THE VISUAL PIGMENTS 



easy to establish the homogeneity of the *red-insensitive' 507 pig- 

 ment: by prolonged exposure to long- wave light all, or virtually all, 

 of the red-sensitive material was removed, leaving sufficient 507 

 pigment in the solutions for subsequent examination. But it was not 

 possible to bleach the 'red-sensitive' component of the mixture with- 

 out bleaching a little of the 507 pigment at the same time. Conse- 

 quently all the difference spectra for the red-sensitive component 

 (1-4 in table) were 'contaminated' by contributions from pigment 507. 

 We can check whether the tabulated data conform with the con- 

 clusion that the solutions contained a mixture of pigments 533 and 

 507 by applying the theory of bleaching of a binary mixture developed 

 in the previous pages. In the first instance, by adding together, in 

 various proportions, curves for the two pigments, a graph relating 

 the Amax of a mixture with its composition can be obtained. From 

 this graph it is found that curves with Amax at 532, 529, 522 and 

 515 m^a consist of 6, 12|, 33 and 60 per cent of pigment 507 respec- 

 tively. Applying these results to the data the composition of each 

 difference spectrum can be calculated as follows : 



Pigment 507 Pigment 533 



1. 0-0014 (1-9%) 0-0214(38%) 



2. 0-0018(4-3%) 0-0123(60%) 



3. 0-0100(17-8%) 0-0204(97%) 



4. 0-0024(21%) 0-0016(100%) 



5. 0-0207(49%) — (100%) 



6. 0-0380(100%) — (100%) 



0-0743 0-0557 



The figures in brackets give the cumulative percentages bleached 

 at each stage. When the percentages for one pigment are plotted 

 against those for the other, the resulting curve is one of the family 

 shown in Fig. 6.5, specifically that for a photosensitivity ratio of 

 about 20:1. Finally, since the ratio of pigment 533 to 507 in the 

 original mixture was 0-0557 to 0-0743, i.e. 0-75, we should expect 

 that the best obtainable difference spectrum for pigment 533 (see 

 p. 168) would consist of 20 x 0-75 = 15 parts of 533 and 1 part of 

 507. Such a mixture would have Amax = 532 m//. This was, in fact, 

 the Amax of the first difference spectrum. Thus the difference spectra 

 of the first four bleachings can be simulated by mixtures of pigments 

 533 and 507; the proportions of the two pigments so required agree 

 with the bleaching of a binary mixture at a photosensitivity ratio of 



170 



