550 SENSORY EQUILIBRIUM AND DARK ADAPTATION 



from the position of the points that the progress of dark adaptation 

 in My a follows a decidedly regular course. 



2. The hypothesis which was suggested to explain the photic re- 

 sponses of Ciona (Hecht, 1918, b), involves the existence of a revers- 

 ible reaction sensitive to Hght. The action of light is to decompose 

 a photosensitive substance into its precursors. The recombination 

 of the precursor substances to form sensitive material is, however, 

 independent of the presence of light, and proceeds whenever any 

 fresh precursors are formed. This photosensitive system not only 

 adequately accounts for the behavior of Ciona, but serves as well 

 to rationalize the two characteristics of the photic sensitivity of Mya 

 with which we are at present concerned. 



It will be remembered that the sensitization period represents the 

 amount of photochemical activity. In terms of our hypothesis this 

 is equivalent to the amount of precursor substances formed by the 

 decomposition of the photosensitive material. It was shown for 

 Ciona that in order to produce a response the amount of precursor 

 substance which must be formed by the light is a constant fraction 

 of the amount of precursor already present in the reaction system. 

 Therefore the duration of the sensitization period may be consid- 

 ered as a direct measure of the amount of precursor substances 

 present at any moment in the sense organ. 



When Mya is placed in the dark, any precursor material present 

 will at once react to form sensitive substance. Since none of the 

 latter is being decomposed, the rate of disappearance of the precursor 

 material will depend on their concentration, on the order of the 

 reaction, and on the value of the velocity constant. During dark 

 adaptation then, the change in the sensitization period should reveal 

 the rate of formation of photosensitive substance. 



In view of this reasoning it is highly significant that the curve 

 drawn in Fig. 1 is an isotherm of a bimolecular reaction. 



That the points fit the theoretical curve is apparent from mere 

 inspection. However, in order to bring this out in the customary way, 

 Table III is presented, giving the average observed values for the 

 reaction time and those calculated accordingfto 



1 X 



k = 



at a—x 



the equation for a bimolecular reaction. 



