THE VISUAL PIGMENTS 



the *totar difference spectrum which had been obtained by bleaching 

 another sample in a single exposure to white light. 



These experiments showed that the extracts of tench retinae were 

 not homogeneous; they contained two light-sensitive components. 

 One of these — the 'red-sensitive' component — showed density 

 changes on bleaching which were maximal at 535 mju and the other 

 — the 'red-insensitive' component — changes which were maximal at 

 470 mju. Apparently in the tench extracts of previous workers 

 (kottgen and abelsdorff, 1896; bayliss, lythgoe and tans- 

 ley, 1936) only the red-sensitive component had been present. The 

 reason for this is not known, but the presence of a red-insensitive 

 component in dartn all's tench extracts was a fortunate occurrence 

 for it led to the realization that the homogeneity of all retinal extracts 

 ought to be systematically investigated. This had not been done in 

 the past. 



THE METHOD OF PARTIAL BLEACHING 



The demonstration that dartn all's tench extracts were not 

 homogeneous was a simple matter because the two Hght-sensitive 

 components absorbed in very different parts of the spectrum. Thus 

 (Fig. 6.2) the red-insensitive one does not appreciably absorb light of 

 wavelengths longer than about 560 m/u. Consequently by exposing 

 the original solution to light of wavelength 610 m//, the red-sensitive 

 component readily bleached whereas the other was hardly affected. 

 At the end of 4J hr exposure practically all the red-sensitive com- 

 ponent, but hardly any of the red-insensitive component, had 

 bleached. Thus by the selection of a suitable bleaching wavelength 

 a virtually complete 'separation' had been effected. 



But how do we know that the two components are themselves 

 homogeneous ? May they not both be a mixture of pigments — red- 

 sensitive in the one case and red-insensitive in the other? The 

 experiment just described throws no light on these questions. Both 

 components had been completely bleached, the one by orange-red 

 light, and then the other by white light. Just as the total bleaching of 

 the tench extract (Fig. 6.1) yielded no information about the homo- 

 geneity of the extract, so the total bleaching of each of its two com- 

 ponents tells us nothing of their individual make-up. Clearly, the 

 test of partial bleaching, which disclosed the heterogeneity of the 

 extract as a whole, can be applied also to its components. 



162 



