THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



however, complicates the interspecies comparison of density spectra, 

 and wide hmits were tolerated by wald when assigning a visual 

 pigment to the rhodopsin or porphyropsin groups (see Figs. 2.3 

 and 2.4). 



The density spectrum of a photometrically pure solution of a visual 

 pigment has a minimum value of about 20 per cent on the blue side 

 of the maximum. The presence of yellow impurities (which absorb 

 with increasing strength towards the short-wave Hmit of the visible 

 spectrum) results in a raising of this minimum and a displacement of 

 the Amax of the extract to a shorter wavelength. 



The purity of an extract can be roughly expressed by the ratio of 

 its optical density at the minimum (Dmm) to that at the maximum 

 (^max)- The purer the extract, the lower is this ratio. In an investi- 

 gation of carp retinae, crescitelli and dartnall (1954) prepared 

 extracts with DmmlDm&x ratios ranging from 0-98 to 0-57, the Amax 

 ranging correspondingly from 500 to 520 mju. They did this by 

 varying the number of pre-extraction washings and by using different 

 procedures. In Fig. 2.6 the /)miii/^max ratio is plotted as a function 

 of the Amax of the extracts. Data from saito (1938) and wald 

 (1939b) for carp are also included in the figure. 



Rayleigh scattering in a slightly opalescent extract has an effect 

 similar to that of yellow impurities. According to collins and 

 MORTON (1950a) the Amax displacement is 1 mju when the scattering 

 is such as to raise the DminlDm&x ratio to between 0-33 and 0-45, 

 2 m/u if it raises the ratio to 0-45-0-55, and 3 m/j, if to 0-55-0'63. 

 These hmits are shown in Fig. 2.6 by the vertical bars and they accord 

 with the other data in showing a fairly close correlation between the 



-Dmin/^max and the Amax- 



It is reasonable to suppose that the relationship between the 

 DminlDmsLx ratios and the absorption maxima of carp extracts will 

 hold good for visual pigment extracts made from other fish, provided 

 the same visual pigment is involved or, at least, if its absorption 

 characteristics are not very different from those of visual pigment 

 523, the carp pigment. 



In Fig. 2.6 an inset wavelength scale, cahbrated in m/Li displace- 

 ments from the pigment maximum, has been provided. This allows 

 the figure to be used for interpreting the crude extract data of other 

 species. For example, collins and morton (1950a) and dartnall 

 (1952) both prepared visual pigment solutions from the pike. 

 COLLINS and Morton's extract had Amax = 525 m/x and i)min/^max 



39 



