132 'Joiumal of Comparative Neurology and Psychology. 



The direction of the ray may be the important factor in orientation 

 irrespective of difference of intensity of light upon different parts 

 of the organism, as has been maintained by Sachs for the photo- 

 tropic movements of plants. I am not ready to adopt the theory 

 of Sachs, but I feel that it is a view that is not entirely out of court." 



The following experiments on the movement of Volvox when 

 exposed to light from two different sources, and on the orientation 

 of Volvox in light graded in intensity seem to me to settle this ques- 

 tion conclusively. 



On August 18, 1904, a single 222 volt Nernst glower was fixed 

 in a vertical position 70 cm. from the middle of the plate glass 

 aquarium, so that the lower end of the glower was level with the 

 bottom of the aquarium and the rays perpendicular to the side at 

 a point 4 cm. from one end. A single no volt glower was arranged 

 like the 222 volt glower, but in such a position that the light rays 

 were perpendicular to the end of the aquarium at the middle and, 

 therefore, perpendicular to the rays from the 222 volt glower at a 

 point 4 cm. from the end, and halfway between the two sides, as 

 represented in Fig. 7. The 222 volt glower was stationary, but 

 the no volt glower could be moved to any desired distance from 

 the aquarium. These glowers were both carefully screened so 

 that the only light which escaped passed through a rectangular 

 slit a trifle larger than the glower. The side and end of the aqua- 

 rium facing the glowers was also screened, with the exception of 

 an opening one centimeter wide and six centimeters long, at the 

 bottom of the aquarium, as indicated in Fig. 7. The aquarium 

 contained thoroughly filtered water 1.5 cm. deep. Thus, practically 

 all reflection from the sides of the aquarium and the surface of the 

 water was eliminated. 



The direction of movement of Volvox was ascertained, first with 

 the 222 volt glower exposed alone, then with both glowers exposed, 

 the 222 volt glower 66 cm. from the side, and the no volt 

 glower 24, 49, 99, and 199 cm. from the end of the aquarium. 

 In order to ascertain the direction of motion under the various 

 light conditions, a considerable number of colonies were carefully 

 dropped into the corner of the light area farthest from the glowers. 

 Among the specimens used in this experiment there were about 

 as many that deflected to the right as to the left, so that when one 

 gloweronly was exposed the center of the group of colonies moved 

 across the aquarium practically parallel with the light rays. Sev- 



