briefly, but it is hoped that this little treatise will enable any intelligent practitioner to form correct conclusions respecting the condition of the eyes of his patients in this respect.

REFRACTION AND ACCOMMODATION OF THE EYE.

The eye may be regarded as an optical instrument, similar, in some respects, to a camera-obscura, such as is used by photographers, in which rays of light are concentrated by means of convex lenses in such manner as to fall upon a screen at the rear of the dark chamber. If the screen is of white ground glass, an image of an object from which the rays emanate may be seen upon the glass by an observer looking from behind the screen.

In the eye, rays of light pass through transparent media, where they are so bent or refracted as to be concentrated upon the retina, where the impression is recognized as the form of the object perceived.

Rays of light passing from space into the eye are refracted, according to Donders, by the anterior surface of the cornea, the anterior surface of the lens, and the anterior surface of the vitreous. The transparent media through which the rays must pass to the retina, and in which the refraction is accomplished, form the dioptric system.

The ideally normal eye is so constructed that rays from an infinite distance, that is to say, parallel rays, in traversing the dioptric system, are brought to a focus at the retina without an effort of accommoda

tion. This normal condition of the eye is called emmetropia.

The diagram (Fig. 3) shows the arrangement of the different structures of the eye and the relations of the

membrane, the choroid (Ch.), having an expanse about equal to that of the sclera. The cornea is transparent, permitting rays of light to pass into the eye, where they make their way through the aqueous humor (A. H.) and pass through the opening in the iris (I.), which is the pupil. The rays then traverse the crystalline lens (C. L.) and the vitreous humor (V. H.), at length falling upon the retina (R.), the delicate nervous membrane which extends from the optic nerve and lies between the choroid and the vitreous humor. If the rays are brought to a focus on the retina, this focus lies at a point somewhat external to the point of en

trance of the optic nerve, where the retina becomes even more thin and delicate than in its general expanse. This point, which is exactly in the visual axis, is called the macula lutea (M. L.). The point at which the optic nerve enters the eyeball is called the optic disc. The crystalline lens is held in position by an extremely delicate enveloping membrane called the capsule (Cap.), which is connected with the muscular ring, the ciliary muscle (C. M.).

If an object which is clearly defined upon the screen of a camera be moved nearer to the instrument or carried farther from it, the image upon the screen will be no longer well defined, but indistinct. In this case the clear definition may be restored by changing the relation of the lenses to the screen, by moving them backward or forward, or the lenses may be replaced by others having greater or less refracting power.

If the eye were so constructed that its focal adjustment was always the same, objects only within a certain range would be well seen, and all objects removed beyond or brought within shorter range would be indistinctly perceived. This condition is provided against by the faculty possessed by the eye of changing, within certain limits, its refractive state. This is called the faculty of accommodation, and it must be brought into action whenever the eye regards objects nearer than the most distant point of clear vision; and thus during waking hours it is almost constantly exercised. The theory of the mechanism of accommodation of the eye was long one of the most interesting of

physiological inquiries, and many suppositions and speculations were from time to time accepted. The first to discover and to demonstrate the actual changes which occur in the exercise of this important function was Dr. Thomas Young.

From the era of Kepler until the time of Dr. Young's contributions to the "Philosophical Transactions" in 1801, much had been written and but little had been known of the nature of this faculty possessed by the normal eye of adapting itself to bring to a focus rays of light emanating from points at different distances. Young, by experiments, and by what, had they been properly understood, should have been regarded as conclusive arguments, showed that the change of focal adjustment of the eye in accommodation depends upon alteration in the degree of convexity of the crystalline lens. A similar hypothesis had previously been held, but no demonstrations had been adduced.

Little attention was paid to Young's theory until Helmholtz and Cramer, working independently, proved by mathematical and ocular demonstrations the truth of the theory. This important physiological problem having been solved, it remained to others, and notably to the illustrious Professor Donders, to develop the theories of accommodation and refraction in respect to individual defects. The result of Professor Donders's labors in this direction were given to the world in his great work, "On the Anomalies of Accommodation and Refraction of the Eye," published in 1864.

According to the present knowledge of the function of accommodation, the ciliary muscle, a small muscular ring situated in the interior of the eye and surrounding the border of the crystalline lens, acting upon the lens in such a manner as to modify its curvatures, and hence its refracting power, is the seat of the faculty of accommodation.

According to the investigations of Cramer and Helmholtz, it is shown that in the act of accommodating the eye for near points the lens becomes convex, its anterior surface advancing toward the cornea, while the posterior surface remains nearly stationary, a change produced by the contraction of the ciliary muscle. When this contraction is discontinued, the lens resumes its original form, and the eye is adjusted for distance. The modification of the convexity of the lens, when accommodated for distance and near points, is well shown in the accompanying diagram:

FIG. 4.

In Fig. 4 parallel rays are shown by the solid lines which enter the eye, where they undergo refraction