444 Dynamic Theory. 



the purple film termed the visual purple, which covers this delicate 

 membrane, loses its color on exposure to light. He has indeed proved 

 that it is possible to obtain upon this film a so-called optogram, or 

 visible image fixed on the retina, of the object to which the eye may 

 be directed. In the living eye the sensitive surface is quickly being 

 renewed and consequently the eye constantly recovers its power." 

 ( Koscoe, Spectrum Analysis 32. ) 



CHAPTER XL VII. 



COMPARATIVE ANATOMY OF THE EYE. 



The construction of the mammal e} r e is in a general way essentially 

 that described above even down to the rods of the retina. But in regard 

 to the cones there are exceptions. And first, those nocturnal mammals, 

 such as moles, bats, mice, &c. , whose activities are chiefly confined to 

 twilight, have no cones, and therefore presumably no sense of color. 

 But they are provided with an immense number of rods, and must be 

 sensitive to the faintest glimmerings of light. The great mass of the 

 mammals, however, possess cones distributed among the rods as in man, 

 and therefore must have a color sense. But with one exception no mam- 

 mals beside man have the yellow spot. The exception is in the monkey 

 and ape tribes. The possession of this peculiarity by the monkey, while 

 separating him from the lower mammals, to an equal degree brings him 

 nearer to man. In general, among the lower mammals, the proportion 

 of cones to rods is about as one to three. The peculiar habits of ani- 

 mals have caused more or less variety of detail in the structure of the eye 

 as of other parts. Thus, the cat and other animals have in the back part 

 of the eye behind the retina, and either taking the place of the pigment 

 coat or mingled with it, a carpet of glittering fibres called the tapetum. 

 It is a powerful reflector and a very little light gives it a luminous ap- 

 pearance. It intensifies the stimulus of the light upon the rods and 

 enables the animal to see with a small amount of light. The pupil of 

 the eye, as before mentioned, is a mere opening in the curtain called the 

 iris, and it is changed in size by the expansion and contraction of the 

 iris. In certain animals, as the cat, fox, &c. , the pupil is elliptical in- 

 stead of round and contracts to a vertical line. This arrangement al- 

 lows a greater variation in the size of the opening, closing tighter in the 

 day and opening wider in the darkness, and is a manifest advantage to 

 night prowlers in the same manner as the tapetum. But some have sup- 

 posed that the vertical position of the opening, which gives a great ver- 

 tical range of vision, indicates an animal living in the woods. Thus 

 the pupil of the Genet, a timber animal, closes to a vertical line, while 



