18 STUDIES IN GENERAL PHYSIOLOGY 
There are two methods by which the second fact, that only 
the more refrangible rays bring about orientation, can be 
proved, namely, by experimenting with prismatic spectra or 
with colored screens. 
All authors who have studied the behavior of plants 
behind colored screens have obtained the same result—that 
it is only, or more especially, the more refrangible rays which 
are heliotropically active. Studies on the behavior of plants 
in prismatic spectra have led to harmonious results, in so far 
as they confirm the gross results obtained by using colored 
screens; yet opinions differ as to the efficacy of the more 
limited portions of the spectrum. Since for the present I 
wish to show only that the laws governing the orientation 
of an animal toward light correspond to the laws governing 
the orientation of plants toward the same stimulus, it was 
necessary to use as a basis the really established data of plant 
physiology, and I therefore shall confine myself to the proof 
of the fact that the more refrangible rays of the spectrum 
are exclusively, or almost exclusively, effective. To do this 
I proceeded as is usual in plant physiology. In order to 
have only the less refrangible rays act on the animals, I 
passed the diffuse daylight through a solution of potassium 
bichromate or ruby glass; to study the influence of the more 
refrangible rays, I chose cobalt glass or an ammoniacal solu- 
tion of copper. The screens were examined spectroscopically. 
The dark-red glass which I used completely absorbed the 
more refrangible rays, and let through only the red, yellow, 
and a part of the green rays. The dark-blue glass absorbed 
the less refrangible red and yellow and a part of the green 
rays, with the exception of a small region in the outer red. 
Since, however, the heliotropic phenomena appear only 
weakly or not at all behind dark-red glass, while they occur 
just as in diffuse daylight behind dark-blue glass, the few 
red rays which penetrate the dark-blue glass cannot be 
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