348 HENRY LAURENS AND HENRY D. HOOKER, JR. 



effect, the results obtained from the lights in which the organ- 

 isms were visible being the same when it was lighted as when it 

 was not. 



MATERIAL 



Volvox globator was found in the spring and summer and 

 early fall of 1917 in Mill River near New Haven. Material was 

 collected two to three times a week, and kept fresh and cool by 

 placing the glass vessel containing the colonies in running water 

 in a battery jar. The colonies were dark-adapted for at least 

 an hour before exposure to the light, the stimulating effect of 

 which was to be tested. Only photopositive colonies were used. 



EXPERIMENTAL 



The reactions, orientation, etc., of Volvox to white light have 

 been carefully observed and analyzed (Holmes, '03; Mast, '07, 

 '11). We are not primarily interested at the present time in 

 the problem of orientation, that is, as to whether Volvox orients 

 directly or indirectly; nor in the question as to whether the 

 response to light is occasioned by a change in light intensity 

 (time rate of change) or by the continuous action of light, al- 

 though our results have a bearing on these questions, particu- 

 larly the latter. 



The experiments may be divided into two main parts: a) those 

 dealing with the determination of the presentation or action 

 time, and, b) those dealing with the determination of the relative 

 rate of locomotion. 



a. Determination of the presentation or action time 



By the presentation time is meant the minimum time for which 

 the organism must be stimulated or acted upon, by a stimulus of 

 constant strength in order that a motor reaction be elicited. 

 For the human eye, the action time, the time required for a stimu- 

 lus to produce a sensation of maximum luminosity, is a function 

 of intensity, not of color. 



