The Book of Cats. 69 



in common with most animals, the Cat is furnished 

 with a nictitating membrane, which is, in fact, a 

 third eyeUd, sHding over the transparent cornea be- 

 neath the common eyehds. 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 ex- 

 ternal 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 bright- 

 ness 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 



