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 Muller—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. 
