SUPPLEMENT.

IF the doctrines set forth in this work are worthy of acceptance, it must follow that a knowledge of the refractive and muscular anomalies of the eyes is essential to the most successful treatment of a very large proportion of nervous complaints. The supplemental portion of this work is prepared with the view of af fording the practitioner who does not profess to be a specialist in eye-affections, a general understanding of the anomalies to be sought for and the means for their correction. No effort is here made to present an exhaustive or a critical treatise. One who would pursue these subjects in a technical manner will find abundant material for study in a part of this field in the superb works of Donders and of Landolt upon "The Refraction and Accommodation of the Eye." Unfortunately, there are no text-books in which that class of anomalies of the ocular muscle known as "insufficiencies," is fully discussed. For the most part, the literature of this subject is confined to a single condition of "insufficiency," and even this receives, as a rule, but a passing notice. The reader will, in the pages devoted to this subject, find it treated very

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