SUMMARY. 171 



These, however, tnke in different parts of the field of 

 vision. 



The principal reasons, then, which seem to favour 

 Miiller's theory of mosaic vision are as follows : — 



(1) in certain cases — for instance, in Hyperia— there 

 are no lenses, and consequently there can be no image ; 



(2) the image would generally be destroyed by the 

 crystalline cone ; (3) in some eases it would seem that 

 the image would be formed completely behind the eye, 

 while in others, again, it would be too near the cornea ; 

 (4) a pointed retina seems incompatible with a clear 

 image ; (5) any true projection of an image would in 

 certain species be precluded by the presence of im- 

 penetrable pigment, which only leaves a minute central 

 passage for the light-rays ; (6) even the clearest 

 image would be useless, from the absence of a suit- 

 able receptive surface, since both the small number 

 and mode of combination of the elements composing 

 that surface seem to preclude it from receiving more 

 than a single impression; (7) no system of accommoda- 

 tion has yet been discovered, finally, (8) a combina- 

 tion of many thousand relatively complete eyes seems 

 quite useless and incomprehensible. 



On the Powek of Vision in Insects, etc. 



As regards the practical vit^ion of insects, our know- 

 ledge is still very imperfect. No one, indeed, who has 

 observed them can doubt that in some the sight is 

 highly developed. It is impossible, for instance, to 

 watch a dragon-ily hawking over a pond, — to see the 

 rapidity and accuracy of its movements, and doubt 

 that it can see well. 



