48 STUDIES IN GENERAL PHYSIOLOGY 
tube when diffuse daylight entered through the window. 
The more refrangible rays reflected from the walls of the 
room were more effective than the rays from the window 
which had passed through the light-red glass. This pre- 
vented the animals from going under the red glass. But 
when I used direct sunlight, the animals moved under the 
red glass to the window side of the vessel and remained 
there. When the animals were collected at the room side of 
the test-tube lying horizontally on a table and with its lon- 
gitudinal axis perpendicular to the plane of the window, and 
an opaque cover was placed over the room side of the tube, 
while a dark-red glass was placed over the rest of the tube, 
the animals went under the red glass and gradually collected 
there on the window side of the tube. But when I placed 
the opaque cover over the window side and the red glass over 
the room side of the tube, and the animals were under the 
opaque cover at the beginning of the experiment, they did 
not collect under the red glass. The rays reflected from the 
wall of the room had lost their directing power in filtering 
through the red glass. The experiments were made in the 
diffuse light of a dark day. On a bright day the animals 
moved to the room side of the tube under the red glass. 
The rays which pass through red glass have therefore the 
same effect, only they are weaker than the rays which pass 
through blue glass. 
I have already mentioned the fact that the day Lepi- 
doptera begin to fly as soon as direct sunlight falls upon 
them, while in diffuse light their heliotropic movements con- 
sist chiefly in creeping. The same difference in the effects 
of different intensities of light can be easily demonstrated 
in winged plant lice. In diffuse light of low intensity they 
move forward by creeping; when brought into the sun they 
Hy. 
To obtain a measure of the difference in the activity of 
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