342 INTENSITY AND PHOTORECEPTION 



which the hne crosses the y axis. The Hne in Fig. 3 crosses the coordi- 

 nates at (0, 0). This is true graphically, and also follows from the 

 fact that at zero intensity the velocity of the latent period reaction 

 is zero. Therefore & = 0, and the equation becomes 



y = a X. (2) 



Using the notation of Fig. 3, >> is the velocity F, and % is the logarithm 

 of the intensity. The numerical value of the constant a is found by 

 dividing a given value of the ordinate by the corresponding value of 

 the abscissa. In Fig. 3, a = 2.2. Equation (2) may therefore be 

 written 



F = 2.2 log/. (3) 



It will be remembered that we have been using common logarithms. 

 The factor for converting common into natural logarithms is 2.3. 

 It is highly probable that within experimental errors, our constant 

 a = 2.2 is the same thing as the factor for converting Briggsian into 

 Naperian logarithms. The equation for the straight line in Fig. 3 

 should therefore be 



V ^Inl (4) 



in which In means logarithms to the base e. 



Equation (4) not only demonstrates the logarithmic connection be- 

 tween the incident light and the velocity of the latent period, but it 

 shows that this relation is of the simplest mathematical nature. 

 Before making any theoretical deductions from equation (4), it will 

 clarify matters if we first consider its direct connection with the reac- 

 tions which underlie photic sensitivity. 



IV. 



The two terms of the equation which we have just deduced repre- 

 sent the initial step and the final result of the double process of light 

 sensitivity. The light decomposes a photosensitive substance into 

 its precursors. These precursors, according to our hypothesis, then 

 catalyze the latent period reaction, the end-product of which initiates 

 the nervous impulse. We have discovered a simple mathematical 

 relation between the intensity of the light and the velocity of the 



