THE VISUAL PIGMENTS 



One, found in four mammals, a bird and three amphibians, has 

 maximum absorption in the blue green at about 500 m^. Human 

 visual purple, as shown by the earher measurements of konig (1894) 

 is also in this class. The other kind, found in eight fishes, has 

 maximum absorption in the yellow green (at about 540 m//), thus 

 accounting for the more violet colour of fish retinae, as first noticed 

 by kOhne. 



To distinguish between these two pigment groups the term visual 

 purple (or rhodopsin) is now reserved for pigments with absorption 

 maxima at around 500 m.ju and the term visual violet (or porphyrop- 

 sin) for the fish pigments. Occasionally — when it is desired to specify 

 a particular pigment — the name of the species is added, as in *human 

 visual purple' or 'carp porphyropsin.' Alternatively, it is sometimes 

 convenient to use the wavelength at which there is maximum 

 absorption (Amax) as the means of identification. Thus human and 

 rat visual purples, which both absorb maximally at 497 m// and seem 

 to be identical, may be called 'visual pigment 497.* 



THE VISUAL PIGMENTS AND THE VITAMINS A 



FRiDERiciA and HOLM (1925) and tansley (1931) showed that in 

 the retinae of vitamin A-starved rats, the visual purple was syn- 

 thesized more slowly than in normal animals. This suggested that 

 vitamin A might be concerned in the retinal production of visual 

 purple. 



Direct evidence for the presence of vitamin A in eye tissues was 

 first obtained by wald (1933, 1935a). Vitamin A was identified, in 

 the yellow oil obtained from various eye tissues, (a) by its density 

 spectrum (Amax = 328 m^) in chloroform, (b) by the blue colour 

 (Amax = c. 620 mju) given with the Carr -Price reagent (antimony 

 trichloride), and (c) by the curative property of the oil when fed to 

 rats suff'ering from vitamin A deficiency. The curative eff*ect of 

 adding retinal tissue to the diet had been noted by previous workers, 

 first by HOLM (1929). 



THE RHODOPSIN (VISUAL PURPLE) CYCLE 



The intimate relationship between visual purple, vitamin A and a 

 new substance — retinene — was then demonstrated by wald (1935b, 

 1936a) in experiments with the retinae of frogs (Rana esculenta, R. 

 pipiens and R, catesbiana). wald found that when dark adapted 

 retinae were extracted in darkness with a homopolar solvent, such as 



30 



