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 effective — 

 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 lights. 



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 wliich 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 dift'erent 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 



