84 VISION. 



as in insects ; and in both instances each rod re- 

 ceives its own ray of light which produces a separate 

 sensation, such as I have presumed to be produced 

 at the bottom of each tube of the insect's eye, and 

 in both the number of those separate sensitive points 

 is enormously greater than in the insect's eye. 

 To illustrate the difference in sensitiveness of the 

 higher forms of eyes and the insect's eye, I may 

 mention that in a fly's eye there are about 8,000 

 tubes, whereas in the most sensitive spot of the 

 human eye there are probably about 700,000 sensi- 

 tive points crowded into about the two hundreth 

 of a square inch. But the resemblance between 

 the retina of the cuttle fish and that of verte- 

 brates is carried no farther than I have men- 

 tioned. The rods of such a retina as ours are 

 structures which originated, as already mentioned, 

 in the earliest development, from the lining of 

 the interior of the brain ; while those of the in- 

 vertebrata are derived from the skin ; and in conse- 

 quence of that, there is this great difference, that in 

 the invertebrata the rods are on the surface of the 

 retina looking towards the light, while in our eye 

 they are turned away from it. 



There is a way in which those two forms of eyes 

 may be more closely compared morphologically ; 

 and it is one which gives promise of more beautiful 



