[AS.] The human eye is a nearly spherical ball, which in an adult is about nine-tenths of an inch in diameter. The external coating, known in common language as the white of the eye, is a tough, horny membrane, having about four-fifths of its circumference opaque, and named the sclerotica. The front portion of this coating, called the cornea, is transparent and more curved than the sclerotica. Behind the cornea is a flat circular membrane called the iris. It is colored, and at its centre there is a circular opening called the Pupil, which is capable of becoming contracted or enlarged on exposure to light or darkness. The color of the iris gives the characteristic color to each person's eyes. Behind the pupil is the crystalline lens. The cavity between the cornea and the crystalline lens is called the anterior chamber, and contains the aqueous humor. The cavity behind the crystalline lens, called the posterior chamber, contains the vitreous humor.
The sclerotica is lined by a dark-colored membrane called the choroid coat, saturated with a black mucus. The choroid is lined with a membrane called the retina, which is traversed by a system of nerve filaments coming from the optic nerve. Light falling on the retina produces the sensation of vision, and this is the only part of the eye which possesses this property. When the rays of light from an object enter the eye, they undergo refraction at the cornea and the crystalline lens, and come to a focus on the retina ; if the image formed on the retina is distinct, the object'is seen clearly. The eye can accommodate itself so as to be able to see objects at different distances ; this is supposed to be brought about by a change in the focal length of the crystalline lens. The prawn has a pair of gleaming eyes standing out upon short stalks, which are composed of a number of six-sided facets in the shape of a hemisphere, by which the prawn keeps a sharp look-out. The snail also has eyes set on long stalks. Bees; butterflies, beetles, ants, flies, house-crickets, and other insects, have compound eyes. The eye of the grasshopper often consists of 12,000 lenses, with a glass-like cone and thread-like rods forming the image. Many of the molluscs and other low forms of life have eyes.