HELIOTROPISM OF ANIMALS 29 
pillars of Porthesia chrysorrhea 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 zt 7s the more refrangible 
rays of the visible spectrum which are chiefly concerned in 
bringing about the orientation of the caterpillars of Por- 
thesia chrysorrheea. 
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 
Digitized by Microsoft® 
