LAURENS: MONOCHROMATIC LIGHTS. 250 
any attempt was made to measure the relative intensities — and by 
this I mean the energy contained in each light — 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, 1. 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- 
