THE VISUAL PIGMENTS 



and the next one are given in Fig. 7.3 (B). They show that the changes 

 from curve 1 to curve 4 are substantially the same in type and repre- 

 sent the bleaching of the main pigment, visual purple. However, the 

 final exposure to white light resulted in small density losses, maximal 

 at 430-440 m^ (x 10, for clearness, curve 4-5, Fig. 7.3 (B)) which 

 could be due to bleaching of the green-rod pigment. According to 

 DENTON and WYLLIE, whcn the green rods are bleached they 

 become darker in green light. This suggests that the photoproduct 

 absorbs long wavelength light more strongly than the parent 

 430 mfji pigment. This, if true, would be a most unusual phenome- 

 non. The difference spectrum in Fig. 7.3 (B) does not support this 

 observation. 



At 500 m.fA, the loss in density which occurred when the solution 

 was bleached by yellow hght was 0-115 (see Fig. 7.3 (A), curves 1 

 and 4). The loss at 430 m/^ on subsequent exposure to white light 

 was about 0-0064 (Fig. 7.3 (A), curves 4 and 5). In terms of optical 

 density, therefore, the amount of 430 m/^ pigment was about 6 per 

 cent of that of the main pigment, visual purple, denton and wyllie 

 found that the green rods amounted to about 8 per cent of the total 

 (see table, p. 188). If the 430 m/^ pigment present in solution is 

 indeed the photosensitive pigment of the green rods, the approximate 

 agreement between these figures suggests that the optical density of 

 pigment 430 in situ is similar to that for visual purple. 



THE AVERAGE DIFFERENCE SPECTRUM FOR THE FROG'S RETINA 



In Fig. 7.4 the filled and empty circles give the difference spectrum 

 (max = 100) on bleaching, as determined for two retinae. The 

 continuous curve is lythgoe's difference spectrum for visual purple 

 in solution at a pH of 8-5. As denton and wyllie concluded, this 

 comparison shows that the principal pigment involved in their 

 measurements was visual purple. 



Now the Amax of the retinal data is at 510 m/i rather than 500 mfx. 

 This cannot be due to the green rods for the Amax of the photo- 

 sensitive pigment, which they contain, is at 430 m^. 



The retinal difference spectrum closely agrees with that, found by 

 arden, for suspensions of the frog's photoreceptors (Fig. 7.2). This 

 suggests that the 'extra' pigment (p. 185) which arden considered 

 was present in his suspensions could likewise be responsible for the 

 displacement to Amax 510 m/i of the retinal data. Now denton and 

 WYLLIE did not mention any rods other than the 'pink' (visual 



192 



