60 STUDIES IN GENERAL PHYSIOLOGY 



before exposing them to the sunlight I covered them with a 

 box of dark-blue glass. Within ten to twenty seconds these 

 animals had also placed their median planes sharply and 

 precisely in the direction of the rays of light, in which 

 direction they moved toward the room side. When I took a 

 third lot of fresh animals and covered them with red glass, 

 the orientation of the animals into the direction of the rays 

 did not occur. They crept to the right and to the left, occa- 

 sionally moving a short distance toward the source of light ; 

 but even after minutes under the red glass the precise orien- 

 tation of the animals, which followed under the blue glass in 

 a few seconds, did not occur. Under red glass the animals 

 behaved toward direct sunlight just as they did under blue 

 glass toward very weak <l(tijlight. That the rays which pass 

 through red glass are not absolutely without effect seems to 

 be shown by the fact that the animals avoided going to the 

 window side, and that they finally collected at the room side 

 of the board. The directing force of the red rays seems 

 therefore to be limited to this, that the animals will not 

 move for long distances toward the source of light. In con- 

 sequence, the animals must collect ultimately on the room 

 side of the vessel. 



In all the previous experiments the animals were on a 

 plane board. When at the beginning of an experiment the 

 animals were collected 011 the window side of a test-tube 

 which lay horizontal and perpendicular to the plane of the 

 window, in direct sunlight and under blue glass all the ani- 

 mals turned their oral poles within ten seconds toward the 

 room side of the tube. In about twenty seconds they 

 migrated to the room end of the tube. When the same ani- 

 mals were exposed in the same way to direct sunlight, but 

 under red glass, they neither oriented themselves nor moved 

 toward the room side of the tube during the next four 

 minutes, even though they were very restless. 



