712 LIGHT AND LIFE 



the facts of increment threshold, and Hecht's theory gained immense 

 prestige by giving so simply an accurate explanation of this relation. 

 It is fundamentally different from the sort of explanation we have 

 derived in our own electrophysiological approach. 



According to the photochemical theory, a steady adaptation light 

 / bleaches the pigment to some fraction 1/p of the initial concentration. 

 It will now need p times the original flash intensity to photolyse a 

 fixed "threshold" amount of pigment. It follows that the pigment 

 must be half bleached away by an intensity / which causes A^ to be 

 twice the absolute threshold. It also follows that if A^ is to be 

 measured at this level, after taking a reading where A^ was 20 times 

 the absolute threshold, we must wait for the pigment to regenerate 

 from 95 per cent bleached back to 50 per cent. 



Neither of these conclusions is at all true. In increment threshold 

 determinations we do not have to wait for the slow regeneration 

 of pigment in changing from one level to another. To a first approxi- 

 mation, the relation of equation (2) is established instantly and re- 

 mains unaffected by subsequent changes in the level of concentration 

 of the visual pigment. 



It is a remarkable thing that the chief proponents of the theory that 

 rod function hangs upon the quantitative relations of rhodopsin 

 bleaching appear neither to have themselves measured this rate of 

 bleaching nor paid attention to the results of Dartnall, Goodeve, and 

 Lythgoe (1) , who did. Otherwise it would have been obvious that 

 this goes on so slowly at a background field where A^ is twice thresh- 

 old, that even in the absence of any regeneration, the required 

 bleaching of half the total rhodopsin would take 30 years! 



The electrophysiological interpretation seems to be closer to the 

 facts. Here the increment threshold depends upon a more or less 

 instantaneous logarithmic transformation of the light intensity, and 

 upon the nerve responding to a constant increment in the trans- 

 formed signal. The nature of the transformation is certainly still 

 obscure, but it is an observed fact in the Limulus eye, and even in the 

 photosensitive ganglion of Aplysin. 



There is an entirely different aspect of visual function which the 

 photochemical theory has always related to the concentration of 

 visual pigments. The raising of the absolute threshold as a result of 

 exposure to light, and its gradual fall subsequently in tlie dark, have 

 long been considered to be due to bleaching and regeneration of the 

 visual pigment. Until our own recent quantitative studies, the nature 



