156 Journal oj Comparative Neurology and Psychology. 



turned about and moved in the opposite direction. When it 

 reached c^ the glowers were exposed and the colony promptly 

 changed its direction of motion and proceeded on a course directed 

 from a point between the two sources of light. This point, how- 

 ever, was much nearer the arc than the glowers, the light fromthe 



o 



a 



oU 



Fig. 14. The lines A and B represent the course taken by single colonies as seen in water 2 cm* 

 deep in the plate glass aquarium, e (the paths are represented in approximately accurate propor- 

 tions); g, a group of three 222 volt Nernst glowers in a vertical position; a, carbon arc; /, direction of 

 light rays; d, opaque screens; n n', path with glowers exposed and arc shaded ; c c', path with arc 

 exposed and glower shaded; (/ n, path with both glowers and arc exposed. 



former being much more intense than that from the latter. When 

 the light from the arc was cut off at n, the colony was found to 

 be negative to the comparatively weak light from the glowers. It 

 consequently changed its course and moved from this source; but 



