CHAPTER SIX 



The Homogeneity of Visual 

 Pigment Solutions 



In 1949 DARTNALL was attempting to estimate the retinal densities 

 of the scotopic pigments in various animals. Having done this for 

 the frog, a representative of the 'rhodopsin* class, he then decided to 

 examine a representative of the *porphyropsin' class. The tench was 

 chosen since its scotopic sensitivity had been measured (granit, 

 1941). 



Aqueous digitonin extracts of dark-adapted tench retinae were, 

 therefore, prepared (dartnall, 1950, 1952a). On exposing these 

 extracts to Hght it was found, however, that bleaching was maximal 

 at approximately 520 m/i instead of at 535-540 mjn as reported for 

 the tench by kottgen and abelsdorff (1896) and by bayliss, 

 LYTHGOE and TANSLEY (1936). On the other hand a similar extract 

 made from another fresh- water fish — the pike — behaved in the 

 expected way. 



dartnall' s (1952a) results are shown in Fig. 6.1. The density 

 spectra, before and after bleaching, of the pike extract are given in 

 the upper half of the figure. The Amax of the unbleached solution was 

 530 m/x. The difference spectrum — obtained by subtracting the 

 *bleached' curve from the 'unbleached' shows that, on bleaching the 

 solution, density was lost maximally at 535 m/u and gained maximally 

 at 405 mju. Such behaviour is generally accepted as typical of a 

 *porphyropsin' (visual violet) preparation (wald, 1939b). The loss 

 in density was due to bleaching of the visual pigment and the gain, to 

 the consequent formation of a photoproduct having Amax about 

 405 mju. 



The density spectra, before and after bleaching, of a tench extract 

 which had been prepared in exactly the same way are given in the 

 lower half of Fig. 6. 1 . The 'unbleached' curve had Amax at 495 mix 

 instead of at 530 mju as in the pike. 



The dissimilarity between the pike and tench extracts is best shown, 



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