Fig. 62.—SECTION OF THE EYE SEEN FROM BEHIND.
A. The pupil of the eye through which the light enters.
B. The iris, which dilates or contracts, and thereby increases or lessens the size of the pupil.
C. The three coats of the eye, called the sclerotic, choroid, and retina.
D. The ciliary processes, or hair-like muscles, which have a slight vibratory motion which they impart to the fluids of the eye.
E. The dark coat of the choroid, the coat forming the retina removed.
Because the field of vision is thereby much extended; the intensity of sight is also increased, the impressions upon the brain being clearer and better defined, just as in a stereoscope the effect of vision is heightened by a double picture; the sense of sight being more constantly exercised than any other sense during our waking moments, one eye is frequently called upon to give rest to the other; and the important faculty of vision, being endangered by the necessary exposure of some parts of the eye, and the equally necessary delicacy of an organ formed to receive impressions from so ethereal an element as light, is rendered the more secure to us, since though one eye may become enfeebled, diseased, or wholly lost, the other eye will retain the blessing of sight.