140 ^Journal of Comparative Neurology and Psychology. 



change in the grade of intensity caused an average increase in 

 deflection of if degrees. 



Cause of Deflection Toward the More Strongly Illuminated Side 

 in Graded Light. — If a colony of Volvox deflects to the right in 

 light of uniform intensity it will deflect more in a field of light 

 graded in intensity, provided the more highly illuminated end 

 of the field is to the right, but not as much if this end is to the 

 left. This fact is clearly expressed in Fig. 8. Under the condi- 

 tions of the experiments described above, this diff'erence in deflec- 

 tion must have been primarily due to one of three factors: (i) 

 diff^erence in total light intensity under the two conditions; 

 namely, with the more highly illuminated end of the field to the 

 right and with this end to the left; (2) refraction or reflection as 

 the light passes through the aquarium; (3) diff'erence in light 

 intensity on opposite sides of the colony. A discussion of these 

 three factors follows. 



1. We have demonstrated (see Table IV) that an increase in 

 light intensity, without change of grade, causes a decrease in 

 deflection. Now, as represented in Fig. 8, the colonies, as they 

 deflect in crossing the aquarium with the brighter end of the field 

 to the right, gradually pass into regions of higher light intensity, 

 but when the brighter end of the field is to the left, they gradually 

 pass into regions of lower intensity. This consequently tends to 

 cause a decrease in deflection under the former conditions and an 

 increase under the latter, but the angle of deflection is greater 

 under the former condition than under the latter. The diff^erence 

 in deflection under the two conditions, therefore, cannot be due to 

 the higher light intensity to which the organisms are exposed when 

 the more strongly illuminated end of the field is to the right than 

 when it is to the left. 



2. As the light passes through the glass wall of the aquarium 

 and the water in it, some is reflected and some refracted thus 

 producing lateral rays. This reflection and refraction cannot be 

 entirely eliminated even with the utmost precaution. May not 

 these lateral rays have been of suflBcient intensity to cause deflec- 

 tion toward the brighter end of the field as was found to be true 

 in case of Oltmanns' apparatus ?' 



^ Oltmanns ('92) produced a field of light graded in intensity by placing a hollow prism filled with 

 a mixture of gelatine and India ink between the source of light and the aquarium. He assumed that 

 the rays in the aquarium were all perpendicular to the wall facing the source of illumination. This, 

 however, is not true, for the particles of ink in the prism disperse the light before it gets into the aquarium. 



