THE VISUAL PIGMENTS 



purple, which lythgoe found to be unafifected by pH, is also shown 

 for comparison. 



lythgoe conjectured that the density spectrum of pure visual 

 purple approached close to the zero axis at wavelengths shorter than 

 400 mju (Fig. 2.1 1). We now know, however, that the density of pure 

 visual purple at 400 mju is fully 20 per cent of that at the maximum. 

 LYTHGOE* s Solutions were, in fact, much more nearly pure than he 

 supposed. The impurity 'corrections' which, at the time, he thought 

 necessary to his visual purple curves were, of course, appUed also to 



0-15 



£ 0-1 



005 - 



\(m;i.) 



Fig. 2.12. Density spectra of acid and alkaline indicator yellow and of 



the parent (frog) visual purple. #, visual purple; O, indicator yellow 



in acid solution; x , indicator yellow in alkaline solution. 



(Collins and Morton (1950b)) 



the indicator yellow curves. Consequently, they too are uniformly in 

 error at the shorter wavelengths. Nevertheless the results in Fig. 2. 11 

 clearly show the dependence on pH of the density spectrum. In acid 

 solution (pH 5-2) the indicator yellow maximum is at c. 440 mju 

 while in alkahne solution it shifts into the ultra-violet, beyond the 

 reach of lythgoe's apparatus. At c. 400 m/u, the indicator yellow 

 curves all have approximately the same ordinate value. It is probable 

 that were it not for the fading of indicator yellow at some pH's, there 

 would have been a true isosbestic point at c. 400 mju. This suggests 

 that indicator yellow exists in two forms and that the curves in 

 Fig. 2.1 1 represent mixtures of these in different proportions, depend- 

 ing on the pH. 



The full density spectra of the two forms of indicator yellow in 

 relation to the spectrum of the original visual purple solution are 



52 



