284 bulletin: museum of comparative zoology. 



toads each were tested, making a total of 384 trials in each single pair 

 of lights or 2304 trials in all. One of the sets of twelve toads were the 

 remainder of the lot used for the first three sets of reactions, when 

 both the eyes and skin acted as receptors, though seven individuals 

 died during the course of the experiments, and their places had to 

 be supplied by seven new toads. The other lot employed was one of 

 the new lots selected for the last two sets of tests in the same series 

 of experiments. 



By referring to Table 8, it will be seen that the results were, in 

 general, the same as those obtained when the whole body was exposed 

 (Table 7). In any pair of lights there were movements toward both 

 lights, with the larger percentage of responses to the blue, or to the 

 light of a given pair which, in the spectrum, is nearer the blue. There 

 were only a few more movements to red light, however, in the pairs 

 with green and yellow, than there were negative responses to these 

 lights, when they were used singly; though, when paired with blue, 

 there was 9% more (Table 2, and Figs. 3 and 4). Red light under 

 these circumstances was, therefore, not much more effective than 

 darkness in the production of responses. 



When blue and green were paired, the percentage of responses to 

 the blue was considerably higher than that to the green, the ratio 

 being 62 to 38. But, when blue or green was paired with yellow 

 (see also Fig. 6), there was only 4 % more responses to blue than to 

 green; and when paired with red (see also fig. 4), only 6 % more. 

 There was 1 1 % more responses to blue than to green when they were 

 used singly (Table 2). Green also was considerably more effective 

 than yellow when these two lights were paired, the percentage of 

 responses to them being in the ratio of 66 to 34. But when paired 

 with blue consecutively (see also Fig. 5), the difference in effectiveness 

 was only 8 %, and when paired with red (see also Fig. 4), also only 8 %. 

 The difference in effectiveness between green and yellow, when used 

 singly, was 9 % (Table 2). 



By way of summarizing the results of the experiments with balanced 

 pairs of monochromatic lights in which only the eyes served as recep- 

 tors, it may be said that they were essentially the same as when both 

 the skin and the eyes acted as receptors, and in the main, the same as 

 those obtained with single monochromatic lights, when only the eye 

 was exposed. Blue, green, and yellow were effective in the production 

 of positive responses, but red, in comparison with any other color, 

 never showed this. Blue was the most effective stimulus, and red 

 the least, and the decrease in effectiveness followed the order of the 



