ences its shape. The lens is held in its position by the suspensory ligament, a thickened part of the hyaloid membrane, which is continued forward and attached to the anterior surface of the capsule, near its margin. The lens and its capsule, together

Diagram of lens viewed from the side at different periods of life. a, At birth; b, Adult; e, Old age. (Allen Thomson.)

with the vitreous humor, may be said to be enclosed in the hyaloid membrane, which, in front, is thickened and attached to the ciliary part of the choroid and the capsule. Thus, any tension exercised by the suspensory ligament tends to tighten the ante

Showing early stages of the development of the lens. c, Epithelial tissue about to form the lens; o, Optic cup; a, Epidermis. (Cadiat.)

rior part of the capsule and flatten the anterior surface of the lens.

The shape of the lens varies at different times of life, being nearly spherical in the infant and tending to become less convex in old age (Fig. 218). The lens is developed from the outer

layer of the embryo by the gradual thickening and growing inward of the epithelium, which meets the optic cup, and after a time is cut off from the parent tissue. The stages of its development may be followed in the preceding woodcuts (Fig. 219).

The lens is composed of a number of peculiar band-like cells, derived from the epithelium. These are cemented together in

A further stage of the development of the lens. (Cadiat.)

a, Elongating epithelial cells forming lens; b, Capsule; c, Cutaneous tissue becoming conjunctiva; d, e, Two layers of optic cup forming retina; f, Cell of mucous tissue of the vitreous humor; g, Intercellular substance; h, Developing optic nerve.

parallel rows, eccentrically arranged in layers. These bands are hexagonal in transverse section, and in the younger periods of life may be seen to contain nuclei.

In the living state the lens is perfectly transparent, but after death it becomes slightly opaque. The nutriment for the adult lens is derived from the vessels of the choroid, which, however,

do not come into direct communication with its texture. On this account the nutrition of the lens is not so perfect as that of many other tissues, and is but imperfectly repaired after injury,

Fragment of lens teazed out to show the separate fibres. (Cadiat.)-a, b, and fibres with different sized nuclei.

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

FIG. 222.

A

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

FIG. 223.

F

-A

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