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 



