288 bulletin: museum of comparative zoology. 



toward the lights with which it was paired, was the same for the bhie 

 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 



I % 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 effecti^'eness, 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 



II % 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 



