26 STUDIES IN GENERAL PHYSIOLOGY 
The animals follow the path of these rays and arrive at the 
uncovered portion of the tube. As soon, however, as the 
strong rays of diffuse light fall upon them at A, they turn 
about and direct their heads toward the window, until they 
come again under the pasteboard which shuts out the diffuse 
light. They are then again attracted by the light of the 
room, and so on, until they come to rest at the boundary 
between the two regions at A. 
At the beginning of the experiment, before the animals 
stop moving it can really be seen that they are driven around 
in a narrow circle. 
If at the beginning of the experiment the animals are 
collected, not on the window side, but on the room side of 
the test-tube at C, they move toward the window until they 
reach the pasteboard at A. If the tube is pulled away from 
the window for some distance, while the pasteboard remains 
stationary, the animals begin to move, until they reach the 
edge of the pasteboard. 
If the tube is placed horizontally with the longitudinal 
axis parallel to the window, the animals distribute themselves 
over the whole length of that portion of the tube which is 
not covered by the pasteboard, collecting, however, always 
on the window side of the tube. 
According to the prevailing views of zodlogists and ani- 
mal physiologists, the movement of caterpillars toward the 
light is determined by the animals’ “fondness for light.” 
They, therefore, move from a region of less intense light to 
one of greater intensity. That the essential feature, how- 
ever, is the direction of the rays, and not a difference in 
their intensity,’ is evident from the following experiments. 
Experiment 4.— The animals are in a glass cylinder a, 
some 38cm. in diameter. Light can enter it from all sides 
(Fig. 2). The inside of a second test-tube b, which has the 
1Jn different parts of the tube. [1903] 
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