Page:Handbook of Ophthalmology (3rd edition).djvu/17



to the lucid treatment of this subject by Donders, the Anomalies of Refraction and Accommodation have become one of the clearest and most complete chapters of ophthalmology. Even the first step was decisive. The sharp distinction which Donders made between Refraction and Accommodation was sufficient to banish all obscurity. By refraction we understand that optical adjustment of the eye which depends upon its anatomical structure; the accommodation includes those changes of the optical adjustment which are effected by the ciliary muscle. Hence we can say that the refraction is that optical condition of the eye which we have when the ciliary muscle is wholly relaxed.

The anatomical components which determine the optical structure of the eye are as follows: (1) the curved surfaces of the dioptric apparatus,—i.e., the surface of the cornea, the surfaces of the lens, together with the distance between these surfaces; (2) the refractive index of thfe transparent media,—i.e., of the cornea, aqueous humor, lens, and vitreous body; and (3) the length of the axis of the eye.

The great number of these components, and their liability to variations, naturally cause different conditions of refraction in different individuals. Donders, however, easily succeeded in arranging the possibilities, since he started out with the behavior of the eye toward rays of light which fall parallel upon the cornea; for such rays after refraction in the eye must be focused either upon the sensitive layer of the retina or before or behind it.

Hence there may be three different conditions of refraction, perfect rest of accommodation—that is, total relaxation of the ciliary muscle—being always assumed.

When rays of light which proceed from a point lying at an infinite distance, and which are therefore practically parallel, fall 11