SELIG HECHT 507 



catalyzed by the products formed in the photochemical reaction. 

 It is the initial photochemical reaction that is of interest in the pres- 

 ent context, and with it alone we shall be concerned. Without going 

 into details, — for which the original work must be consulted, — the 

 results are as follows. A photosensitive substance S is decomposed 

 by the action of light into two products of decomposition P and A. 

 This reaction is reversible, and in the absence of light the reverse 

 reaction goes on unopposed by the light reaction. The equation for 

 the complete process is thus 



light 



S ^ P + A. 



"dark" 



It is apparent that the substances F and A are the precursors as 

 well as the decomposition products of the photosensitive substance S. 

 The velocity of the light reaction, 5 — » P -{-A, is entirely depen- 

 dent on the intensity of the incident hght. The exact relation be- 

 tween the two is a logarithmic one. If E is the photochemical effect, 

 as measured by the amount of P and A formed in unit time, and / 

 the intensity of the light, then 



E=k-lnl 



In being the sign of natural logarithms. In Mya, k has a value of 

 1. This quantitative relation is the first of the two aspects of the 

 sensory process to which reference has been made. Its significance 

 lies in its ability to describe the action of light in the objective terms 

 of a physicochemical mechanism, rather than in terms of sensory 

 effects. 



The second feature of the photosensory mechanism as postulated 

 for Mya and Ciona is also concerned with products of the light reac- 

 tion S -^ P + A. This second principle states that the degree of 

 irritabihty of the sense organ depends entirely on the concentration 

 of the precursor decomposition products present in the sensory 

 mechanism. To be more precise: before it can cause a sensory effect,, 

 the incident light must produce such an amount of freshly decom- 

 posed precursors P and A that a definite ratio is attained between 

 the freshly formed and the residual precursors present in the sense 

 organ. 



