Page:The fairy tales of science.djvu/133

Rh the cornea or bow-window is the aqueous humour, a perfectly limpid liquid resembling water; the second in situation is the crystalline humour, which is a little capsule of transparent membrane, holding a small quantity of fluid; and the third, termed the vitreous humour, is a transparent jelly which fills the inner chamber of the eye, and contributes chiefly to preserve the globular figure of the organ.

Each eye is placed in a basin-shaped cavity in the skull, called the orbit, and there are various muscles attached to different parts of the orbit, which by their contraction give a lateral or rolling motion to the eyeball, and thus assist in directing the sight towards particular objects. Eyelids, also moved by muscles, and fringed by the eyelashes, serve to guard the eyes from dust, and to screen them from the access of too intense a light.

So much for the anatomy of the eye; let us now consider its functions. As already observed, the eye may be compared to a camera obscura, for the rays of light from any object entering the pupil form an image on the retina, just as the picture is painted on the ground glass of the camera. The various humours of the eye form a wonderful compound lens, far excelling the achromatic lenses of the opticians. The seat of vision is generally supposed to be the retina, though some philosophers regard the choroid coat as the sensitive tablet upon which the impression is made. We may trace the phenomena of