VISION. SI 
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 can 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 
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