338 INTENSITY AND PHOTORECEPTION 



stant intensity has already been studied. The results show that 

 within the investigated range of exposure the velocity of the latent 

 period reaction is a linear function of the exposure time (Hecht, 1918- 

 19, b). It is assumed that the velocity of the latent period reaction 

 is directly proportional to the concentration of the catalytic substances 

 P and A. Therefore the photochemical effect of the Hght is a linear 

 function of its time component. 



The limits between which the time factor may be varied in these 

 experiments are only a few hundredths of a second apart. This is 

 due to the short exposure which is required for a response (Hecht, 

 1919-20, c). The intensity factor, however, may be varied over a 

 much greater range, and with more precision even than the time fac- 

 tor. The relation between the source of energy and the primary and 

 secondary reactions may therefore be determined for a wider grada- 

 tion of energy application than has been done heretofore. 



With this in mind, the present series of experiments was performed. 

 The animal used is Mya arenaria. For a description of the sensory 

 properties of this animal the reader is referred to previous papers of 

 this series (Hecht, 1919-20, c). The work was done at the Marine 

 Biological Laboratory at Woods Hole, Massachusetts, during the 

 summer of 1919. 



n. 



The experiments are very simple. A number of animals are thor- 

 oughly dark-adapted by being kept in a dark room for 24 hours. 

 Using a constant source of illumination and a constant exposure time, 

 the reaction time of each animal is determined at different distances 

 from the hght. After each observation, the animal is maintained in 

 complete darkness for 15 minutes before the next observation is made 

 on it. As the duration of the exposure to light is known, the latent 

 period is found by subtracting the exposure from the reaction time. 

 Also, since the source of illumination is a concentrated-filament, in- 

 candescent lamp, the intensities may be computed from the distances 

 on the inverse square law. The exposure used was 0.133 second, and 

 the source of hght a 250 watt Mazda lamp. The arrangement and 

 construction of the apparatus will be found in detail in a recent pubU- 

 cation (Hecht, 1919-20, c). 



