THE PERCEPTION OF LIGHT 



605 



We have noted that the ocular characteristics of a nocturnal and 

 a dinrnal eye are essentially incompatible ; in the arhythmic eye a 

 compromise is reached. The nocturnal eye is attuned to a high develop- 

 ment of the light-sense ; in the diurnal eye this gives place to the 



Figs. 7'40 to 749. — Xocturnal, Diurnal and Arhythmic Types of Eye. 

 (In each case the lens is unshaded.) The eyes are not drawn to scale. 



Note the huge size of the lens and its set-back position in fully nocturnal 

 types (Figs. 740-3), and its small size and anterior position in fully diurnal 

 types (Figs. 747-9). 



Fig. 744. — A diurnal 

 gecko. 



Fig. 745. — A Ivnx. 



Fig. 746. — An owl. 



Fig. 



chanie- 



FiG. 748. — A chimpanzee. 



Fig. 749. — A pigeon. 



apparatus required for keen visual acuity. In order to attain efficiency 

 a 7iocturnal eye evolves special ijecuUarities both in its optical system 

 and in the organization of the retina. 



I. The opticeil system demands a large eye to gather as much light 

 as possible, a widely dilated pupil to allow the maximum amount of 

 light to enter, and a large sjjherical lens set far back from the cornea 

 to place the optical centre near the retina so that light transmitted 



