82 STUDIES IN GENERAL PHYSIOLOGY 
3. The dependence of the orientation on the intensity of 
the rays of light.—It is a peculiarity of all animal as well as 
plant structures that only external stimuli of a certain inten- 
sity can call forth reactions. It can easily be shown that at 
the approach of twilight there comes a time when the rays of 
diffuse daylight coming through a window no longer attract 
caterpillars of Porthesia chrysorrhcea. 
If the animals are between two sources of light of differ- 
ent intensities, that having the greater intensity is the more 
effective. This can easily be shown by bringing the animals 
into a room into which light enters from opposite directions. 
Other conditions being the same, the animals move to the 
window nearest them. A maximum limit for the intensity 
of the light cannot be established, as direct sunlight is in 
itself effective. Artificial sources of light above a certain 
intensity and containing the more refrangible rays affect the 
animals in the same manner as the natural sources of light. 
In a dark room caterpillars are attracted by a kerosene flame 
as markedly as moths; the caterpillars, however, are not 
burned, because they move so slowly that they have time to 
turn back before the zone of fatal temperature is reached. 
Such animals as are attracted by direct sunlight may also be 
attracted by the candle flame, exactly as is the case in posi- 
tively heliotropic plants. 
4. At a constant intensity light acts as a continuous 
source of stimulation.—If the test-tube which is placed with 
its longitudinal axis perpendicular to the window is left 
undisturbed, the animals remain permanently on the side 
nearest the window. Under these conditions we can also 
safely open the room side of the vessel without a single 
animal changing its position or escaping from its cage. It is 
remarkable, however, that when the test-tube has been left 
undisturbed all day, the animals keep.their position during 
the night. In this way I have kept animals for several days 
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