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 " 



In the seventh minute - 13 



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 - 



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. 



