THE VISUAL PIGMENTS 



maximal at 545 m.fx in the first exposure, at 538 m// in the second and 

 at 512 m^w in the third. The second exposure yielded a difference 

 spectrum (curve 2 in Fig. 6.7) Uke that for visual pigment 533, 

 suggesting that this pigment might be a component of the solution. 

 If so, then the difference spectrum which had been obtained by the 

 first exposure to 610 m^* (curve 1 in Fig. 6.7) would certainly have 

 been 'contaminated' by that for visual pigment 533. This would 

 indicate an even longer wavelength than 545 m// for the true absorp- 

 tion maximum of the first 'pigment.' Similarly the third and final 

 bleaching with white light (curve 3 in Fig. 6.7) would include any 

 visual pigment 533 not removed by the previous exposures to 610 m/i 

 light. In this event, the true absorption maximum of the third 

 'pigment' would be at a shorter wavelength than 512 m/^. 



As in the cases of the tench and the rainbow trout, the photo- 

 sensitivity of the bleak extracts may be regarded as due to a red- 

 sensitive and a red-insensitive component. By prolonged exposures 

 to fights of long wavelength, the sensitive component could be almost 

 entirely removed, leaving some insensitive component behind for 

 investigation with short-wavelength bleaching lights. In this way 

 DARTNALL (1952b) found that the red-insensitive component was a 

 single pigment having Amax at 510 i 3 vcifji. This pigment is probably 

 identical with that, having Amax at 507 i 3 m/^, present in the rain- 

 bow trout. 



The interpretation of the difference spectra obtained by partially 

 bleaching the bleak extracts with long wavelength light was decidedly 

 more difficult, however. The curves obtained had Amax ranging from 

 545 vail to 522 m,a, the majority lying between 539 m/z and 536 m/i. 

 After analysing these results, dartnall (1955) concluded that the 

 red-sensitive component consisted of two pigments ; one with Amax 

 at 533 m// and the other with Amax at about 550 m//. 



In Fig. 6.8 the density spectra of pigments 510, 533 and 550 are 

 scaled to the proportions in which they were present in the extract of 

 Fig. 6.6. If the three density spectra are added together, the resulting 

 curve is quite smooth and has a maximum at about 530 m// ; in fact 

 (see dotted curve. Fig. 6.8) it closely resembles the density spectrum 

 of pure pigment 533. This accounts for the close resemblance 

 between the 'total' difference spectrum of this extract and that of a 

 pike extract (Fig. 6.6) and emphasizes the need for testing the homo- 

 geneity of all retinal extracts, even in unpromising cases. 



Over a period of twenty months five extracts were prepared from 



174 



