CHAPTER THREE 



The Physical Chemistry of 

 Visual Purple 



1. PHOTOCHEMISTRY 



In 1904 TRENDELENBURG measured the relative ejBiciency of 

 different parts of the spectrum of a Nernst lamp in bleaching visual 

 purple solutions. Trendelenburg did not allow for the energy 

 distribution in his Nernst lamp spectrum but, instead, compared the 

 results with the relative sensitivity of human vision (in the dark 

 adapted state) when determined with the same arrangement of 

 apparatus. Good agreement was obtained between the *bleaching 

 factors' and the *twilight factors' (relative scotopic sensitivities). In 

 1911 TRENDELENBURG Concluded that the bleaching of visual purple 

 was independent of wavelength, if due allowance was made for the 

 light actually absorbed by the solutions ; and in the same year this 

 was confirmed by henri and larguier des bancels who showed 

 that when Trendelenburg's original 'bleaching factors' were 

 corrected for energy distribution they agreed approximately with 

 konig's (1894) difference spectrum for visual purple. 



Further studies of the bleaching kinetics of visual purple were 

 made by hecht (1920, 1921, 1924). His approach to the problem 

 was direct and simple though, unhke Trendelenburg, he used only 

 *white' Hght for bleaching. For the bleaching experiments, the visual 

 purple (extracted from frogs' retinae with 3-4 per cent bile salts 

 solution) was transferred to small test tubes about 50 mm long and 

 of 2 mm inside diameter. About 0- 1 ml of solution was put into 

 each tube, the length of the liquid column being adjusted to 35 ± 

 0-2 mm. Concentrations were estimated by comparing the depth of 

 colour, when the tube was viewed end on, with that of standards. 



The standards were prepared by making mixtures of an unbleached 

 and bleached visual purple solution. The first consisted of 20 

 drops of the unbleached solution alone, the second of 18 drops of 



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