THE VISUAL PIGMENTS 



shown by the unfilled circles. They are in reasonably good agreement 

 with the modern values. 



When GOODEVE, lythgoe and Schneider's measurements were 

 made the density spectrum of visual purple was known accurately 

 only at wavelengths longer than about 430 mfx. The density spec- 

 trum of a visual purple solution determined at this time by goodeve, 



I I 1 1 I M 1 I 1 I I I I I I I I I I I I I I I I I I I I I I ' ' *' 1 ' » ' ' 



1-, 



c 



> 



4 - 



o 3 



X 

 Q. 



2 



1 



o 



t I I I I I I I I I » I ' I t I I I I I I I I I I I I I I I I I I I 1 I I I I 



*' 



250 300 350 400 450 500 550 600 



Wavelength in mp 



Fig. 3.6. Comparison of the photosensitivity and density spectra of 

 visual purple. #, Goodeve, Lythgoe and Schneider's values for the 

 photo-sensitivity; O, Trendelenburg's 'bleaching values,' after correc- 

 tion as described in text. Full line curve, density spectrum of a 

 visual purple solution equated at 500 m/i to the photosensitivity. 

 {After Schneider, Goodeve and Lythgoe, 1939 and 

 Goodeve, Lythgoe and Schneider, 1 942) 



LYTHGOE and WOOD is shown by the full line in Fig. 3.6. It will be 

 seen that there is an accurate coincidence between the photo- 

 sensitivity (ay) and the density (a) at all wavelengths above 430 vcifx 

 indicating that 7, the quantum efficiency, is independent of wave- 

 length over at least this range. 



Although, because of yellow impurities, the density spectrum does 

 not coincide with the ay data below 430 m//, a maximum at 350- 

 360 m// and a minimum at 400 m^ is common to both sets of 

 measurements. From this goodeve, lythgoe and Schneider 

 concluded that the band at 350-360 m^ is closely connected with 

 the visual purple chromophore, and corresponds to a transition 



78 



