniting to form an image at the same point in space as the luminous object. The pupil of an eye appears black to an observer, because the eye of the obseiver does not receive any of those reflected rays. If, however, we strongly illuminate the retina, and hold a lens in front of the eye, so as to bring the reflected rays to a focus nearer the eye, then a virtual and erect, or a real and reversed, image of the retina will be seen. Such is the principle of the ophthalmoscope, invented by Helmholtz in 1851. Eyes deficient in pigment, as in albinos, appear luminous, reflecting light of a red or pink colour; but if we place in front of such an eye a card perforated by a round hole of the diameter of the pupil, the hole will appear quite dark, like the pupil of an ordinary eye. In many animals a portion of the fundus of the eyeball has no pigment, and presents' an iridescent appearance. This is called a tapetum. It probably renders the eye more sensitive to light of feeble intensity.

6. Functions of the Iris. The iris constitutes a diaphragm which regulates the amount of light entering the eyeball. The aperture in the centre, the pupil, may be dilated by contraction of a system of radiating fibres of involuntary muscle, or contracted by the action of another system of fibres, forming a sphincter, at the margin of the pupil. The radiating fibres are controlled by the sympathetic, while those of the circular set are excited by the third cranial nerve. The variations in diameter of the pupil are determined by the greater or less intensity of the light acting on the retina. A strong light causes contraction of the pupil; with light of less intensity, the pupil will dilate. In the human being, a strong light acting on one eye will often cause contraction of the pupil, not only in the eye affected, but in the other eye. These facts indicate that the phenomenon is of the nature of a reflex action, in which the fibres of the optic nerve act as sensory conductors to a centre in the encephalon, whence influences emanate which affect the pupil. It has been ascertained that if the fibres of the optic nerve be affected in any way, contraction of the pupil follows. The centre is in the anterior pair of the corpora quadrigemina, as destruction of these bodies causes immobility of the pupil. On the other hand, the dilating fibres are derived from the sympathetic; and it has been shown that they come from the lower part of the cervical, and upper part of the dorsal, region of the cord. But the iris seems to be directly susceptible to the action of light. Thus the pupil of the eye of a dead animal will contract if exposed to light for several hours, whereas, if the eye on the opposite side be covered, its pupil will remain widely dilated, as at the moment of death.

The pupil contracts under the influence (i) of an increased intensity of light; (2) of the effort of accommodation for near objects; (3) of a strong convergence cf the two eyes; and (4) of such active substances as nicotine, morphia and physostigmine; and it dilates under the influence (i) of a diminished intensity of light; (2) of vision of distant objects; (3) of a strong excitation of any sensory nerve; (4) of dyspnoea; and (5) of such substances as atropine and hyoscyamine. The chief function of the iris is to so moderate the amount of light entering the eye as to secure sharpness of definition of the retinal image. This it accomplishes by (i) diminishing the amount of light reflected from near objects, by cutting off the more divergent rays and admitting only those approaching a parallel direction, which, in a normal eye, are focused on the retina; and (2) preventing the error of spherical aberration by cutting off divergent rays which would otherwise impinge near the margins of the lens, and would thus be brought to a focus in front of the retina.

3. SPECIFIC INFLUENCE OF LIGHT ON THE RETINA The retina is the terminal organ of vision, and all the parts in front of it are optical arrangements for securing that an image will be accurately focused upon it. The natural stimulus of the retina is light. It is often said that it may be excited by mechanical and electrical stimuli; but such an observation really applies to the stimulation of the fibres of the optic nerve. It is well known that such stimuli applied to the optic nerve behind the eye produce always a luminous impression; but there is no proof that the retina, strictly speaking, is similarly affected. Pressure or electrical currents may act on the eyeball, but in doing so they not only affect the retina, consisting of its various layers and of Jacob's membrane, but also the fibres of the optic nerve. It is possible that the retina, by which is meant all the layers except those on its surface formed by the fibres of the optic nerve, is affected only by its specific kind of stimulus, light. This stimulus so affects the terminal apparatus as to set up actions which in turn stimulate the optic fibres. The next question naturally is What is the specific action of light on the retina? A. F. Holmgren, and also J. Dewar and J. G. M'Kendrick, have shown that when light falls on the retina it excites a variation of the electrical current obtained from the eye by placing it on the cushions of a sensitive galvanometer. One electrode touches the vertex of the cornea and the other the back of the eyeball. The corneal vertex is positive to the back of the eye, or to the transverse section of the optic nerve. Consequently a current passes through the galvanometer from the cornea to the back. Then the impact of light causes an increase in the natural electrical current during the continuance of light the current diminishes slowly and falls in amount even below what it was before the impact and the withdrawal of light is followed by a rebound, or second increase, after which the current falls in strength, as if the eye suffered from fatigue.

It was also observed in this research that the amount of electrical variation produced by light of various intensities corresponded pretty closely to the results expressed by G. T. Fechner's law, which regulates the relation between the stimulus and the sensational effect in sensory impressions. This law is, that the sensational effect does not increase proportionally to the stimulus, but as the logarithm of the stimulus. Thus, supposing the stimulus to be 10, 100 or 1000 times increased, the sensational effect will not be 10, 100 or 1000 times, but only i, 2 and 3 times greater.

Such electrical phenomena probably result either from thermal or chemical changes in the retina. Light produces chemical changes in the retina. If a frog be killed in the dark, and if its retina be exposed only to yellow rays, the retina has peculiar purple colour, which is at once destroyed by exposure to ordinary light. The purple matter apparently is decomposed by light. An image may actually be fixed on the retina by plunging the eye into a solution of alum immediately after death. Thus it would appear that light affects the purplematter of the retina, and the result of this chemical change is to stimulate the optic filaments; if the action be arrested, we may have a picture on the retina, but if it be not arrested, the picture is evanescent; the purple-matter is used up, and new matter of a similar kind is formed to take its place. The retina might, therefore, be compared to a sensitive photographic plate having the sensitive matter quickly removed and replaced; and it is probable that the electrical expression of the chemical changes is what has been above described.

(a) Phosgenes. Luminous impressions may also be produced by pressure on the eyeball. Such impressions, termed phosgenes, usually appear as a luminous centre surrounded by coloured or dark rings. Sometimes they seem to be small bright scintillations of various forms. Similar appearances may be observed at the moments of opening or of closing a strong electrical current transmitted through the eyeball.

(b) The Retina's Proper Light. The visual field, even when the eyelids are closed in a dark room, is not absolutely dark. There is a sensation of faint luminosity which may at one moment be brighter than at another. This is often termed the proper light of the retina, and it indicates a molecular change, even in darkness.

(c) The Excitability of the Retina. The retina is not equally excitable in all its parts. At the entrance of the optic nerve, as was shown by E. Mariotte in 1668, there is no sensibility to light. Hence, this part of the retina is called the blind spot. If we shut the left eye, fix the right eye on the cross seen in fig. 15, and move the book towards and away from the eye, a position will be found when the round spot disappears, that is CK ^ when its image falls on the en- trance of the optic nerve There FIG . , 5 ._ D ; agram for the is also complete insensibility to Study of the Blind Spot.

colours at that spot. The diameter of the optic papilla is about 1-8 mm., giving an angle of 6; this angle determines the apparent size of the blind spot in the visual field, and it is sufficiently large to cause a human figure to disappear at a distance of two metres.

The yellow spot in the centre of the retina is the most sensitive to light, and it is chiefly employed in direct vision. Thus, if we fix the eye on a word in the centre of this line, it is distinctly and sharply seen, but the words towards each end of the line are vague. If we wish to see each word distinctly, we " run the eye " along the line that is, we bring each successive word on the yellow spot. This spot has a horizontal diameter of 2 mm., and a vertical diameter of -8mm.; and it corresponds in the visual field to an angle of from 2 to 4. The fossa in the spot, where there are no retinal elements except Jacob's membrane, consisting here entirely of cones (2000 in number), is the area of most acute sensibility. This fossa has a diameter of only -2 mm., which makes the angle ten times smaller. Thus the field of distinct vision is extremely limited, and at the same moment we see only a very small portion of the visual field. Images of external objects are brought successively on this minute sensitive area, and the different sensations seem to be fused together, so that we are conscious of the object as a whole.

Towards the anterior margin of the retina sensitiveness to light becomes diminished; but the diminution is not uniform, and it varies in different persons.

(d) Duration and Persistence of Retinal Impressions. To excite the retina, a feeble stimulus must act for a certain time; when the retina is excited, the impression lasts after the cessation of the stimulus; but if the stimulus be strong, it may be of very short duration. Thus the duration of an electrical spark is extremely short, but the impression on the retina is so powerful, and remains so long, as to make the spark visible. If we rotate a disk having white and black sectors we see continuous dark bands. Even if we paint on the face of the disk a single large round red spot, and rotate rapidly, a continuous red band may be observed. Here the impressions of red on the same area of retina succeed each other so rapidly that before one disappears another is superadded, the result being a fusion of the successive impressions into one continuous sensation. This phenomenon is called the persistence of retinal impressions. An impression lasts on the retina from -5*5- to -fa of a second. The cinematograph owes its effects to persistence of retinal impressions.

(e) The Phenomena of Irradiation. If we look at fig. 16, the white square in the black field appears to be larger than the black square in the white field, although both are of precisely the same size. This is due to irradiation. The borders of dear surfaces advance in the visual field and encroach on obscure surfaces. Probably, even with the most exact accommodation, diffusion images form round the image of a white surface on a black ground, forming a kind of penumbra, thus causing it to appear larger than it really is. (/) Intensity of Light required to excite the Retina. Light FIG. 16. Illustrating the Effect of Irradiation.

must have a certain intensity to produce a luminous impression. It is impossible to fix the minimum intensity necessary, as the effect will depend, not only on the intensity of the stimulus, but on the degree of retinal excitability at the time. Thus, after the retina has been for some time in the dark, its excitability is increased; on the other hand, it is much diminished by fatigue. Aubert has stated that the minimum intensity is about 300 times less than that of the full Moon. The sensibility of the eye to light is measured by the photometer.

(g) Consecutive Retinal Images. Images which persist on the retina are either positive or negative. They are termed positive when the bright and obscure parts of the image are the same as the bright and obscure parts of the object; and negative when the bright parts of the object are dark in the image, and vice versa. Positive images are strong and sharply marked when an intense light has acted for not less than $ of a second. If the excitation be continued much longer, a negative and not a positive image will be seen. If, when the positive image is still visible, we look on a very brilliantly illuminated surface, a negative image appears. Negative images are seen with greatest intensity after a strong light has acted for a considerable time. These phenomena may be best studied when the retina 1 is very excitable, as in the morning after a sound sleep. On awakening, if we lox>k steadily for an instant at the window and then close the eyes, a positive image of the window will appear; if we then gaze fixedly at the window for one or two minutes, close the eyes two or three times, and then look at a dark part of the room, a negative image will be seen floating before us. The positive image Qranee is due to excitation of the retina, and the negative' to fatigue. If we fatigue a small Yellow area of the retina with white light, and then allow a less intense light to fall on it, the fatigued area responds feebly, and conse- Green quently the object, such as the window pane, appears to be dark. Greenish blue 4. SENSATIONS OF COLOUR Cyanic blue i. General Statement. Colour (q.v.) is a special sensation excited by the action on the retina of rays of light of a definite wave-length. On the most likely hypothesis as to the physical nature of light, colour depends on the rate of vibration of the luminiferous aether, and white light is a compound of all the colours in definite proportion. When a surface reflects solar light into the eye without affecting this proportion, it is white, but if it absorbs all the light so as to reflect nothing, it appears to be black. If a body held between the eye and the Sun transmits light unchanged, and is transparent, it is colourless, but if translucent it is white. If the medium transmits or reflects some rays and absorbs others, it is coloured. Thus, if a body absorbs all the rays of the spectrum but those which cause the sensation of green, we say the body is green in colour; but this green can only be perceived if the rays of light falling on the body contain rays having the special rate of vibration required for this special colour. For if the surface be illuminated by any other pure ray of the spectrum, say red, these red rays will be absorbed and the body will appear to be black. As a white surface reflects all the rays, in red light it will be seen to be red, and in a green light, green. Colour depends on the nature of the body and on the nature of the light falling on it, and a sensation of colour arises when the body reflects or transmits the special rays to the eye. If two rays of different rates of vibration, that is to say, of different colours, affect a surface of the retina at the same moment, the effects are fused together and we have the sensation of a third colour different from its cause. Thus, if red be removed from the solar spectrum, all the other colours combined cause a sensation of greenish yellow. Again red and violet give purple, and yellow and blue, white. Yellow and blue, however, only give white when pure spectral colours are mixed. It is well known that a mixture of yellow and blue pigments do not produce white, but green; but, as was explained by Helmholtz, this is because the blue pigment absorbs all the rays at the red end of the spectrum up to the green, while the yellow pigment absorbs all the rays at the violet end down to the green, and as the only rays reflected into the eye are the green rays, the substance appears green. Finally, if colours are painted on a disk in due proportions and in a proper order, the disk will, when quickly rotated, appear white, from the rapid fusion of colour effects.

When we examine a spectrum, we see a series of colours merging by insensible gradations the one into the other, thus: red, orange, yellow, green, blue and violet. These are termed simple colours. If two or more coloured rays of the spectrum act simultaneously on the same spot of the retina, they may give rise to sensations of mixed colours. These mixed colours are of two kinds: (i) those which do not correspond to any colour in the spectrum, such as purple and white, and (2) those which do exist in the spectrum. White may be produced by a mixture of two simple colours, which are then said to be complementary. Thus, red and greenish blue, orange and cyanic blue, yellow and indigo blue, and greenish yellow and violet all produce white. Purple is produced by a mixture of red and violet, or red and bluish violet. The following table by Helmholtz shows the compound colours produced by mixing other colours:

Violet Indigo blue Cyanic blue Greenish blue Green Yellowish green Yellow Purple Deep White White Whitish Golden Orange Deep rose White rose White Whitish yellow Yellow yellow Yellow rose rose yellow White White Whitish Whitish Yellowish rose White Green green Green green Green green Blue Water Greenish blue blue Water Water blue blue Indigo blue This table shows that if we mix two simple colours not so far separated in the spectrum as the complementary colours, the mixed colour contains more white as the interval between the colours employed is greater, and that if we mix two colours farther distant in the __ spectrum than the complementary colours, the mixture is whiter as the FIG. 17. Form of Double Slit for the Partial Superposition of Two Spectra.

interval is smaller. By mixing more than two simple colours, no new colours are produced, but only different shades of colour.

2. Modes of Mixing Colour Sensations. Various methods have been adopted for studying the effect of mixing colours.

(a) By Superposing Two Spectra. This may be done in a simple way by having a slit in the form of the letter V (see fig. 17), of which the two portions ab and be form a right angle; behind this slit is placed a vertical prism, and two spectra are obtained, as seen in fig. 18, in which bfea is the spectrum of the slit ab, and cefd that of the slit cd; the coloured spectra are containec in the triangle gef, and by arrangement, the effects of mixture of any two simple colours may be observed.

(b) By Method of Reflection. Place a red FIG. 18. Diagram of Double Spectrum wafer on j m fig. IO and partolly superposed. * so angle a small glass plate a as to transmit to the eye a reflection of the blue wafer on d in the same line as the rays transmitted from the red wafer on b. The sensation will be that of purple; and by using wafers of different colours, many experiments may thus be performed.

(c) By Rotating Disks which quickly superpose on the same Area of Retina the Impres, sions of Different Wave-lengths. Such disks may be constructed of cardboard, on which coloured sectors are painted, as shown in fig. 20, representing diagrammatically the arrangement of Sir Isaac Newton. The angles of the sectors were thus given by him:- Orange Yellow 60" 45-5' 34 10-5' 54 41 Violet Green Blue . Indigo . 6o c 45-5 60 45-5' 54 4i' 34 10-5' With sectors of such a size, white will be produced on rotating the disk rapidly. This method has been carried out with great efficiency by the colour-top of J. Clerk-Maxwell. It is a flat top, on the surface of which disks of various colours may be placed. Dancer has added to it a method by which, even while the top is rotating rapidly and the sensation of a mixed colour is strongly perceived, the eye may be able to see the simple colours of which it is composed. This is done by placing on the handle of the top, a short distance above the FIG. 20. Diagram of the Colour Disk of Sir Isaac Newton.

weighted a little on one side.

coloured surface, a thin black disk, perforated by holes of various size and pattern, and This disk vibrates to and fro rapidly, and breaks the continuity of the colour impression; and thus the constituent colours are readily seen.

3. The Geometric Representation of Colours. Colours may be arranged in a linear series, as in the solar spectrum. Each point of the line corresponds to a determinate impression of colour; the line is not a straight line, as regards luminous effect, but is better represented by a curve, passing from the red to the violet. This curve might be represented as a circle in the circumference of which the various colours might be placed, in which case the complementary colours would be at the extremities of the same diameter. Sir Isaac Newton arranged the colours in the form of a triangle, as shown in fig. 21. If we place three of the spectral colours at three angles, thus green, violet and red the sides of the triangle include the intermediate colours of the spectrum, except purple.

The point S corresponds to white, consequently, from the intersection of the lines which join the complementary colours, the straight lines from green to S, RS and VS represent the amount of green, red and violet necessary to form white; the same holds good for the complementary colours; for example, for blue and red, the line SB = the amount of blue, and the line SR = the amount of red required to form white. Again, any point, say M ; on the surface of the triangle, will represent a mixed colour, the composition of which may be obtained by mixing the three fundamental colours in the proportions represented by the length of the lines M to green, MV and MR. But tU the line VM passes on to && Purjdt Vu reolaceThJ red" ST.SS FIG ' ^--Geometrical Representation of the Relations of Colours as shown by " Indigo portion of the length of the line MY, and mix it with violet in the proportion of SV. The same colour would also be formed by mixing the amount MY of yellow with MS of white, or by the amount RM of red with the amount MD of greenish blue.

The following list shows characteristic complementary colours, with their wave-lengths (X) in millionths of a millimetre:

Red, X 656. Orange, X 608. Gold-yellow, X 574. Yellow, Xs67. Greenish yellow, X 564.

Blue-green, X 492.

Blue, X4QO.

Blue, X 482.

Indigo-blue, \464- Violet, X 433.

By combining colours at opposite ends of the spectrum, the effect of the intermediate colours may be produced; but the lowest and the highest, red and violet, cannot thus be formed. These are therefore fundamental or primary colours, colours that cannot be produced by the fusion of other colours. If now to red and violet we add green, which has a rate! of vibration about midway between red and violet, we obtain a sensation of white. Red, green and violet are therefore the three fundamental colours.

4. Physiological Characters of Colours. Colour physiologically is a sensation, and it therefore does not depend only on the physical stimulus of light, but also on the part of the retina affected. The power of distinguishing colours is greatest when they fall on, or immediately around, the yellow spot, where the number of cones is greatest. In these regions more than two hundred different tints of colour may be distinguished. Outside of this area lies a middle zone, where fewer tints are perceived, mostly confined to shades of yellow and blue. If intense coloured stimuli are employed, colours may be perceived even to the margin of the periphery of the retina, but with weak stimuli coloured objects may seem to be black, or dark like shadows. In passing a colour from the periphery to the centre of the yellow spot, remarkable changes in hue may be observed. Orange is first grey, then yellow, and it only appears as orange when it enters the zone sensitive to red. Purple and bluish green are blue at the periphery, and only show the true tint in the central region. Four tints have been found which do not thus change: a red obtained by adding to the red of the spectrum a little blue (a purple), a yellow of 574-5 X, a green of 495 X and a blue of 471 X.

The question now arises, How can we perceive differences in colour? We might suppose a molecular vibration to be set up in the nerve-endings synchronous with the undulations of the luminiferous aether, without any change in the chemical constitution of the sensory surface, and we might suppose that where various series of waves in the aether corresponding to different colours act together, these may be fused together, or to interfere so as to give rise to a vibration of modified form or rate that corresponded in some way to the sensation. Or, to adopt another line of thought, we might suppose that the effect of different rays (rays differing in frequency of vibration and in physiological effect) is to promote or retard chemical changes n the sensory surface, " which again so affect the sensory nerves as to give rise to differing states in the nerves and the nerve centres, with differing concomitant sensations." The former >f these thoughts is the foundation of the Young-Helmholtz heory, while the latter is applicable to the theory of E. Hering.

5. Theories of Colour-Perception. A theory widely accepte( by physicists was first proposed by Thomas Young anc H Y G B -y afterwards revived by Helmholtz. It is basec on the assumption that three kinds of nervous elements exist in the retina the excitation of which give respectively sensations of red, green and violet. These may be regarded as fundamental sensations. Homogeneous light excites all three, but with different intensities according to the length of the wave. Thus long waves will excite most strongly fibres sensitive to red, medium waves those sensitive to green, and short waves those sensitive to violet. Fig. 22 shows graphically the irritability of the three R O Y U B V FIG. 22. Diagram showing the Irritability of the Three Kinds of Retinal Elements.

I, red; 2, green; 3, violet. R, O, Y, G, B, V, initial letters of colours.

sets of fibies. Helmholtz thus applies the theory:

" i. Red excites strongly the fibres sensitive to red and feebly the other two sensation : Red.

2. Yellow excites moderately the fibres sensitive to red and green, feebly the violet sensation : Yellow.

3. Green excites strongly the green, feebly the other two sensation : Green.

4. Blue excites moderately the fibres sensitive to green and violet, and feebly the red sensation : Blue.

5. Violet excites strongly the fibres sensitive to violet, and feebly the other two sensation : Violet.

6. When the excitation is nearly equal for the three kinds of fibres, then the sensation is White."

The Young-Helmholtz theory explains the appearance of the consecutive coloured images. Suppose, for example, that we look at a red object for a considerable time ; the retinal elements sensitive to red become fatigued. Then (i) if the eye be kept in darkness, the fibres affected by red being fatigued do not act so as to give a sensation of red ; those of green and of violet have been less excited, and this excitation is sufficient to give the sensation of pale greenish blue ; (2) if the eye be fixed on a white surface, the red fibres, being fatigued, are not excited by the red rays contained in the white light ; on the contrary, the green and violet fibres are strongly excited, and the consequence is that we have an intense complementary image; (3) if we look at a bluish green surface, the complementary of red, the effect will be to excite still more strongly the green and violet fibres, and consequently to have a still more intense complementary image; (4) if we regard a red surface, the primitive colour, the red fibres are little affected in consequence of being fatigued, the green and violet fibres will be only feebly excited, and therefore only a very feeble complementary image will be seen; and (5) if we look at a surface of a different colour altogether, this colour may combine with that of the consecutive image, and produce a mixed colour; thus, on a yellow surface, we will see an image of an orange colour.

Every colour has three qualities: (i) hue, or tint, such as red, green, violet; (2) degree of saturation, or purity, according to the amount of white mixed with the tint, as when we recognize a red or green as pale or deep; and (3) intensity, or luminosity, or brightness as when we designate the tint of a red rose as dark or bright. Two colours are identical when they agree as to these three qualities. Observation shows, however, that out of one hundred men ninety-six agree in identifying or in discriminating colours, while the remaining four show defective appreciation. These latter are called colour-blind. This defect is about ten times less frequent in women. Colour-blindness is congenital and incurable, and it is due to an unknown condition of the retina or nerve centres, or both, and must be distinguished from transient colour-blindness, sometimes caused by the excessive use of tobacco and by disease. When caused by tobacco, the sensation of blue is the last to disappear. Absolute inability to distinguish colour is rare, if it really exists; in some rare cases there is only one colour sensation; and in a few cases the colour-blind fails to distinguish blue from green, or there is insensibility to violet. Daltonism, or red-green blindness, of which there are two varieties, the red-blind and the green-blind, is the more common defect. Red appears to a redblind person as a dark green or greenish yellow, yellow and orange as dirty green, and green is green and brighter than the green of the yellow and orange. To a green-blind person red appears as dark yellow, yellow is yellow, except a little lighter in shade than the red he calls dark yellow, and green is pale yellow.

According to the Young-Helmholtz theory, there are three fundamental colour sensations, red, green and violet, by the combination of which all other colours may be formed, and it is assumed that there exist in the retina three kinds of nerve elements, each of which is specially responsive to the stimulus of waves of a certain frequency corresponding to one colour, and much less so to waves of other frequencies and other colours. If waves corresponding to pure red alone act on the retina, only the corresponding nerve element for red would be excited, and so with green and violet. But if waves of different frequencies are mixed (corresponding to a mixture of colours), then the nerve elements will be set in action in proportion to the amount and intensity of the constituent excitant rays in the colour. Thus if all the nerve elements were simultaneously set in action, the sensation is that of white light ; if that corresponding to red and green, the resultant sensation will be orange or yellow; if mainly the green and violet, the sensation will be blue and indigo. Then red-blindness may be explained by supposing that the elements corresponding to the sensation of red are absent; and green-blindness, to the absence of the elements sensitive to green. If to a red-blind person the green and violet are equal, and when to a green-blind person the red and violet are equal, they may have sensations which to them constitute white, while to the normal eye the sensation is not white, but bluish green in the one case and green in the other. In each case, to the normal eye, the sensation of green has been added to the sensations of red and blue. It will be evident, also, that whiteness to the colour-blind eye cannot be the same as whiteness to the normal eye. No doubt this theory explains certain phenomena of colour-blindness, of after-coloured images, and of contrast of colour, but it is open to various objections. It has no anatomical basis, as it has been found to be impossible to demonstrate the existence of three kinds of nerve elements, or retinal elements, corresponding to the three fundamental colour sensations. Why should red to a colour-blind person give rise to a sensation of something like green, or why should it give rise to a sensation at all ? Again, and as already stated, in cases of colourblindness due to tobacco or to disease, only blue may be seen, while it is said that the rest of the spectrum seems to be white. It is difficult to understand how white can be the sensation if the sensations of red and green are lost. On the other hand, it may be argued that such colour-blind eyes do not really see white as seen by a normal person, and that they only have a sensation which they have been accustomed to call white. According to this theory, we never actually experience the primary sensations. Thus we never see primary red, as the sensation is more or less mixed with primary green, and even with primary blue (violet). So with regard to primary green and primary violet. Helmholtz, in his last work on the subject, adopted as the three primary colours a red bluer than spectral red, (a) a green lying between 540 X and 560 X (6, like the green of vegetation), and a blue at about 470 X (c, like ultramarine), all, however, much more highly saturated than any colours existing in the spectrum.

In Handbuch der J> hysiologischen Optik (Hamburg and Leipzig, 1896) Helmholtz pointed out that luminosity or brightness plays a more important part in colour perception than has been supposed. Each spectral colour is composed of certain proportions of these fundamental colours, or, to put it in another way, a combination of two of them added to a certain amount of white.

Hering's theory proceeds on the assumption of chemical changes in the retina under the influence of light. It also assumes that certain fundamental sensations are excited by light or occur during the absence of light. These fundamental sensations are white, black, red, yellow, green and blue. They are arranged in pairs, the one colour in each pair being, in a sense, complementary to the other, as white to black, red to green, and yellow to blue. Hering also supposes that when rays of a certain wave-length fall on visual substances assumed to exist in the retina, destructive or, as it is rermed, katabolic changes occur, while rays having other waveengths cause constructive or anabolic changes. Suppose that in a red-green substance katabolic and anabolic changes occur in equal amount, there may be no sensation, but when waves of a certain wave-length or frequency cause katabolic changes in excess, there will be a sensation of red, while shorter waves and of greater frequency, by exciting anabolic changes, will cause a sensation of [reen. In like manner, katabolism of a yellow-blue visual subtance gives rise to a sensation we call yellow, while anabolism, >y shorter waves acting on the same substance, causes the sensation of blue. Again, katabolism of a white-black visual substance gives white, while anabolism, in the dark, gives rise to the sensation of blackness. Thus blackness is a sensation as well as whiteness, and the members of each pair are antagonistic as well as complementary. In the red end of the spectrum the rays cause katabolism of the red-green substance, while they have no effect on the yellowblue substance. Here the sensation is red. The shorter waves of the spectral yellow cause katabolism of the yellow-blue material, while katabolism and anabolism of the red-green substance are here equal. Here the sensation is yellow. Still shorter waves, corresponding to green, now cause anabolism of the red-green substance, while their influence on the yellow-blue substance, being equal in amount as regards katabolism and anabolism, is neutral. Here the sensation is green. Short waves of the blue of the spectrum cause anabolism of the yellow-blue material, and as their action on the red-green matter is neutral, the sensation is blue. The very short waves at the blue end of the spectrum excite katabolism of the red-green substance, and thus give violet by adding red to blue. The sensation orange is experienced when there is excess of katabolism, and greenish blue when there is excess of anabolism in both substances. Again, when all the rays of the spectrum fall on the retina, katabolism and anabolism in the red-green and yellow-blue matters are equal and neutralize each other, but katabolism is great in the white-black substance, and we call the sensation white. Lastly, when no light falls on the retina, anabolic changes are going on and there is the sensation of black.

Hering's theory accounts satisfactorily for the formation of coloured after-images. Thus, if we suppose the retina to be stimulated by red light, katabolism takes place, and if the effect continues after withdrawal of the red stimulus, we have a positive after-image. Then anabolic changes occur under the influence of nutrition, and the effect is assisted by the anabolic effect of shorter wave-lengths, with the result that the negative after-image, green, is perceived. Perhaps the distinctive feature of Hering's theory is that white is an independent sensation, and not the secondary result of a mixture of primary sensations, as held by the Young-Helmholtz view. The greatest difficulty in the way of the acceptance of Hering's theory is with reference to the sensation of black. Black is held to be due to anabolic changes occurring in the white-black substance. Suppose that anabolism and katabolism of the white-black substance are in equilibrium, unaccompanied by stimulation of either the red-green or the yellow-blue substances, we find that we have a sensation of darkness, but not one of intense blackness. This " darkness " has still a certain amount of luminosity, and it has been termed the " intrinsic light " of the retina. Sensations of black differing from this darkness may be readily experienced, as when we expose the retina to bright sunshine for a few moments and then close the eye. We then have a sensation of intense blackness, which soon, however, is succeeded by the darkness of the " intrinsic light." The various degrees of blackness, if it is truly a sensation, are small compared with the degrees in the intensity of whiteness. In the consideration of both theories changes in the cerebral centres have not been taken into account, and of these we know next to nothing.

6. The Contrast of Colours. If we look at a small white, grey or black object on a coloured ground, the object appears to have the colour complementary to the ground. Thus a circle of grey paper on a red ground appears to be of a greenish-blue colour, whilst on a blue ground it will appear pink. This effect is heightened if we place over the paper a thin sheet of tissue paper; but it disappears at once if we place a black ring or border round the grey paper. Again, if we place two complementary colours side by side, both appear to be increased in intensity. Various theories have been advanced to explain these facts. Helmholtz was of opinion that the phenomena consist rather in modifications of judgment than in different sensory impressions; J. A. F. Plateau, on the other hand, attempted to explain them by the theory of consecutive images.

5. THE MOVEMENTS OF THE EYE i. General Statement. The globe of the eye has a centre of rotation, which is not exactly in the centre of the optic axis, but a little behind it. On this centre it may move round axes of rotation, of which there are three an antero-posterior, a vertical and a transverse. In normal vision, the two eyes are always placed in such a manner as to be fixed on one point, called the fixed point or the point of regard. A line passing from the centre of rotation to the point of regard is called the line of regard. The two lines of regard form an angle at the point of regard, and the base is formed by a line passing from the one centre of rotation to, the other. A plane passing through both lines of regard is called the plane of regard. With these in*.

definitions, we can now describe the movements of the eyeball, which are of three kinds: (i) First position. The head is erect, and the line of regard is directed towards the distant horizon. (2) Second position. This indicates all the movements round the transverse and horizontal axes. When the eye rotates round the first, the line of regard is displaced above or below, and makes with a line indicating its former position an angle termed by Helmholtz the angle of vertical displacement, or the ascensional angle; and when it rotates round the vertical axis, the line of regard is displaced from side to side, forming with the median plane of the eye an angle called the angle of lateral displacement. (3) Third order of positions. This includes all those which the globe may assume in performing a rotatory movement along with lateral or vertical displacements. This movement of rotation is measured by the angle which the plane of regard makes with the transverse plane, an angle termed the angle of rotation or of torsion.

The two eyes move together as a system, so that we direct the two lines of regard to the same point in space.

The eyeball is moved by six muscles, which are described in the article EYE (Anatomy). The relative attachments and the axes of rotation are shown in fig. 23.

The term visual field is given to the area intercepted by the extreme visual lines which FIG. 23. Diagram of the Attachments tv, tt a thrninrh tk*. ront of the Musclesof the Eye and of their pass through the centre of Rotationi th ^ latter bei of the pupil, the amount shown by dotted lines. (Pick.) of dilatation of which The axis of rotation of the rectus determines its size. It internus and rectus externus being follows thp rrmwmpntc vertical, that is, perpendicular to the movements , ane of the canno( . be shown of the eye, and is displaced with it. Each point in the visual field has a corresponding point on the retina, but the portion, as already explained, which secures our attention is that falling on the yellow spot.

2. Simple Vision with Two Eyes. When we look at an object with both eyes, having the optic axes parallel, its image falls upon the two yellow spots, and it is seen as one object. If, however, we displace one eyeball by pressing it with the finger, then the image in the displaced eye does not fall on the yellow spot, and we see two objects, one of them being less distinct than the other. It is not necessary, however, in order to see a single object FIG. 24. Diagram with two eyes that the two images fall to illustrate the on the two yellow spots; an object is Physiological Realways single if its image fall on corre- R e I t n| e ft spending points in the two eyes.

The eye may rotate round three possible axes, a vertical, horizontal and antero-posterior. These movements are effected by four straight muscles and two oblique. The four straight muscles arise from the back of the orbit, and pass forward to be inserted into the front part of the eyeball, or its equator, if we regard the anterior and posterior ends of the globe as the poles. The two obliques (one originating at the back of the orbit) come, as it were, from the nasal side the one goes above the eyeball, the other below, while both are inserted into the eyeball on the temporal side, the superior oblique above and the inferior oblique below. The six muscles work in pairs. The internal and external recti turn the eye round the vertical axis, so that the line of vision is directed to the right or left. The superior and inferior recti rotate the eye round the horizontal axis, and thus the line of vision is raised or lowered. The oblique muscles turn the eye round an axis passing through the centre of the eye to the back of the head, so that the superior oblique muscle lowers, while the inferior oblique raises, the visual line. It was also shown by Helmholtz that the oblique muscles sometimes cause a slight rotation of the eyeball round the visual axis itself. These movements are under the control of the will up to a certain point, but there are slighter movements that are altogether involuntary. Helmholtz studied these slighter movements by a method first suggested by F. C. Bonders. By this method the apparent position of afterimages produced by exhausting the retina, say with a red or green object, was compared with that of a line or fixed point gazed at with a new position of the eyeball. The ocular spectra soon vanish, but a quick observer can determine the coincidence of lines with the spectra. After producing an after-image with the head in the erect position, the head may be placed into any inclined position, and if the attention is then fixed on a diagram having vertical lines ruled upon it, it can easily be seen whether the after-image coincides with these lines. As the after-image must remain in the same position on the retina, it will be evident that if it coincides with the vertical lines there must have been a slight rotation of the eyeball. Such a coincidence always takes place, and thus it is proved that there is an involuntary rotation. This minute rotation enables us to judge more accurately of the position of external objects.

3. The horopter is the locus of those points of space which are projected on retinal points. While geometrically it may be conceived as simple, as a matter of fact it is generally a line of double curvature produced by the intersection of two hyperboloids, or, in other words, it is a twisted cubic curve formed by the intersection of two hyperboloids which have a common generator. The curves pass through the nodal point of both eyes. An infinite number of lines may be drawn from any point of the horopter, so that the point may be seen as a single point, and these lines lie on a cone of the second order, whose vertex is the point. When we gaze at the horizon, the horopter is really a horizontal plane passing through our feet. The horopter in this instance is the ground on which we stand. Experiments show " that the forms and the distances of these objects which are situated in, or very nearly in, the horopter, are perceived with a greater degree of accuracy than the same forms and distances would be when not situated in the horopter " (M'Kendrick, Life of Helmholtz, 1899, p. ijietseq.).

An object which is not found in the horopter, or, in other words, does not form an image on corresponding points of the retinae, is seen double. When the eyeballs are so acted upon by their muscles as to secure images on non-corresponding points, and consequently double vision, the condition is termed strabismus, or squinting, of which there are several varieties treated of in works on ophthalmic surgery. It is important to observe that in the fusion of double, images we must assume, not only the correctness of the theory of corresponding points of the retina, but also that there are corresponding points in the brain, at the central ends of the optic fibres. Such fusion of images may occur without consciousness at all events, it is possible to imagine that the cerebral effect (except as regards consciousness) would be the same when a single object was placed before the two eyes, in the proper position, whether the individual were conscious or not. On the other hand, as we are habitually conscious of a single image, there is a psychical tendency to fuse double images when they are not too dissimilar.

4. Binocular Perception of Colour. This may be studied as follows, Take two No. 3 eye-pieces of a Hartnack's microscope, or two eye-pieces of the same optical value from any microscope, place one in front of each eye, direct them to a clear window in daylight, keep them parallel, and two luminous fields will be seen, one corresponding to each eye. Then converge the two eye-pieces, until the two luminous circles cross, and the central part, like a bi-convex lens, will appear clear and bright, while the outer segments will be much less intense, and may appear even of a dim grey colour. Here, evidently, the sensation is due to a fusion of impressions in the brain. With a similar arrangement, blue light may be admitted by the one eye-piece and red by the other; and on the convergence of the two, a resultant colour, purple, will be observed. This may be termed the binocular vision of colours. It is remarkable that by a mental effort this sensation of a compound colour may be decomposed into its constituents, so that one eye will again see blue and the other red.

6. THE PSYCHICAL RELATIONS OP VISUAL PERCEPTIONS i. General Characters of Visual Perceptions. All visual perceptions, if they last for a sufficient length of time, appear to be external to ourselves, erect, localized in a position in space and more or less continuous.

(a) Visual Sensations are referred to the Exterior. This appears to-be due, to a large extent, to habit. Those who have been born blind, on obtaining eyesight by an operation, have imagined objects to be in close proximity to the eye, and have not had the distinct sense of exteriority which most individuals possess. Slowly, and by a process of education, in which the sense of touch played an important part, they gained the knowledge of the external relations of objects. Again, phosgenes, when first produced, appear to be in the eye, but when conscious of them, by an effort of imagination, we may transport them into space, although they never appear very far off.

(b) Visual Sensations are referred to Erect Objects. Although the images of objects are inverted on the retina we see them erect. The explanation of the effect is that we are conscious not of the image on the retina, but of the luminous object from which the rays proceed, and we refer the sensation in the direction of these rays. Again, in running the eye over the object, say a tall pole, from base to apex, we are not conscious of the different images on the retina, but of the muscular movements necessary to bring the parts successively on the yellow spot.

(c) Visual Sensations are referred to a Position in Space. The localization of a luminous point in space can only be 'determined by observing its relations to other luminous points with a given position of the head and of the eye. For example, in a perfectly dark room, if we look at a single luminous point, we cannot fix its exact position in space, but we may get some information of a vague character by moving the head or the eye. If, however, a second luminous point appears in the darkness, we can tell whether it is nearer or farther distant, above or below the first. So with regard to other luminous points we observe their reciprocal relations, and thus we localize a number of visual impressions. There are three principal directions in space: the transverse (breadth), the vertical (height) and the sagittal (depth). Luminous points may be localized either in the transverse or vertical directions. Here we have to do simply with localization on a surface. A number of points may be observed simultaneously (as when the eye is fixed) or successively (as when the eye moves). If the movement of the eye be made rapidly, the series of impressions from different points may be fused to- g gether, and we are conscious of f a line, the direction of which is e indicated chiefly by the muscular d sensations felt in following it. A .<j...J The case is different as regards points in the sagittal direction. 4 .... .^ We see only a single point of. ^ this line at a time; it may be 9.....

a transverse series of retinal 4 elements, A B, and each of these ^ formed by a number of smaller FIG. 25. Diagram illustrating elements, i, 2, 3, 4, situated in e jl^jjf tion f Visual the axis of each principal element ; it may be, on the other hand, the transverse line a b situated in space and formed by a series of points in juxtaposition.

Each of these points will impress a retinal element, and the result will be the perception of a transverse line; but this will not be the same for the points c, d, e, f, g, situated in space in a linear series, in the sagittal direction; only one of those points, c, will impress the corresponding retinal element, and we can see only one point at a time in the line eg. By accommodating successively, however, for the various points at different and considerable distances along the line eg, we may excite retinal elements in rapid succession. Thus, partly by the fusion of the successive impressions on the retina, and partly from the muscular sensations caused by repeated accommodations and possibly of ocular movements, we obtain a notion of depth in space, even with the use of only one eye. It is, however, one of the chief effects of binocular vision to give precision to the notion of space in the sagittal direction.

(d) Visual Sensations are Continuous. Suppose the image of a luminous line falls on the retina, it will appear as a line although it is placed on perhaps 200 cones or rods, each of which may be separately excited, so as to cause a distinct sensation. Again, on the same principle, the impression of a superficial surface may be regarded as a kind of mosaic, made up of individual portions corresponding to the rods or cones on which the image of the surface falls. But in both cases the sensation is continuous, so that we see a line or a surface. The individual images are fused together.

2. Notions derived from Visual Perceptions. When we look at any object, we judge of its size, the direction of its surfaces (unless it be a point), its distance from the eye, its apparent movement or fixedness and its appearance of solidity.

(a) Apparent Size. This, so far as regards a comparatively small object, depends on the size of the retinal image, as determined by the visual angle. With a very large object, ** there is an appreciation of ? 6 - Diagram to illustrate s ; ze f ro m the muscular Illusions of Size and Distance.

sensations derived from the S45*9'*i' Si *ilH* liili 1 1 ill* FIG. 27. Zoellner's Figure showing an Illusion of Direction.

movements of the eyeball as we " range " the eye over it. It is difficult to appreciate the distance separating two points between which there are other points, as contrasted with an apparently similar distance without intermediate points. For example, the distance A to B appears to be greater than from B to C, in fig. 26.

(b) Direction. As the retina is a curved surface, a long straight line, especially when seen from a distance, appears curved. In fig. 27 a curious illusion of direction, first shown by J. K. F. Zoellner, is depicted. If these lines be looked at somewhat obliquely, say from one corner, they will appear to converge or diverge, and the oblique lines, on each side of the vertical lines, will appear not to be exactly opposite each other. But the verA -A kB^ tical lines are parallel, 5 4 ^ h anc ^ ^ ne bliQ ue ' mes are continuous across them. The effect is evidently due to an error of judgment, as it may be controlled by an intense effort, when the lines will be seen as they really are.

(c) Apparent Distance. We judge of distance, as regards large objects at a great distance from the eye (i) from their apparent size, which depends on the dimensions of the visual angle, and (2) from the interposition of other objects between the eye and the distant object. Thus, at sea, we cannot form, without great experience, an accurate estimate of how many miles we are off the coast, and all know how difficult it is to estimate accurately the width of a river. But if objects be interposed between the eye and the distant object, say a few vessels at different distances at sea, or a boat in the river, then we have certain materials on which to form a judgment, the accuracy of which, however, even with these aids, will depend on experience. When we look at a near object, we judge of its distance chiefly by the sense of effort put forth in bringing the two lines of regard to converge upon it.

(d) The Movement of a Body. It the eye be fixed, we judge of movement by successive portions of the retina being affected, and possibly also by a feeling of an absence of muscular contractions necessary to move the eyeballs. When the eye moves, so as to "follow" the object, there is a sense of muscular effort, which is FIG. 28. Illustrating Stereoscopic Vision.

increased when, in addition, we require to move the head.

(e) The Apparent Solidity of an Object. If we look at an object, say a cube, first with the right eye and then with the left, it will be found that the two images of the object are somewhat different, as in fig. 28. If, then, by means of a stereoscope, or by holding a card between the two eyes, and causing a slight convergence of the eyes, the two images are brought upon corresponding points of the two retinae, the image will at once be seen in relief.

See also article " Vision " by W. H. R. Rivers in Schafer's TextBook of Physiology, vol. ii. p. 1026. (J. G. M.)

7. ERRORS OF REFRACTION AND ACCOMMODATION AND THEIR CURATIVE TREATMENT The following is a classification of the diseases of vision, from a medical point of view (see also EYE : diseases) :

a. Errors of refraction: hyperopia, myopia, astigmatism, aniso- metropia, aphakia.

b. Errors of accommodation:

(1) Loss of accommodation (a) From advancing years (presby- opia), or from debility. (b) From paralysis (cycloplegia) due to 1. Drugs such as atropine.

2. Systemic poisons: diph- theria.influenza.syphilis, etc.

3. Diseases of the nervous system, concussion of the brain.

(2) Spasm of accommodation.

(3) Meridional asymmetrical accommodation by means of which low errors of astigmatism are corrected, producing eye-strain.

Hyperopia or Hypermetropia (H.) (Far-sight; German = Uebersicht). This is a condition of the refraction of the eye in which, with the eye at rest, parallel rays of light focus beyond the retina, which means that the image of a distant object is not in focus when it meets the retina, because the eye is too short antero-posteriorly. Most eyes at birth are hyperopic, but as the child grows the eye also grows; when, however, this does not take place, or does not take place sufficiently, normal development is thus arrested. There are other conditions that cause hyperopia. but this shortening of the anteroposterior axis is by far the commonest.

Hyperopia is corrected by convex glasses (fig. 29), and the measurement of the hyperopia is that convex glass which enables the hyperopic eye, at rest, to see distinctly objects at a distance. When the hyperopia is not too high it can also be corrected by the eye itself by means of the ciliary muscle (muscle of accommodation) which causes the crystalline lens to become more convex, and thus brings about the same result as placing a convex glass before the eye.

In young people when the error is not too high this work is done unconsciously, vision appears to be perfect, and it is only by placing the eye under the influence of atropine that the defect is revealed. In the normal eye distant objects are focused on the retina without the use of the ciliary muscle, FIG. 29. Showing Parallel Rays focused on the Retina of a Hyperopic Eye by means of a Convex Lens.

which is only employed when looking at near objects; but the hyperope has to use this muscle all his waking hours for both near and distant vision, so that his eyes are never at rest. Fortunately he has some compensation for this extra work, for in most hyperopes the ciliary muscle becomes more or less hypertrophied; but even so, if near work is at all excessive, or if the defect is associated with astigmatism or anisometropia, symptoms of eye-strain will sooner or later show themselves (see Eye-strain, below).

In older people a very common symptom is blurring of the type while reading; the book has to be put down and the eyes rested for some minutes before reading can be resumed. This is due to the fatigued ciliary muscle giving way and becoming unable to focus.

As we advance in years we lose accommodation power (see Presbyopia, below), so that the time comes to every hyperope, if he live long enough, when he not only has to use glasses for reading (at an earlier period than the normal person), but he also finds that he is gradually losing his distant vision. This is very alarming to many, until it is explained that all that has happened is the loss of power to correct the defect, which defect, of course, has always existed, and which in future will have to be corrected by suitable glasses. The higher the hyperopia the sooner will these symptoms manifest themselves.

In quite young children, sometimes the earliest sign of the presence of hyperopia is a convergent strabismus (internal squint). As a rule, this squint is nothing more than an overconvergence brought about by over-accommodation in those who cannot dissociate their convergence and accommodation; if we remove the necessity for over-accommodation by correcting the defect with suitable glasses, the over-convergence disappears and the squint is cured.

The total hyperopia of the eye is divided into manifest hyperopia and latent hyperopia. Manifest hyperopia is expressed in amount by the strongest convex glass that allows clear distant vision when the eye is not under atropine. Latent hyperopia is the additional hyperopia which is revealed under atropine. With advancing years the latent hyperopia becomes more and more manifest, and between the ages of 45 and 50 the total hyperopia is entirely manifest.

In addition to the symptoms already described, a very common one among young hyperopes is spasm of the ciliary muscle. This cramp of the muscle causes distant objects to be very indistinct, improvement only taking place with a concave glass, and near work has to be approached very close to the eyes, thus giving a wrong idea that the child is suffering from myopia; by paralysing the ciliary muscle with atropine the spasm disappears and the true nature of the defect is revealed.

The treatment essentially consists in ascertaining the total hyperopia of the eye, and this can only be done satisfactorily, when latent hyperopia is present, by paralysing the accommodation, using atropine for those under 25, and homatropine for those between the ages of 25 and 35 or 40. Over 40 (and when the hyperopia is high, even at an earlier age) no cyclo- plegic is necessary in fact it is in many cases dangerous, as an attack of glaucoma may be induced. (See EYE: diseases.)

Having found the total hyperopia, we learn the amount of the latent hyperopia, and, roughly speaking, the convex glass required is equal to the whole of the manifest hyperopia added to, from one-third to a half, of the latent; but the treatment varies with the age of the individual and the amount of the hyperopia, and is too complicated to be detailed here.

Myopia (M.) (Short-sight). Typical myopia is due to an elongation of the antero-posterior diameter of the eye, so that the retina is situated behind the principal focus, and only diver- FIG. 30.

gent rays of light from a near point (fig. 30), or parallel rays made divergent by a concave glass (fig. 31), can come to a FIG. 31.

focus on the retina. In other words, the far point of a myope is at a short distance in front of the eye, the distance being the measure of the myopia.

A myope ca'n see distinctly at a distance when the eye is at rest (i.e. when accommodation is not being used), with that concave glass whose focal length is equal to the distance of the far point from the eye, and the converse is true; the measurement of myopia is that concave glass with which the myopic eye sees distinctly objects at a distance, and its focal length is equal to the distance of the myope's far point from the eye.

The Causes of Myopia. Although myopia is hereditary, it is, with few exceptions, not congenital. We have seen that almost all eyes are hyperopic at birth. The savage is rarely myopic: it is civilization that is responsible for it; the necessity for constantly adapting the eye for near objects means undue convergence. We find that myopia generally first shows itself at the age of 8 to 10, when school work begins in earnest that is, when convergence is first used in excess and there is no doubt that it is excessive convergence that is mostly responsible for the development of myopia. The over-used internal recti constantly pulling at the sclerotic tend to lengthen the antero-posterior diameter of the eye, and as this lengthening of the anteroposterior axis necessitates greater convergence still, a vicious circle is produced, and the myopia gradually increases. The hereditary character of myopia is explained by the existence in such eyes of an " anatomical predisposition " to myopia. The sclera is unusually thin, and consequently less able to resist the pull of the internal recti, and the relative position of the recti and the position of the optic nerve, both of which may be hereditary, may be factors in the production of this defect. Anything which causes young subjects to approach their work too near the eyes may be the starting-point. Bad illumination, or the light coming from the wrong direction (for instance, in front), or defective vision produced by corneal nebulae, or lamellar cataract, etc., all necessitate over-convergence in order to obtain clearer images, and myopia may be produced.

It is interesting to note that when the work is approached very near the eye, but convergence is not used, as in the case of watchmakers, who habitually use a strong convex glass in one eye, there is no special tendency to myopia.

Some of the more common symptoms of myopia are: (i) Distant objects are seen indistinctly. (2) Near objects are seen distinctly, and the near point is much nearer than in the normal eye. (3) Acuteness of vision is often lowered, and especially is this the case in high myopia. (4) Eye-strain is often present, due to overuse of the muscles of convergence, and this may lead to (5) an external or divergent squint.

(6) Floating black specks are often complained of, these are generally muscae volitantes, but often, especially in high myopia, may be actual opacities floating in the vitreous.

(7) Myopes often stoop and become " round shouldered " from their habit of poring over their work.

A small amount of myopia, if it is stationary, is in no sense a serious defect of the eye, the possessors of it are often quite unconscious of any deficiency in vision, and in fact brag that they have better vision than their fellows. The reason of this is that they learn in early life to recognize indistinct distant objects by the aid of other senses in a way that the ordinary individual can hardly understand, and in later life they can postpone the wearing of glasses for near work for many years, and sometimes until extreme old age. Unfortunately myopia is, as a rule, not stationary; it almost always tends to increase, and if this increase leads to very high myopia such serious changes may occur in the eyes as to lower the visual acuity enormously and sometimes lead to total loss of vision.

The treatment of myopia is general and local.

General Treatment. The most important part of this is the preventive treatment (prophylaxis), especially in its application to children. AH children who have one or both parents myopic are specially " marked down " for this defect, for they have probably inherited an anatomical predisposition. Bearing in mind that excessive convergence is the most potent cause of myopia, the most rigid attention should be paid to the ophthalmic hygiene of the schoolroom. This room should be large, lofty and well ventilated, and have good-sized high windows on one wall, preferably on the north side. Each scholar should have an adjustable seat and desk so arranged that his head is upright and the work not too near his eyes. These desks should be arranged in rows so placed that the pupils sit with the light on their left. Schoolbooks must be clearly printed and the type should not be too small. The school work that needs close application of the eyes should be continued only for a short period at a time, the period alternating with other work which does not require the use of the eyes, such as mental arithmetic, black-board demonstrations, recitation, or play. Schoolmasters should teach more that is, they should explain and impart knowledge by demonstrations and simple lectures, and reduce as much as possible the time spent in " home preparations, " which is usually work done by bad light and when the student is physically and mentaHy tired. Even in the nursery the greatest care should be taken. The little ones should be supplied with large toys, a large box of plain wooden bricks being the best form; picture books should be discouraged, and close work that entails undue convergence, such as sewing, threading beads, etc., ought to be forbidden. The nursery governess can teach the alphabet, small words and even simple arithmetic with the bricks. No child with a tendency to myopia, or with a myopic family history should be allowed to learn to write or draw until at least seven years old. The child's bed should not be allowed to face the window, preferably it should be back to the light. Students, or those engaged in literary or other work which entails close application for many hours a day, should be advised to regulate their work, if they are free to do so, by working for shorter periods and taking longer intervals of rest, they should be specially careful not to approach their work too near to the eyes and they should always work in a good light.

Local Treatment. This consists in correcting the error with a concave 'glass. The testing must be done when the eye is under atropinc in all those under 25, and under homatropine between the ages of 25 and 35 or 40. Over 40 no cycloplegic is required. Except when playing rough games the glasses must be worn always. The wearing; of glasses for near work produces at first considerable rebellion in children, because they can see near work so much better without a glass. The object of enforcing this treatment is to make the muscle of accommodation do its proper work, and not only do we do this, but we also do away with the excess of convergence over accommodation, and lastly, make excessive convergence impossible, because, with the glasses on, the near work has to be held at some considerable distance from the eyes. In other words, we have practically made the eyes normal, and it is only by doing this that we can prevent the increase of myopia. Adults who have never worn their correction (especially if the myopia is high) must have a weaker glass for near work. Each case must be treated on its own merits. So-called malignant myopia, which is high myopia with serious changes in the eye, must be treated in_a special manner and with the greatest care.

Astigmatism. 1 The principal seat of astigmatism is the cornea, the curvature of one meridian being greater than that of the other. In regular astigmatism, which is the only form that can as a rule be treated by glasses, the meridians of greatest and least curvature are at right angles to each other, and the intermediate meridians pass by regular gradations from one to the other. Rays of light passing through such an astigmatic surface do not focus at one point, but form many points, with the result that the image is more or less indistinct according to the amount of the error. In uncorrected astigmatism a clock-face viewed at a distance of 4 or 5 yds. will appear to have certain figures distinct, and others (at right angles) indistinct; for instance, figures XI and V may appear quite black, while figures II and VIII are grey and indistinct. If one of the principal meridians be emmetropic the astigmatism is simple; if both be hyperopic, or if both be myopic, it is compound; and if one meridian be hyperopic and the other myopic, it is styled mixed astigmatism. Generally the vertical meridian or one near it is the most convex, and this is called direct astigmatism (astigmatism " according to the rule "). When the horizontal meridian or one near it is the most convex, the term inverse astigmatism is used (astigmatism " against the rule ") When the -meridians are oblique, that is, about 45, it is .called oblique astigmatism. Low degrees of astigmatism (of the cornea) are corrected by the ciliary muscle, producing an astigmatism of the crystalline lens, the opposite of that of the cornea, and so neutralizing the defect. This work is done unconsciously, vision is generally quite good and no suspicion is entertained of anything wrong until some symptom of eye-strain shows itself (see Eye-strain, below), and the detection of it is one of the most important duties of the oculist. The only certain method of detecting and consequently correcting a low error of astigmatism, in all below the age of 50, is by paralysing the ciliary muscle with atropine or homatropine and thus preventing it from correcting the defect, and revealing the true refraction of the eye. Astigmatism is corrected by cylindrical glasses combined with spherical convex or concave glasses if hyperopia or myopia co-exist, and the correction must be worn always in the form of rigid pince-nez or spectacles.

Presbyopia (Old Sight). A normal-sighted child at the age of ten has his near point of accommodation 7 cms. from the eye, and as age advances this near point recedes gradually. At the age of 40 it has receded to 22 cms., in other words at this age fine print cannot be read nearer to the eye than 22 cms. Between the ages of 45 and 50 the person who has apparently enjoyed good sight up till then, both for distance and near, finds that by artificial light he cannot read the newspaper unless he holds it some distance from the eyes, and he has to give up consulting " Bradshaw " because he cannot distinguish between 3's and 8's. Another symptom often complained of is the " running together of letters," so that the book has to be closed and the eyes rested before work can be resumed. This loss of accommodation power is due to the gradual hardening of the crystalline lens from age, and convex glasses have to take its place, in order to make reading possible and comfortable. In hyperopia the presbyopic period is earlier, and in myopia it is later than normal (see above).

It is unwise for the presbyope to select the glasses for himself, as astigmatism or anisometropia may be present and must, of course, be -corrected; the eyes should be properly tested, and this testing should be repeated every two or three years, as, not only does the old sight increase, but changes in the static refraction of the eyes are probably taking place. When an error of refraction exists with the presbyopia, glasses for distance, as well as reading, have to be worn, and to avoid the trouble 1 See also A stigmatism, above.

of constantly changing, the two should be combined as bi-focal glasses. The upper portion of the bi-focal corrects the distant, and the lower the near vision, and in the best form the division between the two is invisible. When properly fitted these bi-focals prove the greatest boon to the presbyope.

Anisometropia (Odd Sight) is a condition in which the refraction of the two eyes is different. There are three varieties, (i) Binocular vision exists. As a rule a very small difference is present, and the difference is generally in the astigmatism; consequently eye-strain is very commonly manifested, and the correction by suitable glasses is imperative. (2) The eyes are used alternately. For instance, one eye may be hyperopic or emmetropic, and the other myopic; in such a case the former will be used for distant and the latter for near vision, and although binocular or stereoscopic vision is lost, glasses may never be required and any attempt at a correction of the defect may be useless. However, if eye-strain is present, the attempt should be made. (3) One of the eyes is permanently excluded. When the difference between the eyes is great the most defective eye is little used and tends to become amblyopic (partially blind), if it is not so already. This condition is very common in squint, and the treatment in such cases consists in providing the defective eye with its correcting glass, completely covering up the good eye and practising for certain periods every day, and thus forcing the defective eye to work. This eye may never take its share in binocular vision, but it may become very useful, especially if disease or damage should affect the good eye ; and the improvement of the Vision of the eye materially assists the treatment of the squint. When one eye is irremediably lost, the other should be very carefully tested, and if any error exists it ought to be corrected and the glass worn always.

Aphakia is the absence of the crystalline lens through dislocation, or removal by operation, or injury. A strong convex glass has to be worn in front of such an eye in order to obtain clear vision even for distance, and a still stronger one for near vision; after cataract operation astigmatism is generally present and the convex glass must be combined with a cy Under: these glasses are best worn in the form of bi-focals (see Presbyopia, above).

Eye-Strain. Eye-strain is a symptom, or group of symptoms, produced by the correction, or attempt at correction, by the ciliary muscle of an error of refraction, or a want of balance between the external muscles of the eye (heterophoria). Where gross errors exist either in the refraction or in the muscular equilibrium, the correction cannot be made, and consequently no attempt is made to correct the defect, and eye-strain is not produced. The smaller the error the more likely is the eyestrain to be present, and also, unfortunately, the more likely is it to be overlooked. It is important to recognize what may be the different manifestations of eye-strain. They may be grouped under three headings : (i) manifestations on the eye and lids, such as conjunctivitis, blepharitis, iritis, cyclitis, glaucoma and cataract. (2) Peripheral irritation: (a) with pain: headaches and megrim; (V) without pain: epileptic attacks and choreiform movements of the facial muscles: vertigo, nausea, vomiting. (3) Nerve waste: nerve exhaustion, neurasthenia, brain-fag. This last form of eye-strain is as common as it is subtle. It is subtle because the sufferer never suspects the eyes to be at fault; all his waking hours he is unconsciously correcting a low degree of astigmatism, or anisometropia, or heterophoria, which means a constant nerve waste; and when he begins near work he starts with a big deficit, and further strain results.

Insomnia is a prominent symptom of eye-strain; this leads to depression, which in its turn may lead to the alcoholic or morphia habit. There is no form of functional nerve disorder that may not be caused by, or aggravated by, eye-strain.

The treatment of eye-strain consists in correcting all errors of refraction (and in the case of astigmatism and anisometropia, even the smallest) and in wearing the correction always. A small amount of heterophoria will generally, in a short time, disappear when the error is corrected; if not, it must be corrected by prisms or decentring. (E. C.*)

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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