OTHER METHODS OF STUDYING VISUAL PIGMENTS 



purple) or *green' ones, and the latter were almost transparent to 

 green light. We must therefore suppose that arden's *extra' pigment 

 either coexists with visual purple in the pink rods, or, that it is a cone 

 pigment. The frog's retina is known to contain cones. These, 

 although they were out of focus in denton and wyllie's photo- 

 graphs, would contribute to the changes occurring on bleaching. 



0-4 



0-5 

 Wavelength (n) 



0-6 



Fig. 7.4. Comparison between the difference spectrum of frog visual 



purple in solution at pH 8-5 (continuous line) with that in intact retinae 



(O and • give the spectral density changes of two retinae on bleaching 



with white light). 



{Denton and Wyllie, 1955) 



DENTON and WYLLIE used the spaces between rods in their photo- 

 graphs (Plate 7.1) as a standard for assessing the absorption charac- 

 teristics of the green rods. If, however, the spaces were occupied by 

 out-of-focus cones containing a photosensitive pigment of Amax 

 535 m/^, or thereabouts, the bleaching of this pigment would result 

 in the spaces becoming much more transparent to green Ught. By 

 contrast, the green rods might then seem to have become darker in 

 the green as the result of bleaching. 



193 



