HELIOTEOPISM or ANIMALS 39 



side. On the wall of the room opposite the window I placed 

 a kerosene lamp. At the approach of twilight, when the 

 animals began to fly about, I brought them into the middle 

 of the room, so that they were equidistant from the lamp 

 and the window, and left them alone. They flew to the 

 window. Yet, when I brought them into the immediate 

 neighborhood (within about a meter) of the lamp, they flew 

 into the flame. I repeated this experiment and convinced 

 myself that they always flew to one of the two sources of 

 light, either the window or the lamp ; to the latter, however, 

 only when they were in its immediate neighborhood. 



This experiment shows that the animals do not even pre- 

 fer artificial to the natural light, but that the artificial light 

 attracts them only when its intensity is greater than that of 

 the diffuse daylight, which is the case at night when the 

 animals are within a certain distance of the lamp, varying 

 with the intensity of the flame. The heliotropic sphere of 

 attraction of an electric arc light is therefore larger than 

 that of a candle flame, and the number of moths attracted by 

 it correspondingly greater. 



Experiment 3. It must yet be proved that it is chiefly 

 only the more refrangible rays of light which determine the 

 movements of the moths. I studied the behavior of Sphinx 

 euphorbia, which began to fly at about 9 o'clock in the 

 evening. 



The animals were contained in a large box, 40 cm. long, 

 the upper wall of which was of glass. Whenever I turned 

 the box the animals at once flew to the window side and 

 crowded against the upper glass wall through which the light 

 came. When I placed a red glass over the window side of 

 the box, the animals at once flew to the room side. They 

 collected at the edge of the red glass, but on the room side 

 of it, where they were not covered by it. Here they 

 attempted to fly upward. When I used blue glass instead of 



