HELIOTEOPISM OF ANIMALS 29 



pillars of Porthesia chrysorrhcea is determined by the 

 direction of the rays of light, and not by differences in the 

 intensity of the light in different parts of space. Positively 

 heliotropic animals are compelled to turn their oral pole 

 toward the source of light and to move in the direction of 

 the rays toward this source. 



2. The dependence of orientation on the refrangibility 

 of the rays. I shall now show that it is the more refrangible 

 rays of the visible spectrum which are chiefly concerned in 

 bringing about the orientation of the caterpillars of Por- 

 thesia chrysorrhcea. 



Experiment 1. If we place the test-tube on a table and 

 cover it with a box of dark-blue glass, the animals behave as 

 if the vessel were uncovered. Without exception, they move 

 in a straight line to the window side of the vessel and remain 

 there. If instead of blue glass we use red, which to our 

 eyes seems much brighter than blue glass, no change occurs 

 in the orientation of the animals at first; after a long time, 

 however, the animals collect under the red glass on the win- 

 dow side of the vessel. In direct sunlight, however, orienta- 

 tion takes place more quickly. Exactly the same phenomena 

 are observed if an ammoniacal solution of copper is sub- 

 stituted for the blue glass, or a solution of potassium 

 bichromate for the ruby glass. This is also true in the 

 following experiments, where I may not always call special 

 attention to it. This experiment shows (1) that the more 

 refrangible rays have the same effect as mixed rays, and 

 (2) that the less refrangible rays bring about movements in 

 the same way as the more refrangible ones, only their effect 

 is less intense. The experiment also proves that it is wrong 

 to say, as do the anthropomorphists, that the animals "are 

 fond of" blue and "hate" red; for, were this true, the 

 animals should have been forced to move to the room side 

 of the test-tube when under the red glass, yet they moved 



