With perhaps the exception of the ear, the eye is the most wonderful example of the infinite skill of the Creator. A more exquisite piece of mechanism it is impossible for the human mind to conceive. The annexed diagram (Fig. 1) of a horizontal section of this organ will give a better idea of its general structure than whole pages of letter-press. It will be seen to consist of a globe of three envelopes or coats, which are kept distended by three transparent humours or lenses: the aqueous (_e_), the crystalline (_f_), and the vitreous (_g_). The outer coat (_a_) is dense, white, and fibrous. In front of the eye it gives place to a perfectly transparent one, called the cornea (_d_). The next coat, the choroid (_b_), is vascular, very black on its internal surface, in order that light falling on it through the pupil (_h_) may not be reflected. The pupil is an opening through a diaphragm which is called the iris (_i_), from its colour varying in different individuals. It has the power of expanding and contracting the pupil, for the purpose of regulating the supply of light to the retina (_c_), or third and last coat which lies immediately on the choroid. It is transparent, very complex, and the only part of the eye we shall carefully consider. The following diagram (Fig. 2) represents a section of it magnified 250 diameters, _a_ is called the limitary membrane, and forms its innermost surface, or that which is next the vitreous humour; _b_ consists of the layer of optic nerve fibers; _c_ is a layer of grey nerve cells; _d_, two layers in which the principal retinal blood-vessels are spread out; _e_, two layers of granular matter; _f_, Jacob’s membrane, or layer of rods and cones. Fig. 3 will give some idea of the supposed connexion between these various parts, the same letters referring to the same parts as in Fig. 2. [Illustration: FIG. 2.] [Illustration: FIG. 3.]