SENSIBILITY OF VOLVOX TO SPECTRAL LIGHTS 349 



Between the ocular end of the telescope of the spectrometer 

 and the small glass aquarium there was placed a shutter with 

 stops for 1, 0.5, 0.2, and 0.04 seconds. For exposures of longer 

 duration than one second, the 'bulb' was used and timed by a 

 stop-watch. The aquarium was put in place on the microscope 

 stage, so that the band of spectral light impinged on one wall, 

 and the white light on the opposite wall, the shutter was then 

 closed and a Volvox colony placed in the aquarium. Being 

 stimulated by the white light it moved toward its source, that is, 

 into the portion of the trough of the aquarium farther away 

 from the source of the spectral light. The white light was then 

 screened and the colony observed for a moment or two from 

 above and from the side to make certain that there was no hori- 

 zontal movement. It was easily seen that some of the colonies, 

 after forward motion was thus stopped, swam slowly upward, 

 while others hung apparently motionless. 



The colony was then exposed to the spectral light by opening 

 the shutter set for the shortest exposure, or for one which it was 

 reasonably certain would have no effect. If no reaction fol- 

 lowed, the organism was given the same exposure again, and 

 usually a third time, allowing adequate intervals of time and 

 taking precautions that no fortuitous horizontal movement 

 either in the direction of or away from the source of light was 

 taking place at the moment of exposure. The duration of the 

 exposure was then increased until a reaction — movement toward 

 the source of light — was obtained. In the more effective Ughts 

 the colonies would often continue to the end of the trough, 

 although, as described, they had been exposed to the light for 

 only the short presentation time, which was succeeded by the 

 reaction time. Ten colonies were used for each light and each 

 colony exposed several times and the results averaged. They 

 are shown in table 1, column 3, the figures representing the 

 average minimal duration of exposure below which no effect is 

 produced. 



It is now well established that in order for light to produce a 

 definite degree of effect the time required is inversely propor- 

 tional to the intensity of the light The primary photochemical 



