No. 3.] VISUAL CELLS IN VERTEBRATES. 613 
tion out of the question, for the color of the sheath was pro- 
jected on the less strongly reacting axis. 
It was, therefore, necessary to use nerve fibers without 
protective coverings. The naked axis cylinders radiating trom 
the entering optic nerve in the fiber layer of the retina, met 
this requirement. In order to get a clear demonstration of 
these, I made tests upon the retina from a perfectly fresh 
ox eye, where the large size of the eye made manipulation 
comparatively simple. In this case there was little difficulty in 
identifying the radiating bundles of nerve fibers, which were 
readily distinguishable from small blood vessels and other 
structures of a fibrous nature. The bundles of naked axis 
cylinders proved to be distinctly positive, thus agreeing with 
the rods, and I am consequently forced to conclude that ip 
some way Valentin’s observations were in_ this respect 
erroneous. 
The agreement in reaction to polarized light between the 
outer segments of rods and the axis cylinders of nerve fibers, 
though not necessarily referable to identical structure, may be 
of significance. In the outer segment we would naturally 
look for some association between the polarization and trans- 
mission of light. According to the wave theory of light, the 
vibrations are at right angles to the direction of propagation. 
Since the axis of maximum elasticity is transverse in the rods 
we would seem to have in them a condition most favorable 
for the transmission of light rays in the direction of the 
longitudinal axis. This set of conditions may be simply inci- 
dental and not essential in the physiology of vision, 
The reversal of polarization in the outer segments of rods 
with many reagents is a phenomenon the general occurrence 
of which is surprising, for one would naturally expect that 
killing fluids would destroy polarization (as some do) rather 
than that they would reverse it. The cause, which I only 
surmise, may be the contraction of certain structural elements 
producing a reversal of the conditions of stress in the body; 
1. €., increasing the tension of one axis as compared with 
