[Illustration: _a a_, the _sclerotica_; _b b_, the _choroides_; _c c_, the _retina_; _d d_, the _cornea_; _e_, the _pupil_; _f f_, the _iris_; _g_, the _aqueous humor_; _h_, the _crystalline humor_; _i i_, the _vitreous humor_.] 11. The _sclerotica_ is a membranous coat, to which the muscles are attached which move the eye. The _cornea_ is united to the sclerotica around the circular opening of the latter, and is that convex part of the eye, which projects in advance of the rest of the organ. The space between this and the crystalline lens is occupied by the aqueous humor and the iris. The _iris_ is united to the choroides, and it possesses the power of expanding and contracting, to admit a greater or less number of rays. 12. The _crystalline lens_ is a small body of a crystalline appearance and lenticular shape, whence its name. It is situated between the aqueous and vitreous humors, and consists of a membranous sack filled with a humor of a crystalline appearance. The _vitreous humor_ has been thus denominated on account of its resemblance to glass in a state of fusion. The _retina_ is a membrane which lines the whole cavity of the eye, and is formed chiefly, if not entirely, by the expansion of the optic nerve. 13. The rays of light which proceed from objects pass through the cornea, aqueous humor, crystalline lens, and vitreous humor, and fall upon the retina in a focal point, to which it is brought, chiefly by the influence of the cornea and the crystalline lens. The image, in an inverted position, is painted or thrown on the cornea, which perceives its presence, and conveys an impression of it to the brain, by means of the optic nerve. 14. _Optical instruments._--The art of constructing optical instruments is founded upon the anatomical structure, and physiological action of the eye, and on the laws of light. They are designed to increase the powers of the eye, or to remedy some defect in its structure. In the cursory view which we may give of a few of the many optical instruments which have been invented, we will begin with the _spectacles_, since they are the best known, and withal the most simple. 15. The _visual point_, or the distance at which small objects can be distinctly seen, varies in different individuals. As an average, it may be assumed at eight or nine inches from the eye. In some persons, it is much nearer, and in others, considerably more distant. The extreme, in the former case, constitutes _myopy_, or _short-sightedness_, and, in the latter case, _presbyopy_, or _long-sightedness_. 16. _Myopy_ is chiefly caused by too great a convexity of the cornea and the crystalline lens, which causes the rays to converge to a focus, before they reach the retina. Objects are, therefore, indistinctly seen by myoptic persons, unless held very near the eye to throw the focus farther back. This defect may be palliated by the use of concave glasses, which render the rays proceeding from objects more divergent.