HELIOTROPISM OF ANIMALS 81 
the tube. If at the beginning of the experiment the animals 
were on the room side of the test-tube, they move until they 
reach this boundary. 
We therefore get the same results by using red glass 
that we got by using opaque pasteboard in a previous 
experiment. Taken together with the preceding ones, this 
experiment proves that pre-eminently the more refrangible 
rays of mixeddaylight are heliotropically effective. Although, 
as we have just seen, the rays passing through red glass or a 
red solution are not absolutely ineffective, yet the weak light 
which is reflected from the walls of the room, and which 
contains some blue rays, is more effective than the diffused 
light reflected from the sky after it is filtered through red 
glass. It is for this reason that the animals on the window 
side under the red cover migrate to the boundary of the red 
screen where they are held by the rays of diffuse daylight. 
Experiment 4.—If, as before, we place the test-tube with 
the longitudinal axis perpendicular to the window, and cover 
it with red glass on the window side and with blue glass on 
the room side, the animals collect under the blue glass at its 
boundary with the red glass. 
Experiment 5.—If we place the test-tube with its longi- 
tudinal axis parallel to the window, the animals scatter over 
the whole length of that part of the tube which is covered by 
blue glass. 
From all these experiments tt follows that it is chiefly the 
more refrangible rays which determine the orientation of 
the caterpillars of Porthesia chrysorrhea toward light. 
The only difference between the heliotropism of these 
animals and the heliotropism of plants is this, that the less 
refrangible rays are not so completely imeffective im the 
case of the caterpillars of Porthesia chrysorrhoea as they 
apparently are in many plants. This point must, however, 
be studied more accurately with the aid of a spectrum. 
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