THE VISUAL PIGMENTS 



however, by their respective difference spectra (Fig. 6.1). The shape 

 of these is not the same even after allowing for the height difference 

 (due to the solutions being of different strengths). The main features 

 of the two difference spectra may be summarized as follows : 



Wavelength (mju) of 



Difference maximal maximal Isosbestic 



spectrum of density loss density gain point 



Pike 535 405 451 



Tench 520 395 440 



Since the experiments had been carried out under almost identical 

 conditions, there seemed only two possible explanations of these 

 results : either the visual pigment of the tench was different from that 

 of the pike, or the tench extract contained one or more photosensitive 

 pigments in addition to the *porphyropsin' of pike. 



If tench extracts contain, in addition to porphyropsin, another 

 photosensitive component this would need to be one absorbing 

 mainly in the short wavelength region of the spectrum in order to 

 account for the Amax and the anomalous shape of the difference 

 spectrum. In this event it should be possible to bleach out the por- 

 phyropsin preferentially with light of long wavelength. 



This, in fact, proved to be the case as is shown by the following 

 experiment on another tench extract. First, the changes in density on 

 exposure to white hght were determined. These changes are given by 

 the curve marked 'total' in Fig. 6.2. Two optical cells were then each 

 filled with further samples of the same extract. After measuring the 

 initial differences between the two cells, one of them was exposed for 

 4^ hr to orange-red Hght (610 m//) from a monochromator. The 

 resulting changes in density with respect to the unexposed solution 

 were then measured. These changes (due allowance having been 

 made for the small initial differences) are given by the curve marked 

 *red-sensitive' in Fig. 6.2. This curve shows that bleaching with 

 orange red light caused a maximal density loss at 535 m;^, a maximal 

 gain at 405 m// and no change at 450 mfx ; changes similar to those 

 which took place on bleaching the pike extract with white light. 



The solution which had been exposed to the orange-red light was 

 then exposed to green hght (530 m/<) for 30 min. The object of this 

 was to bleach any traces of the red-sensitive component which might 

 still have remained. In the event, measurement showed that there 

 was no further loss of density at 535 m^, but only a very small 

 amount of bleaching in the short wavelength region of the spectrum 



160 



