266 bulletin: museum of compakative zoology. 



reaction, which I have called " indifferent," that is, locomotion straight 

 ahead, without apparent reference to the light. 



There was some slight inconstancy in the reactions of the same and 

 of different individuals. A given individual seldom showed con- 

 sistently the same percentages of responses to the different lights in 

 the different sets of trials. This inconstancy in the reactions was 

 very noticeable in not more than two or three individuals, though 

 it was very slightly present in several. The precise factors upon 

 which this inconstancy depended were not ascertained, but they were 

 probably functions of the physiological states of the animals, changing 

 slightly from time to time, and therefore changing the nature of the 

 response in any given individual. 



The results of the first series of tests are given in Table 1 . Three 

 sets of twelve toads each were tested in the way described, the total 

 number of trials for each light being, therefore, 288. The same 

 twelve toads were used for the first two sets, and the tests for these 

 followed immediately upon each other. A second lot of twelve toads 

 was, however, selected for the third set of tests. The results with 

 this second lot of twelve toads were so similar to those obtained with 

 the first, that they are not given separately in the Table, but together 

 with the other two sets. 



It will be seen, by referring to Table 1, that all the lights used 

 produced more positive responses than negative. Thus, in the blue 

 light, out of 288 trials, 251 were positive; in the green, 230; in the 

 yellow, 194; and in the red, 167. A comparison of the effectiveness 

 of the four lights can be made more easily if the percentages of positive 

 responses are considered, rather than the actual numbers. This 

 effectiveness was greatest for the blue light (87 %), and decreased for 

 the other lights in the order of the spectrum. The green light (80 %) 

 was, however, not very much less effective than the blue. The 

 decrease in eft'ectiveness between the green and yellow lights was 

 greater than that between the blue and green, the number dropping 

 in yellow to 67 %. The red light was again less effective than the 

 yellow, the percentage of positive responses being only 58 for this 

 light. This percentage of positive responses in red light, if decreased 

 only 8, would make the percentage of positive and negative responses, 

 under this stimulus, equal; that is, the percentage of movements 

 toward the dark would be as great as that toward the light. The fact 

 that there were 58 % of positive responses, and only 42 % of negative 

 responses, shows clearly, however, that the red light must be regarded 

 as more effective than darkness in the production of responses in toads. 



