624 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
changes resulting in fatigue. The long fibrils respond to the red end 
of the spectrum and the short ones to the blue. In rods the fibers are 
of equal length and only monochromatic vision is possible, but in the 
cones their varying length allows a range of color perception. Any 
variation in the form or dimensions of the cones would bring about 
corresponding changes in vision. ‘The increased length of the cones 
at the fovea provides for a greater power of color discrimination. If 
the base of a cone were absent or cylindrical it would be red blind. 
This theory is illustrated in figure 
6. On the right is the diagram of a 
cone and its fibrils; the latter radi- 
ate from an axial filament, the ex- 
istence of which has been discussed 
and denied by other investigators. 
The fibrils in the right half of the 
cone are drawn as responding to 
red, yellowish green, and _ violet 
heht; the Young-Helmholtz curves 
are shown on the left. In nonpolar- 
ized light all of the fibrils in a trans- 
verse section of a cone respond uni- 
formly, but in polarized light only 
such are affected as are indicated in 
the cross sections on the left of the 
figure. Thus the dullness of polar- 
ized light is explained. The cor- 
rectness of this supposition, as 
Doctor Patten states, will be deter- 
mined by extensive measurements, 
much more accurate and detailed 
than any heretofore made, of the 
visual elements in all classes of 
Fic. 6.—Diagram to illustrate the sup- animals. 
posed fibrillar structure of human 6 i 
cones, and the way in which various It will be noted that according 
light waves affect them. (Patten.) to Patten’s and Mrs. Franklin’s 
theories the mechanism for reaction 
to all the colors may exist in a single cone. The Hering theory calls 
for the reaction to at least two colors in one cone; but, according to the 
Young-Helmholtz theory, although the three substances could exist in 
a single cone, each is declared to exist in a cone by itself. This is con- 
sidered to be strongly in favor of the validity of the Young-Helm- 
holtz theory. Since physiologists find no instance in which different 
sorts of impulses are conveyed over a given nerve fiber, it is believed 
that a single cone fiber can transmit only one sort of color sensation. 
The stimuli of the red, green, and blue cones respectively are supposed 
