548 SENSORY EQUILIBRIUM AND DARK ADAPTATION 



mental error. The average of these two sets of determinations is 

 1.29 seconds, practically all of which represents latent period. At 

 lower intensities, of course, the sensitization period becomes larger, 

 and enters perceptibly into the duration of the reaction time as 

 ordinarily measured. 



4. The constancy of the latent period under given conditions of 

 temperature and intensity is of prime significance in the study of 

 dark adaptation. Because of its uniformity, any variations in the 

 reaction time under these conditions signify variations not in the 

 latent period, but in the sensitization period. 



The sensitization period represents the time required for the light 

 to produce its chemical effect. Any variation in its duration means 

 a change in the amount of chemical effect. In terms of the photo- 

 chemical law of Bunsen and Roscoe, — which has been shown to apply 

 to such sensitization processes (Loeb, 1918, Hecht, 1918, h), — the 

 amount of photochemical change at a given intensity varies with the 

 time of exposure to light. For such short periods as occur here it 

 may be safely assumed that the photochemical effect is a linear 

 function of the time of exposure to a given intensity. The duration 

 of the sensitization period may thus be taken to represent the amount 

 of photosensitive substance decomposed in the sense organ by the 

 action of the light. Therefore determinations of the reaction time 

 at constant temperature and intensity furnish us with a measure of 

 the amount of chemical effect necessary to produce a response during 

 various phases of light and dark adaptation in Mya. 



Photic Adaptation. 



1. The exposure of Mya to light results in only a single response. 

 After retracting its siphons, the animal slowly expands them, and 

 remains in this state during the remainder of the exposure. This 

 acquisition of sensory equilibrium was accomplished at all the in- 

 tensities to which Mya was subjected, the highest intensity used 

 being 10,000 meter candles. 



If, after such an adaptation, Mya is placed at a lower intensity, 

 or preferably in the dark, it rapidly becomes sensitive to the light 

 with which it had been in sensory equilibrium. By determining 



