REACTIONS OF AMEBA TO LIGHT. 59 



pod had passed the light, a new pseudopod was thrown out on 

 the left near the light 309. This pseudopod moved straight 

 forward for some distance, when another pseudopod was sent 

 out on the left 312 but this one was finally retracted as the 

 ameba moved away. 



Orange spectral light induced positive reactions in the ameba 

 of this series of experiments, though they were wholly positive 

 only in the first experiment. In the other experiments the ten- 

 dency was toward positive behavior, but the source of stimulation 

 was not definitely sought. The beam of light attracted the ameba 

 only mildly after the first encounter, and the tendency to move 

 forward (Schaeffer, 'i4a) may be presumed to have been about 

 as strong as the tendency to move toward the beam, hence the 

 partial encircling of the beam in the last two experiments. In 

 my laboratory notes there is recorded one experiment with 

 orange light in which the behavior was wholly negative. 



EXPERIMENTS WITH DARK BEAMS. 



When it was seen that white light and spectral light of various 

 wave-lengths had essentially the same effect on ameba, it seemed 

 likely that these results were due to differences in intensity 

 between the beam of light and the diffuse light on the field. The 

 suggestion then presented itself w r hether a decrease in intensity 

 of light in a small area produces a similar result. A dark beam 

 was therefore projected into the microscope. The source of the 

 dark beam was a hole in the screen, leading into a blackened light 

 tight box fastened to the back of the screen. The sides of the 

 hole were blackened to prevent as far as possible the reflection 

 of light. The rest of the screen was illuminated by diffuse 

 light, as in the other experiments, but more brightly so as to 

 increase the contrast between the field and the hole. The hole 

 as viewed through the microscope appeared as a very dark gray 

 spot. 



A proteus was shifted so that the dark spot lay directly ahead 

 of the ameba 314. The tip of the ameba broke up into two 

 pseudopods, one of which turned to the right and the other to 

 the left of the dark spot, indicating a negative reaction. As the 

 right pseudopod moved forward it turned to the left until it 



