184 HOW INSECTS SEE. 



as we do ; though long practice has given us the right 

 impression. The simple eye of insects thus resembles 

 ours in this respect. 



As regards the mode of vision of the compound eyes, 

 there are two distinct theories. According to one 

 the mosaic theory of Miiller each facet takes in only 

 a small portion of the field ; while according to the 

 other, each facet acts as a separate eye. 



This latter view has been maintained by many high 

 authorities, but it is difficult to understand how so 

 many images could be combined into one picture. Some 

 insects have more than 20,000 facets on each side of 

 their head. No ants, indeed, have so many, but 

 in some as, for instance, in the males of Formica 

 pratensis there are not less than 1,000. The theory, 

 moreover, presents some great anatomical difficulties. 

 Thus, in certain cases there is no lens, and conse- 

 quently there can be no image ; in some it would seem 

 that the image would be formed completely behind the 

 eye, while in others again it would be in front of the 

 receptive surface. Another difficulty is that any true 

 projection of an image would in certain species be pre- 

 cluded by the presence of impenetrable pigment, which 

 only leaves a minute central passage for the light-rays. 

 Again, it is urged that even the sharpest image would 

 be useless, from the absence of a suitably receptive 

 surface; since the structure of the receptive surface 

 corresponding to each facet seems to preclude it from 

 receiving more than a single impression. 



