1 66 'Journal of Comparative ISfeurology and Psychology. 



found to be positive to an intensity of 400 candle meters. The 

 following day these organisms were exposed again to light of 1400 

 candle meters and to various lower intensities, but there were no 

 indications of negative reactions. 



I have no explanation to offer with reference to these reactions. 

 The observations were not repeated. 



17, REACTIONS ON REACHING THE OPTIMUM IN A FIELD OF LIGHT 

 GRADED IN INTENSITY. 



Oltmanns ('92) found that Volvox colonies collected and 

 remained in a given light intensity, if put into an aquarium illum- 

 inated by light which first passed through a prism such that the 

 light became gradually more intense from one end of the aquarium 

 to the other. If, however, clouds passed over the sun or if the 

 aquarium was in any way shaded, they hurried (streben) toward 

 the more highly illuminated end of the aquarium, but when the 

 clouds disappeared or the shading was removed, they returned 

 to their former positions. If the prism was put between the source 

 of light and a vessel containing Volvox which had a given direction 

 of motion, the colonies changed their direction of motion almost 

 instantly and moved toward the region of optimum intensity. 

 Oltmanns writes ('82, p. 195): "So kann man leicht constatiren, 

 dass die einzelnen Kugeln ihre urspriingliche Bewegungsrichtung 

 fast momentan verlassen und dann direct auf diejenige Region 

 im Apparat zusteueren, in welcher sie spater verweilen." 



Was the course taken in the apparatus used by Oltmanns due, 

 as he supposed, to difference in light intensity on opposite sides 

 of the organism, resulting from rays perpendicular to the sides of 

 the aquarium t It is impossible to calculate the difference in 

 intensity produced by such rays, at any given point in the appa- 

 ratus but it can be estimated with a sufficient degree of accuracy for 

 our purpose. Let x represent the intensity of the light before 

 entering the prism. Oltmanns states that 80 to 90 per cent of 

 this was absorbed at one end of the prism and 30 to 50 per cent 

 at the other. We shall assume it to have been 90 and 40, respec- 

 tively. The intensity in the aquarium then, due to rays perpen- 

 dicular to the sides, was to ^ candle meters at one end and yV ^ 

 candle meters at the other, a difference oi h x candle meters. The 

 length of the aquarium was 200 mm. The decrease in intensity 

 was, therefore, 4V0 ^ candle meters per millimeter. If the intensity 



