352 HENRY LAURENS AND HENRY D. HOOKER, JR. 



is produced, and during which the primary photochemical effect 

 proceeds to a certain minimal amount, e.g., by transformation of 

 a substance already existing into another, or 'inner stimulus/ 

 which gives rise to the impulse; and the latent period, during 

 which a chemical process, either a continuation of the one 

 started during the presentation time, or others, are taking place. 

 These latter 'processes are not completed until at least a threshold 

 amount of substance has been formed, or transformed, and the 

 excitation, thus set up, conducted to the locomotor organs, or 

 cilia, in Volvox. 



Unfortunately, complete records were not made of the duration 

 of either the reaction time or of the latent period. This is one 

 of the points which, as mentioned above, we desire to clear up 

 later. In our early work on the determination of the relative 

 velocity of locomotion when the colonies were exposed to the 

 various spectral lights, we recorded in a number of cases the time 

 from the beginning of the exposure to the light up to the first 

 evidence of locomotor response, that is the reaction time. We 

 will refer to this again in the portion dealing with the rate of 

 movement. 



Hecht ('18) has obtained some very striking results in connec- 

 tion with the latent period portion of the reaction time of Ciona 

 to light. He demonstrated that the latent period was a constant 

 quantity under considerable variations in intensity, while the 

 sensitization period (the presentation time) varied with the in- 

 tensity of the light. The sensitization period, which is the reac- 

 tion time minus the constant latent period, is the important 

 factor of the reaction in connection with the intensity, since it 

 indicates the quantity of stimulus received by the organism. 

 The product of the sensitization period and the intensity proved 

 to be a constant, and thus the applicability of the reciprocity 

 law of Bunsen and Roscoe to the stimulative action of light has 

 again been confirmed. 



Mention may be made here of certain other points in Hecht's 

 ('19 a, b, c) contributions to the analysis of photochemical re- 

 actions. In the first place, he shows good reason for believing 

 that the photochemical process involved in the sensitization 



