THE HOMOGENEITY OF VISUAL PIGMENT SOLUTIONS 



In a mixture there is ^competition' between the photopigments for 

 the light. The total amount of light absorbed by the mixture is 

 always less than the sum of those amounts which would be separately 

 absorbed by either pigment in the absence of the other. For example, 

 if either pigment separately absorbed 90 per cent (density = 1) of the 

 incident light, the two together would absorb 99 per cent (density = 2), 

 each pigment when in admixture capturing only 49| per cent. How- 

 ever, provided the total density of the solution at the bleaching 

 wavelength does not exceed 0-12, the amount of Hght absorbed by 

 the mixture falls short of the sum of those amounts which would be 

 separately absorbed by less than 5 per cent. In such cases the parti- 

 tioning of the total light absorbed between the two pigments in 

 proportion to their optical densities results in each absorbing very 

 nearly as much light when in admixture as it would have done in the 

 absence of the other. Thus, in solutions of low optical density, each 

 pigment bleaches at practically the same rate as it would if it were 

 alone in the solution. 



Let us suppose that the mixture is exposed to a bleaching Hght of 

 wavelength such that the photosensitivities of the two pigments are 

 not the same. One pigment therefore bleaches faster than the other 

 and consequently, if the bleaching is interrupted at any stage, the 

 unbleached fraction is relatively richer in the less photosensitive 

 pigment than was the original solution. 



In Fig. 6.4 let curve 100 represent the time course of bleaching of 

 the more photosensitive pigment P^, and curve 10 that of the less 

 (Pg)- Then after time 1, 36 per cent of P^ remains in solution and 

 90 per cent of Pg- Thus one time unit of bleaching increases the ratio 

 of P2 to Pi in the unbleached fraction (compared with that of the 

 original solution) by a factor of 90/36 = 2-5. At a later period, say 

 time 5, 0-5 per cent of Pj and 60 per cent of Pg remain, the enrichment 

 factor thus advancing to 120 as a result of the longer bleaching 

 period. Obviously, the longer the bleaching is allowed to proceed, 

 the closer does the unbleached fraction approximate to pure Pg. It is 

 relatively easy, therefore, to obtain 'pure' data for the less photo- 

 sensitive pigment. Thus, even supposing the original solution con- 

 tained 10 parts of Pi to only 1 part of Pg, then by bleaching for time 5 

 (Fig. 6.4), the unbleached residue would contain 0-05 of Pj and 0-6 of 

 P2, i.e. over 90 per cent of Pg. 



Let us now consider the bleached fraction. After time 1 (Fig. 6.4), 

 64 per cent of P^ has bleached and 10 per cent of Pg. Thus one time 



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