THE VISUAL PIGMENTS 



products. KUHNE, on the other hand, had stated that the immediate 

 product of bleaching was 'visual yellow' and that this initial product 

 passed to the colourless 'visual white' only as the result of a second 

 reaction. 



KOTTGEN and ABELSDORFF (1896) admitted 'one of us often saw 

 a yellow colour as a transition stage in the process of bleaching — 

 especially in retinae' but they attached little importance to such 

 appearances. The yellow colour could, presumably, have arisen 

 from mere dilution of the visual pigment colour as bleaching pro- 

 ceeded, particularly since the bile solutions used in extraction were 

 faintly yellow. 



KtJHNE had stated that the light most effective in converting visual 

 yellow to visual white was of short wavelength. One would expect 

 therefore that visual pigment solutions exposed to such Ught would 

 bleach to the visual white stage, while those exposed to long-wave- 

 length hght would go only to the yellow stage. In fact, however, 

 KOTTGEN and ABELSDORFF found that difference spectra obtained 

 by bleaching visual pigment solutions were the same whatever the 

 bleaching light (white, yellow or blue). This was a cogent argument 

 against a yellow stage if, as kuhne had implied, the conversion of 

 visual yellow to visual white was a photochemical rather than a 

 thermal reaction. 



A key to the partial resolution of these differences was provided 

 by GARTEN in 1907. garten remarked that the Hghts used by 

 KOTTGEN and ABELSDORFF for bleaching their solutions were weak, 

 and that an intense bleaching light was necessary to reveal a yellow 

 stage in the bleaching process, garten also observed that when a 

 frog's retina was suddenly bleached at — 20°C its colour changed to 

 bright orange, but that on raising the temperature to — 5°C further 

 fading took place, holm (1922), from the bleaching of intact retinae, 

 concluded that the formation of visual yellow was dependent on the 

 visual purple being bleached by intense Hght. He thought that the 

 function of visual yellow was to act as a filter and thus preserve the 

 eye from the effects of high illumination. 



In 1929 NAKASHiMA observed that 'visual yellow' was a deeper 

 yellow in acid than in alkaline solutions, while in 1936 chase found 

 that 'visual yellow' is an acid-base indicator and thought that the 

 discrepancy between the results of garten (visual yellow formed) 

 and KOTTGEN and abelsdorff (visual yellow not formed) resulted 

 from differences of pH. 



42 



