Il6 LIGHT AND THE BEHAVIOR OF ORGANISMS 



The light intensity from each of the two glowers was equal. 

 The direction of the rays could therefore, without any alter- 

 ing of the intensity, be changed by alternately intercepting 

 the light in each of the two beams. If the ray direction is 

 thus changed after the Stentors are oriented in one of the 

 beams of light, one side will of course be directed toward 

 the light. If it chances to be the aboral side and the 

 Stentors are not very strongly negative, they continue on 

 their course just as though the direction of the rays had 

 not been changed, until in the process of rotation the oral 

 side comes to face the light; then the organism responds in 

 one of two ways: it may stop suddenly and sometimes back 

 a little and turn sharply toward the aboral side; that is, it 

 may respond with the avoiding reaction, or it may merely 

 swerve farther from the source of light on its spiral course 

 as represented in Fig. 14. When the oral side again comes 

 to face the light the organism is again stimulated and it 

 again swerves farther from the source of light. This reac- 

 tion is repeated once during each rotation until the oral 

 side is nearly equally exposed to the light throughout the 

 entire rotation. This is evidently true when the anterior 

 end is directed away from the source of light. If the or- 

 ganism responds with the avoiding reaction it turns more 

 directly from the source of light and thus becomes more 

 rapidly oriented, as represented in Fig. 14. 



Why does Stentor respond when the oral side faces the 

 light and not when the aboral side faces it in the same 

 intensity? If Stentors are oriented in light of a given 

 intensity and the intensity is decreased without any change 

 of ray direction there is no response; but if it is increased 

 they respond in one of two different ways, depending upon 

 the amount of increase. If the increase is relatively slight 

 they merely swerve more strongly toward the oral side; and 

 since this side always faces out when the organism swims 

 on its spiral course, the result is that the course is made 

 wider. If the intensity increase is greater the creature 

 stops suddenly, turns toward the aboral side, sometimes 



