laurens: monochromatic lights. 277 



2. Reactions to Balanced Pairs of Monochromatic Lights. 



A. Lights of Different Wave-lengths. 



a. Reactions with both the Skin and the Eyes as Receptors. 



In testing the reactions of toads to balanced pairs of monochromatic 

 Hghts, the combined generators as described on p. 262-263 were used. 

 These generators were so adjusted as to deUver Hghts of different 

 wave-lengths, which entered the dark-chamber from opposite sides, 

 the toad being placed midway between them, with the lights parallel 

 in direction, but impinging on opposite sides of its body. The lights 

 employed were the same as those used in the tests of the reactions to 

 single monochromatic lights, and are described on p. 259. 



The procedure, method of orientation, etc., were the same as 

 described on p. 264-266, except that, since the lights from both genera- 

 tors were always used at the same time, after the first four trials 

 had been made, the direction of rotation of the toads was simply 

 reversed from clockwise to counter-clockwise. The orientation of 

 each toad was therefore exactly similar to that of every other toad. 

 A period of five minutes was again allowed each toad in which to react. 

 The nature of the reactions was in all cases the same as that described 

 on p. 265, except that, after the toad had turned, instead of heading 

 toward or away from the light, it headed toward one or the other of 

 the two lights. It might be mentioned that the reactions were usually 

 not quite so quick as was found to be the case with the single lights. 



The results of the tests are given in Table 7. The pairs of colored 

 lights were arranged in order, according to their distribution in the 

 spectrum. There are thus seen to be three groups of pairs of lights. 

 The first group, represented by one pair, was made up of the two lights 

 farthest apart in the spectrum, the blue and the red, the wave-lengths 

 of their middle bands differing by 192.5 /jl/jl. In the second group, 

 represented by two pairs, the lights in each pair were nearer each 

 other in the spectrum than were the blue and the red. These pairs 

 were the blue and yellow, and the green and red, the wave-lengths of 

 the middle bands of the lights of each pair differing by 145.0 fx/j. and 

 122.5 MM. respectively. The third group was represented by three 

 pairs of lights, each pair being composed of the lights that are adjacent 

 to each other in the spectrum, viz., the green and yellow, the blue 

 and green, and the yellow and red, the differences of the wave-lengths 



