262 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
shift the lamp until the wave-lengths read 420-480 yy. This was 
then repeated for the light proceeding from generator B. From what 
has been said, the radiant energy in the blue light from generator B 
must now be approximately equal to that in the blue light from genera- 
tor A. The boxes were then so revolved on the pivots (P and P’, Fig. 
1), that the light rays from the two were parallel and opposite. A 
photometer placed midway between the two boxes in the path of the 
lights then showed that the intensity of the two lights was equal. 
The lights were now in such a condition that the animals could be 
introduced for tests. Under the separate sections the special methods 
used and the procedure of the tests themselves will be more fully given. 
It may seem to have been unnecessary to have gone to all of this 
trouble, when the wave-lengths of the lights from generator A could 
have been read, and the animals tested in this. But the use of two 
lights of the same wave-lengths from opposite sources, lessened the 
chance of experimental error. Under the description of the reactions 
to single monochromatic lights, it will be shown how the method of 
rotation and the use of the two lights from opposite sources were 
combined to offset the influence of compensating movements upon 
the reactions. The procedure for the blue, just described, was 
repeated for each of the other three sets of colored lights. 
When the reactions to balanced pairs of monochromatic lights 
were tested, the procedure, while in the main the same as that just 
described, differed somewhat from it. Here there were two lights 
from different sources, midway between which the animal was placed, 
so that the lights impinged at right angles upon opposite sides of its 
body. Further, the lights were of different wave-lengths, except in a 
few experiments that were carried out to check the equality of the 
lights from the two light-generators. For the sake of clearness in 
explaining the procedure, a concrete case again will be taken. Sup- 
pose, for example, that it was desired to test the reactions of a lot of 
toads in blue and in red light. The single-glower lamps were placed in 
both boxes, lighted, and the appropriate diaphragms placed in position. 
The light proceeding from generator A was then scrutinized in a 
spectroscope, and the lamp adjusted until it read 630-655 yu, which 
were the wave-lengths of the red light used. The same process was 
repeated for generator B. The energy contained in the two red lights 
was then assumed to be the same. The boxes were then so revolved 
on the pivots (P and P’, Fig. 1), that the light rays from the two 
were parallel and exactly opposite. A photometric reading showed 
that the intensity of the two lights was approximately equal. The 
