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 window, 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 
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