354 HENRY LAURENS AND HENRY D. HOOKER, JR. 



of the effects of exposures longer than the presentation time, 

 which Hecht describes. In any case, the date given in experi- 

 ment 9.4 (Hecht, '18, p. 153) and figures 1 and 2 (Hecht, '19 b, 

 pp. 659, 660) are insufficient, and the experimental error is too 

 great to justify the setting aside of the Bunsen-Roscoe law. 



b. Determination of the relative rate of locomotion and precision of 



orientation 



The same general procedure was followed in the determination 

 of the relative rate of locomotion as was described for the deter- 

 mination of the presentation time. A colony of Volvox was 

 placed in the trough of the small glass aquarium. 'The spec- 

 trometer was set to deliver wave-lengths of a certain distribution, 

 but the aquarium screened so that the organism was not exposed 

 to their action. By means of the white light of equal radiant 

 power the colony was stimulated to swim to the end of the trough 

 away from the source of the spectral light the stimulating value 

 of which was to be tested. After, or just before, the colony 

 reached the ends of the trough, the white light was screened and 

 the organism, as it stopped forward motion and hung motion- 

 less or moved slowly upward, was observed by the light from the 

 ruby glow-lamp. The colony was then exposed to illumination 

 by the spectral light. At the first indication of movement toward 

 the source of light a stop-watch was started, and when the colony 

 reached the other end of the trough it was stopped. The time 

 it took the colony to swim the distance of the trough was thus 

 obtained. As the colony reached the end of the trough, the light 

 being tested was turned off, and the colony observed for a few 

 seconds as it again hung motionless or moved slowly upward. 

 The white light was then turned on and the time it took the col- 

 ony to reach the other end (the end nearer the source of white 

 light) was taken. The reciprocal of the ratio of these two ' times ' 

 was taken as the index of the relative stimulating value of the 

 wave-lengths in question. By taking in this way alternately the 

 times required to swim toward the source of the white light and 

 toward the source of the spectral light, the stimulating value of 



