SELIG HECHT 369 



is maximal when the amount of P and A has been produced during 

 the exposure occupied by the sensitization period. It is, however, 

 possible to increase the latent period by exposing the animal for periods 

 less than the sensitization period. Submaximal quantities of P and 

 A are then produced and the latent period is prolonged, because the 

 velocity of the latent period reaction is diminished. If merely a 

 minimum exposure is given (presentation time of Laurens and Hooker, 

 1920) the amount of P and A produced is minimal, and the latent 

 period is consequently very much prolonged. In any event, however, 

 a given photochemical effect results in a constant duration of the 

 latent period. 



These findings have been secured by such a variety of methods, 

 and have been demonstrated so frequently to investigators at Woods 

 Hole, that it is somewhat surprising to have them questioned (Lau- 

 rens and Hooker, 1920). The experiments to be described will furnish 

 still another means of corroboration, sufiQcient, I hope, to disarm any 

 criticism. 



The photochemical effect of the light is maximal if it takes place 

 during the sensitization period. An animal which is exposed to light 

 indicates automatically when this maximum has been reached by the 

 retraction of its siphon at the end of the reaction time. Because the 

 latent period is constant, variations in the reaction time to lights of 

 different intensities are the result of variations in the sensitization 

 period. This then furnishes a simple method of testing equation (3). 

 All that is required is to measure the reaction time of animals exposed 

 to lights of different intensities. 



in. 



The intensity of illumination may be varied by placing the animal 

 at different distances from a source of light. As sources of light I 

 have used on different occasions a 40 watt, a 100 watt, and a 250 watt 

 lamp. These are concentrated-filament Mazda lamps, and for our 

 purposes may be considered as point sources. The intensity of illumi- 

 nation therefore varies inversely as the square of the distance from 

 the source. The reaction time is measured with a stop-watch. 



