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 



Ah 



To obtain a measure of the difference in the activity of 



