VISION. 8 1 



the hollow cup, and strike the nerve extremities in 

 different places, while the other colours are absorbed. 

 Therefore only red rays can be appreciated, and 

 even these cannot be discerned as distinctively red, 

 since there are no other colours perceived with 

 which they might compare. Such eyes may guide 

 the animal to localities where there are conditions of 

 light favourable to its wants, just as a dog is guided 

 by smell ; but it is difficult to imagine that they can 

 give information more precise. Nor is it easy to see 

 that much advance cafi be gained in function even 

 where a lenticular transparent body lies in front of 

 a number of distinct nerve-terminations, if as in the 

 case of the emerald eyes that adorn the pecten, a bril- 

 liant pigment surrounds the whole. But when a tran- 

 sparent structure with nerve-termination behind is 

 surrounded by dark pigment, which absorbs the 

 oblique rays and allows only the direct rays to 

 penetrate to the nerve, the effect is very different, 

 and complication suitable for the production of 

 vision may occur either by the crowding together of 

 a number of such organs, or by the multiplication of 

 nerve-terminations of a bacillary character behind 

 one common lens in a dark chamber. 



Of the first sort are the compound eyes met with 

 in crustaceans and insects. If you look, for example, 

 at the large domes which form the greater part of 



