134 THE NATURE OF FOVEAL DARK ADAPTATION 



The relation between the intensity / and the photochemical effect 

 E is linear, and is represented by the equation of a straight line 



The constant h will depend on the units employed; it may therefore be 

 made equal to unity by changing the units. The constant term h 

 represents the intercept on the y axis. In the present case, omitting 

 the factor 10~* , it turns out to have a value of — 6.0. 



It is then necessary to find what the relation is between the photo- 

 chemical effect £, representing the concentration of photolytic 

 products, and the time / during which the eye has been in the dark. 

 Calculation shows this relation to be that of a bimolecular chemical 

 reaction 



1 « 

 X 



at a — X 



in which k has a value of 0.00271. 



In the calculation of the data according to the bimolecular isotherm, 

 a is taken as the value of the first point in Fig. 6. This makes a equal 

 to 39.3 — 6.0, or ZZ.Z units. Correspondingly, / is taken as zero at 

 7.1 seconds, and the other values of t are computed accordingly. 

 This avoids the use of any highly uncertain values of the threshold 

 at the first movement of dark adaptation. Once the constant k has 

 been determined in this way, it is possible to calculate the real value 

 of a at ^ = 0. This extrapolation from the experimental results and 

 the above equation gives a a value of 103.2 imits. It should be noted 

 that the curve in Fig. 6 is drawn from values calculated from the 

 equation of a bimolecular reaction in which k has the value already 

 given. 



Several things follow from the fact that the curve is that of a 

 bimolecular reaction. One is that the gradual disappearance of the 

 decomposition products is not a process of diffusion. Under certain 

 conditions the rate of diffusion may be a linear function of time. More 

 normally it is a function of concentration. In the latter event its 

 course is represented by an equation similar to that of a monomolecular 

 reaction. A bimolecular reaction isotherm represents not a process of 

 diffusion, but one of chemical reaction only. 



