852 THE AMERICAN NATURALIST. (VOL: XXXII. 
long, or about three times as long as the dark red waves. The 
fibrils at the apex of the cones are about one-quarter as long, 
or .0006 mm., or about one and one-half times longer than the 
violet waves. These figures, of course, are comparatively rough 
estimates, for exact and trustworthy measurements of fresh cones 
are wanting ; but they will serve to show the relations existing 
between the probable length of the fibrils and the ether waves 
to which they respond. 
Since there is only one kind of an impulse sent over nerve 
fibrils, the discrimination of different stimuli must be deter- 
mined mainly by the points of departure and arrival of impulses, 
that is, by the parti¢ular order and combination of fibrils stimu- 
lated ; and this in turn is determined by the position and length 
of the peripheral end fibrils. Such an end apparatus as we 
have described as existing in the human retina is apparently 
adequate to receive and differentiate any conceivable combina- 
tion of light waves that may fall within the area of one or more 
cones. But in order to utilize the full capacity of such an end 
apparatus, each and every receiving fibril should be connected 
by a separate wire with the central station in the optic gan- 
glion (or tectum opticum of the mid-brain in lower vertebrates 
or corpora quadrigemmina of higher vertebrates), and that in 
turn should be united in a similar manner with the cerebral 
hemispheres. But the well-known limitations of the human 
visual apparatus clearly demonstrate that its connections cannot 
be as complete as this. 
The same conclusions may also be drawn from the known 
structure of the retina itself. For while there are supposed 
to be about three and a half millions of cones and about one 
hundred and thirty million rods in the retina, there are only 
about half a million fibres in the optic nerve. Now, as every 
one knows, the fibres of the rod and cone cells extend only to 
the inner molecular layer, where they end freely in terminal 
brushes (Fig. 10) ; there the nervous impulses transmitted by 
these cells are probably picked up by a second set, which in 
turn end at the outer molecular layer ; and here the impulses 
are apparently again transferred to a third set, which send their 
fibres along the optic nerve to the optic centres. There are 
