THE HOMOGENEITY OF VISUAL PIGMENT SOLUTIONS 



20; and the highest Amax for a difference spectrum is as expected. In 

 other words the results are consistent with the hypothesis that the 

 solution contained pigments 533 and 507. 



A TERNARY MIXTURE 



The *totar difference spectrum obtained by bleaching an extract of 

 tench retinae with white hght (Fig. 6.1) had an 'anomalous' shape, 

 owing to visual pigment 467 which was present in the extracts in 

 addition to the expected visual pigment 533. It might be thought 

 from this, that the difference spectrum of a non-homogeneous 

 extract would always have an abnormal shape or an unexpected Amax- 



If this were so then the total difference spectra obtained by bleach- 

 ing to completion with white light would provide a valid and rapid 

 means of surveying the visual pigments of a number of species (cf. 

 the investigation of kottgen and abelsdorff, 1896). 



The danger of this assumption is illustrated by the results of an 

 investigation of the visual pigments of the bleak, a common fresh- 

 water fish (dartnall, 1952b, 1955). When a retinal extract, prepared 

 from this fish, was bleached by white light, a difference spectrum 

 almost identical with that for the pike was obtained (Fig. 6.6). In 

 both cases, the maximum loss of density on bleaching was at 535 m//, 

 and the maximum gain (due to formation of a photoproduct) was at 

 400-405. It might have been fairly conjectured, therefore, that the 

 visual pigment of the bleak was the same as that for the pike (pig- 

 ment 533). 



As a careful inspection of Fig. 6.6 shows, however, the two 

 difference spectra are not exactly alike. Had they not been obtained 

 under identical conditions one would hesitate to regard the small 

 differences between them as significant. In the present instance, 

 however, the differences were well outside experimental error. It was 

 decided, therefore, to subject the bleak extracts to a homogeneity 

 test. 



Two cells were filled with further samples of the same extract. One 

 cell was exposed to light of dominant wavelength 610 m^ for 1 hr, 

 while the other was kept in darkness under otherwise identical con- 

 ditions (20°C). The changes in density of the exposed solution were 

 then measured, using the unexposed cell as a 'control.' These changes, 

 maximal at 545 m/z are shown by curve 1 in Fig. 6.7. The cell was 

 then returned to the bleaching apparatus and exposed for a further 

 2 hr to light of wavelength 610 m/n. The density changes, maximal 



171 



