HELIOTROPISM OF ANIMALS 25 
process is repeated, the animals creeping to the window side 
of the glass just as before. If, however, the position of the 
glass remains unchanged, the animals remain permanently 
crowded together on the window side of the test-tube. 
Experiment 2.—If the test-tube is laid on the table with 
the longitudinal axis parallel to the 
plane of the window, the animals Ft 
gradually scatter uniformly over the 
whole of the upper part of the tube. 
The lower portion of the vessel is in x 
consequence again free from animals. 
If the longitudinal axis of the test- 
tube lies at even a slight angle with 
the plane of the window, the animals 
move to the end of the tube nearest 
the window, and remain there in their FIG. 1 
customary position. 
Experiment 3.—The test-tube is placed perpendicular to 
the plane # of the window, and at the beginning of the 
experiment the animals are collected at the window side B 
of the test-tube (Fig. 1). That half of the vessel which lies 
nearest the window is now covered with an opaque paste- 
board box, AK. The following then occurs: The animals 
soon appear at A on the room side of the pasteboard box ; 
as soon, however, as they emerge from the box K into A, 
they turn about, direct their heads toward the window, move 
to the edge of the pasteboard, and remain at the boundary 
between the covered and the uncovered portions of the tube, 
at A and especially at the top of the test-tube. The remark- 
able thing is that they are not distributed evenly over the 
whole brightly illuminated part of the test-tube. The 
explanation is as follows: As soon as the animals near the 
window at B are covered by the pasteboard, the weak rays 
of light reflected from the walls of the room fall upon them. 
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