image formed at the focal point; the iris to the diaphragm, which by cutting off the marginal rays prevents spherical aberration and at the same time regulates the amount of light entering the eye; the ciliary muscle to the adjusting screw by which distinct images are thrown upon the retina in spite of varying distances of the object from which the light rays emanate. The structures just enumerated are those essential for normal vision. The relationship of the various structures composing the eyeball is shown by the following figure:

Petit.

1. Anterior chamber filled with aqueous humor. 3. Posterior chamber. 3. Canal of a. Hyaloid membrane. b. Retina (dotted line). c. Choroid coat (black line). d. Sclerotic coat. e. Cornea. f. Iris. g. Ciliary processes. h. Canal of Schlemm or Fontana. i. Ciliary muscle.-From Holden's Anatomy.

The Dioptric or Refracting apparatus, by which the rays of light entering the eye are so manipulated as to produce an image on the retina, consists of the cornea, aqueous humor, crystalline lens, and vitreous humor. A ray of light in passing through each of these media will undergo refraction at their surfaces and ultimately be brought to a focus at the retina. Inasmuch as the two surfaces of the cornea are parallel and its refractive power practically the same as the aqueous humor, the media may be reduced to three, viz.: I. Corne and aqueous humor. 2. The lens. 3. The vitreous humor. The refract ng surfaces may also be reduced to three.

viz.: I. Anterior surface of the cornea, 2. Anterior surface of lens. 3. Posterior surface of lens.

The refraction effected by the cornea is very great, owing to the passage of the light from the air into a comparatively dense medium, and is sufficient of itself to bring parallel rays of light to a focus about 10 millimeters behind the retina. This would be the condition in an eye in which the lens was congenitally absent. Perfect vision requires, however, that the convergence of the light shall be great enough that the image may fall upon the retina. This is accomplished by the crystalline lens, a body denser than the cornea and possessing a higher refractive power. The manner in which a biconvex lens focuses both parallel and divergent rays is shown in the following figures :

FIG. 26.

LL

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 IN A FOCUS AT A POINT, F.-From Yeo's Text-Book of Physiology.

FIG. 27.

-A

DIAGRAM SHOWING THE COURSE OF DIVERGING RAYS WHICH ARE BENT TO A POINT FURTHER FROM THE LENS THAN THE PARALLEL RAYS IN PRECEDING FIGURE.

From Yeo's Text-Book of Physiology.

The function of the crystalline lens, therefore, is to focus the rays of light with the formation of an image on the retina.

The Retinal Image corresponds in all respects to the object from which the light proceeds. The existence of this image can be demonstrated by removing from the eye of a recently killed animal a circular portion of the sclerotic and choroid posteriorly and then placing at the proper distance in front of the cornea a lighted candle; an inverted image of the candle will

be seen upon the retina. The size of the retinal image depends upon the visual angle, which in turn depends upon the size of the object and its distance from the eye. At a distance of 15.2596 meters the image of an object I meter high would be 1 millimeter, or a thousand times smaller than the object.

Accommodation.-By accommodation is understood the power which the eye possesses of adjusting itself to vision at different distances. In a normal or emmetropic eye parallel rays of light are brought to a focus on the retina; but divergent rays, that is, rays coming from a near luminous point, will be brought to a focus behind the retina, provided the refractive media remain the same; as a result vision would be indistinct, from the formation of diffusion circles. It is impossible to see distinctly, therefore, a near and distant object at the same time. We must alternately direct the vision from one to the other. A normal eye does not require adjusting for parallel rays; but for divergent rays a change in the eye is necessitated; this is termed accommodation. In the accommodation for near vision the lens becomes more convex, particularly on its anterior surface; the increase in convexity increases its refractive power; the greater the degree of divergence of the rays previous to entering the eye, the greater the increase of convexity of the lens and convergence of the rays after passing through it. By this alteration in the shape of the lens we are enabled to focus light rays coming from, and to see distinctly, near as well as distant objects.

Function of the Ciliary Muscle.-Though it is admitted that the change in the convexity of the lens is caused by the contraction of the ciliary muscle and the relaxation of the suspensory ligament, the exact manner in which it does so is not understood. When the eye is in repose, as in distant vision, the suspensory ligament is tense and the lens possesses that degree of curvature necessary for focusing parallel rays. In the voluntary efforts to accommodate the eye for near vision, the ciliary muscle contracts, the suspensory ligament relaxes, and the lens, inherently elastic, bulges forward and once again focuses the rays upon the retina. It is, therefore, termed the muscle of accommodation, and by its alternate contraction and relaxation the lens is rendered more or less convex, according to the requirements for near and distant vision.

Range of Accommodation.-Parallel rays coming from a luminous point, distant not less than 200 feet, do not require adjustment; from this point up to infinity no accommodation is required for perfect vision. This is termed the punctum remotum, and indicates the distance to which an object