120 DIFFEEENT FOEMS OF EYE. 



reach the nerve, but will impinge on the sides and be 

 absorbed by the pigment. Thus, though the light 

 from c will illuminate the whole surface of the eye, it 

 will only affect the nerve at c'. 



In this mode of vision, which was first clearly 

 explained by Johannes Miiller, the distinctness of the 

 image will be greater in proportion to the number of 

 separate cones. "An image," he says,* "formed by 

 several thousand separate points, of which each corre- 

 sponds to a distinct field of vision in the external 

 world, will resemble a piece of mosaic work, and a 

 better idea cannot be conceived of the image of 

 external objects which will be depicted on the retina 

 of beings endowed with such organs of vision, than by 

 comparing it with perfect work of that kind." 



There is, it will presently be seen, reason to suppose 

 that the compound eyes of insects, Crustacea, and 

 some molluscs, are constructed on this plan. 



Thirdly, let L (Fig. 76) be a lens of such a form 



Fig. 76. 



that all the light which falls upon its surface from the 

 point a is re-collected at the point a', that from h at V , 

 from c at c', and so on. If now other light be excluded, 



* " Phys. of the Senses," by Johannes Miiller, iranslated by Dr. 

 Baly. 



