264 bulletin: museum of comparative zoology. 



III. Observations. 



1. Reactions to Single Monochromatic Lights. 



A. Reactions with both the Skin and the Eye as Receptors. 



In testing the reactions of toads to single monochromatic hghts 

 the combined generators, as described on p. 261-262 were used. 

 These generators were so adjusted as to give out Hghts of the same 

 wave-lengths, and screens were so arranged that one or the other light 

 could be cut off in the dark chamber. The lights employed were those 

 already described on p. 259. 



Twelve toads were successively tested in each light, and each animal 

 was given eight trials, of which four were made in the light from one 

 generator, and four in that from the other. The total number of 

 trials in each light was therefore 96. The twelve toads were always 

 brought into the experimental dark-room the afternoon before the 

 experiments were made, and care was taken that they should not in 

 any way be illuminated until they were exposed to the light in which 

 they were to be tested. They were thus subjected to a condition of 

 dark for at least fourteen hours before being tested, and at the time of 

 the test they were undoubtedly "dark adapted." After a toad had 

 been tested in a certain light, it was again placed in the dark, and was 

 not experimented with for at least an hour, after which period its 

 reactions in another light were tested. I do not think that there can 

 be any doubt that the toads were thus always "dark adapted" for 

 each set of tests. 



The procedure used in the experiments was as follows: the lights 

 from both generators were tested spectroscopically and photometri- 

 cally; after they had been found to be equal, that from generator B 

 was screened off, and that from generator A admitted to the dark 

 chamber. Toad No. 1 was then placed on the table, in the beam of 

 light from generator A, and held with its head toward the observer. 

 Before any reaction could be made, it was quickly rotated through 

 ISO degrees, so that its head w^as directed away from the observer, and 

 it was freed in this position, with the light impinging on its left side 

 and at right angles to its long axis. At each of the succeeding trials, 

 the toad was rotated clockwise, so that for the second trial it was 

 headed toward the observer, with the light impinging on its right side; 

 for the third, away from him, etc. After a total of four trials had thus 



