HELIOTROPISM OF ANIMALS 89 
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 
euphorbie, 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 
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