282 bulletin: museum of comparative zoology. 



to the blue decreased as the spectral distance of the other light from 

 the blue decreased (see also Fig. 5). And if the same were done for 

 the red, it was found that the percentage of positive responses to the 

 other three lights increased as the distance from the red increased 

 (see also Fig. 4). 



Although the blue was considerably more effective than the green 

 when these two lights were paired, the percentage of responses being 

 in the ratio of 62 to 38, yet, when these two lights were paired with 

 yellow, there was only 3 % more responses to blue than to green (see 

 Table 7, and Fig. 6); and again, when they were paired with red 

 (see Table 7, and Fig. 4), there was 7 % more responses to blue than to 

 green. By referring to Table 1, it will be seen that there was a differ- 

 ence of 7% between the effectiveness of the blue and green when 

 these lights were used singly. Green was also considerably more effec- 

 tive than yellow when paired with it, the ratio of the percentage of 

 responses being 68 to 30, with two indifferent reactions. But, when 

 green and yellow were paired consecutively with blue (see Table 7, 

 and Fig. 5), the greater effectiveness of green over yellow was much 

 lessened, and there was only 9 % more responses to the green than to 

 the yellow. When these two lights were similarly paired with red (see 

 Table 7, and Fig. 4), the dift'erence in eft"ectiveness was again found 

 to be only 9 %. There was a difference of 13 % between the effec- 

 tiveness of these two lights when used singly in the production of 

 positive responses (Table 1). 



By way of summarizing the results of the experiments with balanced 

 pairs of monochromatic lights in which both the skin and the eyes 

 acted as receptors, it may be said that they were in general similar 

 to those obtained with single monochromatic lights under the same 

 conditions. Blue, green, and yellow were effective, in the order given, 

 in the production of positive responses; red produced only a very 

 slight positive response, if any at all. There were movements toward 

 both lights in any pair, with the larger percentage always toward the 

 light nearer the blue end of the spectrum. Blue was the most effective 

 stimulus in the production of positive responses, green next, and yellow 

 next, while red light had little, or no more effect than darkness. 



b. Reactions with the Eyes as Receptors. 



While the results of the tests of the reactions to single monochro- 

 matic lights, in which only the eye, and only the skin, were exposed 

 to the light showed the same general results as those obtained when 



