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 difference in effectiveness 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 
