298 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
percentages of positive responses when used singly. But the less 
effective light also reduced the effect of the more potent light. The 
less effective lights, yellow and red, made the two more effectgve — 
blue and green — much more nearly similar in their effects, when paired 
with them, than when blue and green were paired together, or used 
singly. When the lights were received through only the skin, this was 
made even more evident, owing to the comparatively greater insensi- 
tiveness of the skin to differences in wave-lengths, particularly in the 
more refrangible hghts. 
The reactions to balanced pairs of monochromatic lights showed, 
therefore, essentially the same relations under the three different 
conditions of exposure, and also the same relations as in the reactions 
to single lights. But the sensitiveness of the skin to differences in 
wave-lengths was not as great as that of the eyes. Moreover, the 
effectiveness of the more potent light in any pair is reduced by that 
of the light with which it is paired, and vice versa; and this has a tend- 
ency to make the differences in effect between the more effective lights 
less, when paired with others, than when they were used singly, or 
when paired with each other. 
The slight lack of agreement between the distribution of the effective- 
ness of the lights and the distribution of the several lights in the spec- 
trum must be due to specific chemical effects called forth by the several 
lights. And the fact that the distribution of the effectiveness of the 
several lights was different for the three conditions of exposure, points 
to the conclusion that the effects on the eye were slightly different 
from those on the skin, and that, therefore, the reactions when both 
the skin and the eyes served as receptors should also be different from 
those when one or the other served alone. The toad probably reacted, 
not to the stimulation of the light, directly, but to the chemical 
changes which were produced in the eyes and the skin by the lights. 
These chemical changes were greatest in the blue, and least in the 
red, and while the sequence of effectiveness followed the spectrum in 
this order, the lights between blue and red had each their own specific 
effects, which were different in amount, though not in kind, on the eyes 
and on the skin. It is not known exactly how light affects chemical 
reactions, or what the chemical changes are that take place upon 
illumination, but that they are a function of the wave-lengths has 
been brought out by the present experiments. The absorption of 
the light surely plays a part, for, if the light were not absorbed, the 
reactions would not have taken place. The effectiveness of the 
several lights, however, cannot be attributed to the energy of the light 
eS eee stale tie 
