HELIOTROPISM OF ANIMALS 49 
rays of different intensities and refrangibilities, I measured 
the time it required for the animals in a test-tube to pass a 
line scratched in the glass, when moving under the influence 
of light from the room side of the tube to the window. 
In these experiments I used some sluggish winged plant 
lice which I had taken from a plum tree. As a rule, the 
heliotropic movements of plant lice took place much more 
quickly than in the experiment to be described here. The 
experiment was made in diffuse light on August 8, 1888. 
At the beginning of the experiment all of the animals were 
on the room side. 
The animals passed the marks as follows: 
In the first minute 11 animals 
In the second minute 17 
In the third minute 19 “ 
In the fourth minute 21 . 
In the fifth minute 10 - 
In the sixth minute 12 xe 
In the seventh minute 13 es 
Only three animals had at this time not yet crossed the 
line. Several minutes later, at 9:20 o’clock, when the sun 
was coming through a fleecy white cloud, I made the following 
experiment in direct sunlight with the same animals. At 
the beginning of the experiment the animals were again on 
the room side of the tube. 
The animals this time passed the mark as follows: 
In the first minute 31 animals 
In the second minute 36 “& 
In the third minute 23 « 
Half a minute later the last sixteen animals had also passed 
the mark. The velocity of the movement was twice as great 
in direct sunlight as in ordinary daylight. These experi- 
ments were repeated and gave practically the same results. 
At 10:17 I placed the animals under a dark-blue glass. 
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