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 zoologists 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, 1 is evident from the following experiments. 



Experiment 4. The animals are in a glass cylinder a, 

 some 3cm. in diameter. Light can enter it from all sides 

 (Fig. 2). The inside of a second test-tube 6, which has the 



1 In different parts of the tube. [1903] 



