THE VISUAL PIGMENTS 



All visual pigments so far examined appear to behave on bleaching 

 in an analogous manner to visual purple. It is fortunate that the 

 alkaline conditions, which favour stabihty of the visual pigment, 

 favour also the stabihty of the product. Consequently, in alkaline 

 conditions (pH 8-10) difference spectra can be reproduced with 

 precision. Some examples are shown in Fig. 2.13. 



Another advantage of alkaline conditions is that the pH-dependent 

 I 1 I I > I I I I 1 I I I I I I I I I I I I I I I I t I I I I t I 



100 



90 



80 



70 



60 



50 



40 



30 



I 20 

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$ 



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400 500 600 700 



Wave-length {mju.}. 



Fig. 2.13. Difference spectra of a visual pigment present in retinal 

 extracts of the tench. pH 8-6, temperature 20'0°C. The symbols indi- 

 cate the results of five independent determinations. All maxima equated 

 to 100 to facilitate comparison. 

 {Dartnall, 1952) 



56 



