THE VISUAL PIGMENTS 



main, visual purple was being regenerated. In a few instances, how- 

 ever, the rate of regeneration was exceedingly fast, the half-return 

 period being less than 1 min. The difference spectrum for the rapid 

 changes (Fig. 7.8) had Amax at c. 550 m// (whale, 1953c). In a paper 

 pubHshed at the time of writing, wald, brown and smith (1955) 

 have reported that at 10°C and pH 6-5 the velocity constant of the 

 O-lOr— T — I — 1 — 1 — I — I — I — I — I — I — I — I — I — I — I 1 j — I — I — I — 





450 500 550 600 



Wavelength in mp 



Fig. 7.8. Difference spectra for the (normal) slow and the (occasional) 



fast regenerative processes in the living cat's retina. 



{Weaky 1953b and private communication) 



synthesis of iodopsin (Amax = 562 m/^) in solution is more than 

 500 times that for rhodopsin (visual purple). 



In an investigation of the guinea-pig, we ale (1955) found that the 

 spectrum of the density changes occurring when retinae were exposed 

 to the unselective action of white light was only roughly similar to 

 that of visual purple. The *irregularities* suggested that other 

 pigments might be involved as well, we ale therefore used the method 

 of partial bleaching in order to test the homogeneity of the retinal 

 pigment. The results of one experiment are shown in Fig. 7.9. The 

 upper part of Fig. 7.9 gives the spectrum density changes when a 

 dark-adapted retina was partially bleached with blue light. In the 



200 



