28 



STUDIES IN GENERAL PHYSIOLOGY 



whole length of the tube, remaining, however, upon its sunny 

 side. Orientation takes place more quickly in direct sun- 

 light than in diffuse daylight. 



l\.i-j>erhnent o. A small pencil SS of direct sunlight is 

 allowed to fall on a table obliquely to the plane of the win- 



dow through the window F (Fig. 

 3). Rays of diffuse daylight fall 

 upon the remaining portions of 

 the table. If at the beginning of 

 this experiment all the animals 

 are at the end a of the test-tube 

 -which is so placed on the table 

 that a is in direct sunlight, while 

 tin- other half l> is in diffuse day- 

 light, and is nearer to the plane 

 of the window than a the fol- 

 lowing occurs: 



The animals move from a 

 through the pencil of direct sunlight into b, which lies in 

 the diffuse daylight, where they remain at the cup of the 

 test-tube. They pass from the direct sunlight into dif- 

 fuse daylight without even attempting to return into the 

 sunlight. 



This experiment can be explained only by the assumption 

 tliat ihe orientation of the annual* /s determined by the 

 direction of the ran*. The animal can and must follow 

 the rai/s of diffuse liyht which hare the <lirection h-*a. 

 If, as is customary with zoologists, we believed that these 

 animals love the light or, more correctly, that they prefer 

 the more intense light --it would be impossible to see why 

 they do not remain in the direct sunlight, or at least why 

 they do not hesitate to go into the diffuse light. 



From irhat has been said, no one, I believe, will doubt 

 that the direction of the progressive movements of the cater- 





