350 



SCIENCE 



[N. S. Vol. LIII. No. 1372 



stance in the receptor. Therefore the amount 

 of decomposed photosensitive material neces- 

 sary for a response during dark adaptation 

 is at first large, and gradually becomes 

 smaller and smaller until it reaches the nor- 

 mal amount for that intensity. 



These phenomena, and many others, can be 

 explained in terms of a simple hypothesis. 

 In producing sensory equilibrium, the light 

 decomposes a photosensitive substance, and at 

 the same time causes a loss of sensitivity. 

 The removal of the light results in a char- 

 acteristic return of sensitivity. This is prob- 

 ably because new photosensitive material is 

 being formed. If we assume that the action 

 of the light is to break up the sensitive 

 material into its precursors, and that in the 

 dark these precursors reunite to form the 

 sensitive substance, all of our data may be 

 explained in terms of the kinetics and 

 dynamics of chemical and photochemical re- 

 actions whose general properties are well 

 known and mathematically predictable. 



Consider the kinetics of the formation of a 

 sensitive substance from its precursors. The 

 velocity of reaction at any moment will be 

 proportional to the concentration of the pre- 

 cursors. Therefore these will disappear at 

 first rapidly, and then more slowly according 

 to the well-known expression 



-- = k{a-xy, 



where (a — -x) represents the concentration 

 of precursors, and n the order of the reaction, 

 the other symbols having their usual meaning. 

 It is certain that the reaction time, and 

 therefore the amount of photochemical action 

 necessary for a response, is not proportional 

 to the concentration of sensitive substance in 

 the sense organ, because during dark adapta- 

 tion the former decreases while the latter in- 

 creases. Moreover, it becomes apparent on 

 second thought that the sensitive substance 

 as such is not the effective agent; it is only 

 after it has been decomposed by the light 

 into something else that it can produce its 



effect. It is therefore more likely that the 

 amount of decomfvosition represented by the 

 reaction time (more accurately, by the sensi- 

 tization period) will depend not on the con- 

 centration of sensitive substance, but on the 

 concentration of its precursors. 



Let us assume this to be true. The changes 

 in the reaction time during dark adaptation 

 should therefore parallel the progress made in 

 the disappearance of precursor material dur- 

 ing tlieir combination to form the sensitive 

 material. A superficial resemblance between 

 the dark adaptation curve and the isotherm 

 of a chemical reaction is at once apparent. 

 The resemblance however is more than super- 

 ficial. The curve which best fits all of the 

 data on dark adaptation is actually the 

 isotherm of a bimolecular reaction, repre- 

 sented by the expression 



k-- 



which is the integral form of the equation 

 above when n^2; a represents the initial 

 concentration of precursors, x the concentra- 

 tion of sensitive substance at the time t, and 

 a — X is the concentration of precursors at 

 the same time. This means that there are 

 two precursors (P and A) whose concentra- 

 tion is decreasing because they are com- 

 bining to form the sensitive substance S. 

 The process which goes on in the sense organ 

 may then be written 



fight 



S:^P + A 



"dark" 



with a full consciousness of the quantitative 

 significance of the expression. 



The dynamics of this reversible photo- 

 chemical reaction account for the prominent 

 characteristics which we have described for 

 the photosensory process. The response to an 

 increase in illumination, the applicability of 

 the Bunsen-Eoscoe law, and the low tem- 

 perature coefiicient are all inherent to the 

 light reaction, 8 -^ P -]- A. Sensory equili- 

 brium corresponds to the well-known station- 



