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ciated with the time-rate of change in illumination rather than 

 with the quantity of light. For example, on August 8 colonies 

 of Pandorina which had been in darkness four days were exposed 

 to direct sunlight at 4.40 p.m. They were inactive for about 

 one minute, then they began to swim, slowly at first and gradu- 

 ally more and more actively, and in practically every case from 

 the light. They were definitely negative. The observation aqua- 

 rium w^as now moved 4 m. from the window. Here the colo- 

 nies were strongly positive. This change in illumination was 

 repeated several times with the same results. The aquarium 

 was then exposed in direct sunlight near the window a few centi- 

 meters from the diffuse light. The colonies soon began to swim 

 rapidly from the light. The microscope was then carefully 

 moved into the diffuse light without changing the distance from 

 the window. The colonies immediately became strongl}^ positive, 

 but aft6r they had proceeded toward the window about 1.5 

 cm., they became neutral, and approximately one minute later 

 they were strongly negative again. The aquarium was now re- 

 turned to direct sunlight. The colonies remained negative. 

 They were then again moved into diffuse light where they im- 

 mediately again became strongly positive, than neutral and later 

 negative. This change in illumination was repeated many times 

 and the same results were always obtained. 



Now, the reversion from negative to positive orientation ob- 

 served in this experiment, after sudden reduction of intensity, 

 was in all probability dependent upon the time-rate of change in 

 illumination, for without any further change the colonies, in the 

 course of about one minute, again became negative. This 

 seems clearly to indicate that if the reduction had consumed 

 sufficient time there would have been no reversion. 



EFFECT OF THE AGE OF THE COLONIES ON REVERSION 



In the preceding section it was pointed out that in the experi- 

 ments on the relation between the intensity of the illumination 

 the smaller colonies of Pandorina invariably became negative 

 before the larger ones did. No observations in reference to this 



