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. 
Experiment 5,—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 
the other half 6 is in diffuse day- 
light, and is nearer to the plane 
a 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 
that the orientation of the animals is determined by the 
direction of the rays. The animal can and must follow 
the rays of diffuse light which have the direction b—a. 
If, as is customary with zodlogists, 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 what has been said, no one, I believe, will doubt 
that the direction of the progressive movements of the cater- 
Wii 
FIG. 3 
Digitized by Microsoft® 
