Page:The Book of Cats.djvu/89

Rh in common with most animals, the Cat is furnished with a nictitating membrane, which is, in fact, a third eyelid, sliding over the transparent cornea beneath the common eyelids. This membrane is not altogether opaque, but translucent, allowing light to fall on the retina, and acting, as it were, like a shade. The nictitating membrane is often seen in the Cat when she slowly opens her eyes from a calm and prolonged sleep: it is well developed in the eagle, and enables him to gaze steadfastly on the sun's unclouded disk.

The illumination of a Cat's eye in the dark arises from the external light collected on the eye and reflected from it. Although apparently dark, a room is penetrated by imperceptible rays of external light from lamps or other luminiferous bodies. When these rays reach the observer direct, he sees the lamps or luminiferous bodies themselves, but when he is out of their direct sight, the brightness of their illumination only becomes apparent, through the rays being collected and reflected by some appropriate substance.

The cornea of the eye of the Cat, and of many other animals, has a great power of concentrating the rays and reflecting them through the pupil. Professor Bohn, at Leipsic, made experiments