LAURENS: MONOCHROMATIC LIGHTS. 255 



any attempt was made to measure the relative intensities — and by 

 this I mean the energy contained in each hght — of different colored 

 lights by any physical apparatus. Kniep and Minder (:09), working 

 on the assimilation curve of chlorophyl-containing plants, came to the 

 conclusion that the reason there were so many conflicting opinions 

 on this subject was that not enough attention had been paid to the 

 relative intensities of the different colored lights. They therefore 

 proceeded to measure the absolute energy of sunlight in the different 

 spectral regions by means of a Ruben's thermopile, in connection 

 with which was a very delicate galvanometer, and to adjust the light 

 until the radiant energy in the different colors was the same for each 

 region. The value of this work is unfortunately much lowered by 

 the fact that the experimenters used screens to obtain their different 

 colored lights, and therefore the dark heat rays must have been 

 present, and have had their effects. 



I shall attempt to summarize, as briefly as possible, the results of 

 the work that has been done on testing the reactions of amphibians 

 to colored lights. 



The first investigator, to my knowledge, to test the reactions of 

 amphibians to different colored lights was Graber, who in two pub- 

 lications ('83 and '84) made an exhaustive series of experiments in 

 an attempt to solve the problem as to whether, and to what extent, 

 animals were able to distinguish intensity and color differences. In 

 all of his work, Graber used colored glasses to obtain his different 

 lights, two colors being used in such a way that the animal was given 

 the choice of two compartments, each illuminated by a different 

 colored light. He tested three species of amphibians, the first of 

 these being the salamander, Triton cristatus. This he found ('83, 

 p. 220, and '84, p. 108) to be strongly negatively phototropic in white 

 light, both in the normal and eyeless condition, though less so in the 

 latter. When Triton was given the "choice" between two different 

 colored lights, Graber found that it "preferred" the less refrangible 

 rays, i. e., the red. This was so for both normal and eyeless indi- 

 viduals. He was sure that these reactions were not due to the in- 

 fluence of temperature, since he used a heat screen for the blue, and 

 none for the red light, and obtained similar results, which, since the 

 salamander is "thermophob," showed that heat was not a determining 

 factor in the reactions. He also ('84, p. 120) found that Rana escu- 

 lenta, which, according to him, was negatively phototropic in white 

 light, when exposed to different colored lights, showed a " preference" 

 for red, as compared with blue and green, and that green was "pre- 



