288 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
toward the lights with which it was paired, was the same for the blue 
and for the green, and only 3 % less for the yellow (see also Fig. 4). 
This did not point, as might be supposed, to the ineffectiveness of 
the red light on the skin alone when paired with others, for that had 
already been shown to be the case not only for the eyes and the skin 
when both were exposed to the light, but also for the eyes alone; 
it did show, however, that the sensitiveness of the skin to differences 
in wave-lengths, at the blue end, when it was exposed to balanced 
pairs of monochromatic lights, was much lower than that of the eyes, 
or of the eyes and the skin together. This fact was further brought 
out when yellow was paired with blue and green. There was only 
1 % more of responses to blue than to green when paired with yellow 
(see also Fig. 6), and only 4 % more to blue than to green when these 
lights were directly paired, the percentage of responses to each being 
in the ratio of 52 to 48. When blue and green were used singly, they 
were also near in their effect on the skin (Table 3). 
The skin, however, did show some differences in sensitiveness to 
lights of different wave-lengths. If the pairs of lights in which blue 
occurred be considered, it will be seen that the percentage of responses 
to the blue decreased as the spectral distance of the other light from 
the blue decreased, until, in the pair blue and green, the movements 
to each light were practically the same, there being only a difference 
of 4 % between them (see also Fig. 5). Green when paired with yellow, 
however, showed a considerably greater effectiveness, the ratio of 
the percentage of responses being 62 to 38; also when these two lights 
were paired with the blue (see also Fig. 5), there was a difference of 
11 % between them, though, when they were paired with red (see 
also Fig. 4), there was a difference of only 3 % between them, which 
was, of course, due in greater part to the ineffectiveness of the red 
light. 
By way of summarizing the results of the experiments with balanced 
pairs of monochromatic lights in which only the skin acted as a recep- 
tor, it may be stated that the results do not correspond entirely with 
those obtained when either the whole body, or only the eyes, were 
exposed to balanced pairs of monochromatic lights, or when the skin 
only was exposed to single monochromatic lights. While blue, green, 
and yellow were effective in the order in which they are given in the 
production of responses, the difference in effectiveness between blue 
and green was hardly noticeable, though that between green and 
yellow was considerable. Red, again, shows itself to be but little 
more effective than darkness. The sensitiveness of the skin to lights 
