THE VISUAL PIGMENTS 



the broadening of the spectra as their location advances to longer 

 wavelengths. 



If, instead of a wavelength scale, a frequency scale is used, the 

 difference spectra become very nearly of the same shape. This is 

 shown in Fig. 6.10 in which the difference spectra have been shifted 

 along the frequency axis so that their maxima correspond. Above 



500 600 



Wave-lengch (m/x.) 



Fig. 6.9. Comparison of the difference spectra (wavelength abscissae) 



of four visual pigments. •, pigment 467; +, pigment 502 (frog visual 



purple); O, pigment 533; x , pigment 562 or iodopsin (Bliss, 1946). 



{Dartnall, 1952a) 



the *base-line,' the curves are almost superimposable. Since the 

 upper portion of a density spectrum approximates (particularly in 

 alkaUne solution) to the density spectrum of the pure visual pigment, 

 this similarity suggests a corresponding agreement in shape between 

 the density spectra of the visual pigments, dartnall (1953) there- 

 fore devised a nomogram by means of which the approximate density 

 spectra of visual pigments of known ^max could be constructed. The 

 nomogram was based on the well-known density spectrum of visual 

 pigment 502 (frog visual purple). 



176 



