THE VISUAL PIGMENTS 



of bleaching is thus governed by the product of time and intensity, 

 that is by the quantum dosage received by the solution. 



Akernatively, we may assume the intensity, /, to be constant and 

 regard the parameter variation as due entirely to variation in the 

 photosensitivity, ay; or, since y is constant in any case, to variation 

 in a, the extinction coefficient. The family of curves in Fig. 6.4 then 



01 234 56789 10 11 12 

 Time scale 



Fig. 6.4. The time courses of bleaching of the visual pigments. Curves 



calculated according to the equation cJcq = e~'^y^^ for various values 



of the parameter ay/ between 1 and 100. 



gives the time courses of bleaching caused by lights of different 

 regions of an equal quantum-intensity spectrum. The curves demon- 

 strate that the time to effect a given degree of bleaching is inversely 

 proportional to the extinction coefficient of the photopigment. 



Visual purple is the only one of the visual pigments whose photo- 

 chemical bleaching has been investigated in much detail. However, 

 BLISS (1946) has shown that iodopsin behaves similarly. Assuming 

 that the bleaching kinetics of visual purple is a pattern for all visual 

 pigments, we can consider the bleaching of a mixture of (say) two 

 photopigments. 



166 



