THE VISUAL PIGMENTS 



rods both provided standards of reference. From them, denton 

 and WYLLiE (1955) were able to estimate roughly the absorption 

 characteristics of the green rods. 



In the unbleached state, the green rods absorbed strongly in the 

 blue (400-440 m/0» moderately in the yellow and red (560-680 m/z) 

 but hardly at all in the green (490-520 m/z). These chai acteristics 

 thus account for their green colour. After being bleached by white 

 light, the green rods became much paler in the blue but darker in the 

 green (see Plate 7.1) while in the yellow and red they hardly changed. 

 This accounts for their grey colour when bleached. 



Using the loss in density in the blue (max. at 430 m/j) as a criterion 

 of bleaching, denton and wyllie found that the green rods were 

 readily bleached by blue light but were hardly affected by green 

 light. They were also insensitive to deep red light (wavelengths 

 longer than 650 m//). From this denton and wyllie concluded 

 that the green rods would act as blue-sensitive receptors, and that 

 the absorption band in the range 560-680 mju was due to the presence 

 of a light-stable pigment in these rods. 



KUHNE (1878), however, had stated that the green rods were 

 especially sensitive to red light. Possibly kuhne's red hght con- 

 tained shorter wavelengths than denton and wyllie's (> 650 mju). 

 Unfortunately the latter authors did not report the effect of light 

 within the range 560-650 m//. If such hght should be effective in 

 bleaching, i.e. in causing loss of density at 430 m//, this would 

 indicate that the light-stable pigment responsible for absorption at 

 the long wavelengths acts as a photosensitizer for the bleaching of 

 pigment 430. In this event the green rods would be sensitive to light 

 within the range 560-650 m^ as well as to blue hght. 



PRESENCE OF GREEN-ROD PIGMENT IN EXTRACTS 



Partial bleaching experiments carried out on a digitonin extract of 

 frogs* retinae have suggested the presence of a small amount of 

 photosensitive pigment with Amax 430-440 m// (dartnall, unpub- 

 lished). The frogs were Rana esculenta, a species closely related to 

 R. temporaria used in denton and wyllie's work. The results are 

 shown in Fig. 7.3. In Fig. 7.3 (A), curve 1 is the density spectrum of 

 the unbleached extract ; curve 2 same after 2 hr exposure to yellow 

 Hght (570 m/z) ; curves 3 and 4 after further exposures of 2 and 1 hr, 

 respectively, to the same yellow light, and finally, curve 5 after 

 exposure to white hght. The difference spectra between each curve 



190 



