GEORGE WALL) 



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'Porphyropsin, cyanopsin, and spectral sensitivity of the tench 



_ porphyropsin 



scotopic 

 ■sensitivity 



cyanopsin 



photopic 

 sensitivity 



SOD 



100 



600 



Wavelenqih - mjj 

 Fig. 6. Absorption spectra of porphyropsin and cyanopsin (lines, small circles) 

 compared with the spectral sensiti\ities of rods and cones in a freshwater fish, 

 the tench (large circles). The spectral sensitivities were measured electrophysiologi- 

 cally bv Granit (1941). The absorption spectrum of tench porphyropsin is that 

 measured by Crescitelli on a preparation from Dartnall (Crescitelli, 1958). The 

 cyanopsin was prepared synthetically (Wald, Brown, and Smith, 52). 



and dark adaptation as some simple function of the visual pigment 

 concentration. For many years Hecht tried to infer the mathematical 

 relationship between these quantities from sensory data alone, in 

 the context of a priori considerations involving the physico-chemical 

 arrangement of photoreceptor systems. He was never satisfied with 

 the result, and after a time he stopped trying to fit theoretical curves 

 to dark adaptation data. 



I think that in recent years, as it became possible to measure the 

 synthesis of visual pigments, first in vitro and then in vivo, it has 

 become increasingly clear that the relation sought involves a paral- 

 lelism between the logarithm of the visual sensitivity and the photo- 

 pigment concentration (46) . This inference was drawn first on the 

 basis of roughly quantitative comparisons of the course of visual pig- 

 ment synthesis in solution with the course of dark adaptation. So, 

 for example, just as in duplex retinas the cones ordinarily dark 

 adapt much more rapidly than the rods, so in extracts of chicken 



