30 STUDIES IN GENERAL PHYSIOLOGY 



toward the window. The animals neither "are fond of" 

 blue nor "hate" red, but they are like plants, simply 

 positively heliotropic, and the blue rays are more effective 

 heliotropically than the red. There is, as I shall state 

 here once for all, no difference in direction between the 

 movements called forth by blue light and red light ; there 

 is only a difference in the velocity and precision with which 

 these heliotropic movements take place. 



Experiment 2. The longitudinal axis of the test-tube is 

 again perpendicular to the plane of the window. The small 

 caterpillars are at the beginning of the experiment on the 

 room side of the tube. The window half of the test-tube is 

 covered with dark-blue glass. The experiment goes on as if 

 the tube were uncovered; the animals move to the window 

 side of the test-tube, where they remain under the blue 

 cover. If the same experiment is repeated, only so that the 

 blue cover is placed over the room side of the test-tube, the 

 animals again move to the winjdow, where they remain. The 

 experiment proves that the more refrangible rays alone have 

 the same effect as mixed light ; and the fact that the animals 

 leave the uncovered portions of the test-tube to creep under 

 the dark-blue cover corroborates what has already been said, 

 that positively heliotropic animals move in the direction of 

 the rays of light even when in so doing they pass from a 

 place of greater intensity of light to one of less intensity. 



Experiment 3. The test-tube again lies horizontally, 

 with its longitudinal axis perpendicular to the window. At 

 the beginning of the experiment the animals are on the 

 window side of the test-tube. If the window half of the 

 tube is covered with red glass (which may seem much 

 brighter to us than the blue glass of the previous experi- 

 ment), immediately after the red glass has been placed over 

 the animals they appear on the room side of it, and collect 

 at the boundary between the covered and uncovered parts of 



