VISION. 83 



points are scattered, while if it is near they are 

 crowded together; and probably in this way a 

 sense of distance is obtained. We may judge also 

 that this form of eye is suitable principally for seeing 

 very near objects. 



In all the higher kinds of animals namely, in all 

 the vertebrates, from fishes up to man, and likewise 

 in the highest group of .invertebrates, including the 

 nautilus and cuttle fishes, the arrangement exists of 

 a camera, a lens, a bacillary layer and a retina or 

 membranous expansion of brain-matter. So far, 

 the eye of the cuttle fish, together with the other 

 less developed camerate eyes found in the inverte- 

 brata is similar to the eye of vertebrate animals ; 

 but while the optical contrivances in the two sets of 

 structures are similar, the sources of their origin are 

 totally different ; so that it is impossible to con- 

 ceive that by any process of modification in succes- 

 sive ages the one kind of eye could have grown out 

 of the other. This is particularly the case as 

 regards the retina or sensitive curtain on which the 

 light is thrown. In both vertebrate and cuttle fish 

 eye, it consists of a sheet of nervous substance con- 

 nected with a covering of microscopically minute 

 rods which receive the rays of light and are affected 

 by them. In both instances it is an inverted pic- 

 ture which is cast on the retina, not an erect picture 



