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 
