No. 383.] BASIS FOR A THEORY OF COLOR VISION. 853 
thus three principal sets of elements, an outer, middle, and 
inner, through which the nervous impulses must pass before 
reaching the optic ganglion. The number of elements in each 
set, as is obvious from even a superficial examination of the 
retina, decreases rapidly from the outer set towards the inner, 
while the territory covered by their terminal brushes increases 
from within outwards. In other words, one element in the 
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Fic. 10. — Diagram to illustrate a ibl din of z f retinal impul 
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. in the human retina. 
middle set appears to receive impulses from many rod and 
cone cells, and one element in the outer set receives impulses 
from many elements of the middle set. There are, therefore, 
no indications whatever that every wave length producing the 
sensation of a distinct color is provided with a private wire 
for its sole use. On the contrary, the diminution in the 
number of retinal cells in each layer, and the increasing 
territory covered by their root-like processes as we pass from 
the outer surface of the retina inward, indicate very clearly 
that the so-called ganglionic layers do not serve to still further 
