LAURENS: MONOCHROMATIC LIGHTS. 263 



lights in generator A, it will be remembered, were standard lights 

 of equal intensity, these being the ones measured with the radio- 

 micrometer, and all that has been just described was done to make 

 sure that the red light in generator B was similar in energy content, 

 intensity, to that of the standard in generator A. The 3-glower 

 lamp was then substituted for the 1-glower lamp in generator A, and 

 the light adjusted until it was found to be 420—180 /jlij., as read in a 

 spectroscope. The boxes were then again so revolved that the Ughts 

 were exactly opposite. It is clear that under such circumstances the 

 radiant energy contained in the red light in generator B must be equal 

 to that contained in the blue light in generator A. The lights were 

 now in such a condition that tests of the reactions of the animals 

 could be made, the two lights being of different wave-lengths, but 

 containing the same amount of radiant energy. The special methods 

 used, and the procedure followed, will be more fully considered under 

 the section devoted to the reactions to balanced pairs of mono- 

 chromatic lights. 



2. IVIaterial. 



The common toads, Bufo americanus and B. fowleri, were the forms 

 employed in all of the experiments described in the succeeding pages. 

 No separate records were made of the reactions of the two species, 

 and there was no reason for belie\ing that they were in any way differ- 

 ent. IMost of the toads used were collected in the vicinity of Cam- 

 bridge, Massachusetts, but a few were procured from Orlando, Florida. 

 The stock was kept in a basement room, in boxes, the floors of which 

 were covered with about eight inches of earth. ^Yhen a lot of toads 

 was selected for experimentation, each individual was kept by itself 

 by being placed alone in a medium-sized battery-jar, the bottom of 

 which was covered with earth. In this way, it was possible to keep 

 track of the individual's reactions during a long series of experiments. 

 While the experiments were in progress, the toads were periodically 

 fed meal worms, which they ate readily. 



The experiments were carried on in the spring months of 1910, and 

 the fall and winter months of 1910-1911. They were all performed 

 in an experimental dark-room in the basement of the Museum. 

 The temperature of the room varied between 19° C. and 24° C, though 

 it was only occasionally that it went as high, or as low as the extremes 

 given, the average temperature being between 20° C. and 21° C. 



