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Chemically, the lens is made up of globulin, and furnishes a ready source for obtaining this form of albumin for examination.

DIOPTRICS OF THE EYE. Light travels through any even transparent body, such as the atmosphere, in a straight line. But when it meets any change in density, particularly when it has to pass obliquely into a

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Diagram showing the course of parallel rays of light from A in their passage through a

biconvex lens L, in which they are so refracted as to bend toward and come to a focus at a point F.

denser medium, the ray is bent so as to run in a direction more perpendicular to the surface of the denser body. The degree of bending or refraction of the rays depends on the difference in optical density of the two media and the angle at which the ray strikes the surface of the more dense.

On its way to the sensitive retina, the light has to pass

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Diagram showing the course of diverging rays, which are bent to a point further from the

lens than the parallel rays in last figure.

through the various transparent media just named, viz., the cornea, the aqueous humor, the crystalline lens, and the vitreous humor. On entering these media, which have different densities, the rays of light emitted by any luminous body become bent or refracted, so that they are brought to a focus on the retina, just in the same way as parallel rays of light from the sun may be focused on a near object by means of an ordinary convex lens.

Only so much light reaches the fundus of the eye as can pass through the opening in the iris, so that a comparatively narrow and varying beam is admitted to the chamber in which the nerve endings are spread out for its reception.

If we hold a biconvex lens at a certain distance from the eye and look out of the window through it, we see an inverted image of the landscape. If we place a piece of transparent paper behind the lens, we can throw a representation of the picture on it, which will be seen to be inverted. This power of convex lenses is employed in the instrument used for taking photographs, called a camera, which consists of a box or chamber into which the light is allowed to pass through a convex lens, so that an inverted image of the objects before it is thrown upon a screen of ground glass within the box. When the sensitive plate replaces the screen, the light coming through the lens makes a photograph.

Just in the same way an inverted image of the things we look at is thrown on the retina by the refracting media of the eye. This may be seen in a dark room, if a candle be placed at a suitable distance in front of the cornea of an eye taken from a recently killed white rabbit. When cleared of fat and other opaque tissues, the sclerotic is transparent enough to act as a screen upon which the inverted candle flame can be recognized.

Though our organ of vision is often compared to a camera obscura, the refractions of light which occur in it are far more complex than those taking place in that simple instrument. In the latter we have only two media—the glass lens and the air ; in the eye, on the other hand, we have several, which are known to have a distinct refractive influence on the rays which pass through the pupil.

THREE MEDIA AND REFRACTING SURFACES. Since the surfaces of the cornea, however, are practically parallel, we may neglect the difference between it and the aqueous humor, and look upon the two as one medium, having in front the shape of the anterior surface of the cornea, and behind, the anterior surface of the lens, so as to form a concavoconvex lens. We thus have only three media to consider, viz., (1) the aqueous humor and cornea; (2) the lens and its capsule ; and (3) the vitreous humor. And only three refracting surfaces need be enumerated, viz., (1) the anterior surface of the cornea ; (2) the anterior surface of the lens; and (3) the posterior surface of the lens.

These refracting surfaces may all be looked upon as portions of spheres whose centres lie in the same right line, and hence may be said to have a common axis. The eye may be regarded as an optical system, centred round an axis which passes through

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Showing the course of the rays of light rom two luminous points to the retina. The rays

from the point a on passing through the cornea, lens, etc., are collected on the retina at b. Those from a' meet at b', and thus the lower point becomes the upper.

the middle point of the cornea in front, and the central depression (fovea centralis) of the retina behind. This is spoken of as the optic axis of the eye.

The rays of light entering the eye are most strongly refracted at the surface of the cornea, because they have to pass from the rare medium, the air, to the denser cornea and aqueous humor. So also more bending of the rays occurs between the aqueous humor and the anterior surface of the lens than between the posterior surface of the lens and the vitreous humor.

The lens is not of the same density throughout, but denser in the centre, and being made up of layers, the central part refracts more than the outer layers.

The manner in which the inversion of the image is produced by a convex lens is shown in the preceding figure, in which the lines correspond to the rays passing from two points through the lens. If the arrow a a' be taken for the object, from either extremity of it rays pass through, and are more or less bent by the lens. It will be sufficient to follow the course of three rays from the head of the arrow. One of these passes through the centre of the lens, and leaves it in the same direction which it entered, because the two surfaces at the points where it entered and left may be regarded as parallel, and so cause no refraction. The rays which do not pass through the centre are bent on entering and on leaving the lens, so that they all meet at the same point and there produce an image of the head of the arrow, at b'. In the same way the feather end of the arrow is produced at b; the position of the image of the object is thus reversed by the light rays passing through the lens.

In a biconvex lens, with two surfaces of the same degree of convexity, the central point through which the rays pass without being refracted is easily made out, as it is the geometrical centre of the lens. This central point is spoken of as the optical centre. With systems of lenses of varying convexity, and more than one in number, as we have in the eye, where the rays of light are bent at different surfaces, it is much more difficult to determine the optical centre. However, by means of the measurements made by Listing, two points close together are known, which may be said to correspond practically with the optical centres of the eye; they lie in the lens, between its centre and posterior surface. The path of the various rays may thus be exactly made

out.*

The

rays which come from a distant luminous point and fall upon the eye, are refracted by the cornea and aqueous humor, so as to be made convergent on their way to the lens; they are then

* The impossibility of making clear the important relationships, such as nodal points, and other constants of the eye in a short text-book, and the deterrent effect exerted upon the mind of a junior student by brief incomprehensible statements, have induced the author to omit this part of the subject. He must refer those who are anxious to learn the cardinal points of the eye, to the more advanced text-books.

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further bent at the surfaces of the lens, so that they are brought exactly to a point on the retina. That is to say, for distant luminous points, the retina lies exactly in the plane of focus of the dioptric media of the normal eye.

This convergence of the rays to a point on the retina, is the first essential for seeing clear and distinct images; for if the rays from each point of a luminous body were not united on the retina as points, the effects of the different rays from the various points of a body would become mixed, and there would be loss of definition of its image.

The rays from any bright point which enter the eye through the pupil may be imagined to form a luminous cone, the point of which lies at the retina, and its base at the pupil. After their union at the point of the cone, the rays would diverge again if the retina were not there to receive them.

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SCHEINER'S EXPERIMENT. It may be seen from the foregoing figure that if the retina, which normally would lie at 2, were placed nearer the dioptric apparatus, say at 1, or further from it, at 3, it would not meet the exact point of the luminous cone, but would receive the rays either before they came to a point, or after they had diverged from it. Thus indistinct rings of light would be seen instead of one luminous point, and an image would be blurred and indefinite.

From this it follows that the eye, when quite passive, can only get an exact image of bodies which are placed at a certain distance from it, just as, for any given state of a camera, only those bodies in one plane come into focus and give a clear picture on the screen. If the dioptric apparatus of the eye were rigid and unalterable, since the relation of the retina to it is permanently the same, we could only see those objects clearly which are at a given distance from the eye. We know, however, that we see as distinct an image of distant as of near objects, and we can look through the window at a distant tree, or can adjust our eyes so as to see a fly walking on the window pane. We cannot see both distinctly at the same moment. This power of focusing may

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