THE VISUAL PIGMENTS 



quite slow; in bright daylight the colour may vanish almost at once. 

 It is easier, too, to see the colour in some retinae than in others. For 

 example, the frog's retina is deeply coloured; that of the rat, only 

 sHghtly so. 



KUHNE found that a retina bleached by Hght regained its colour if 

 left in darkness for some time. But contact of the retina with the 

 pigment epithelium was necessary for this to occur. This was shown 

 by an experiment in which kuhne carefully raised about half of a 

 retina from the pigment epithelium and sUpped a thin piece of 

 porcelain under the raised portion. The whole was exposed to day- 

 light until it was completely bleached. kOhne then removed the 

 porcelain, allowing the raised flap of retina to sink back again. After 

 a few minutes in sodium Ught (not active in bleaching the retina) he 

 drew the entire retina away to find it uniformly red all over. KiJHNE 

 was so struck by the success of this experiment that he *was seriously 

 led to try with a piece of tissue paper whether the cup of the 

 eye did not contain a small quantity of red secretion; the morsel 

 of tissue paper, however, came out moist it is true, but quite 

 colourless.' 



KtJHNE invented the term 'sehpurpur' for the photo-sensitive 

 retinal pigments. He used this word, which has been translated into 

 Visual purple,' for all visual pigments, even though, as he himself was 

 the first to observe, their colour ranged, in different species, from red 

 to purple or violet. 



According to kDhne, when a retina is bleached it becomes succes- 

 sively chamois, orange and yellow in colour before finally becoming 

 colourless, ewald and kOhne (1878) considered that there were 

 three stages in the bleaching: the original pigment visual purple (or 

 rhodopsin); an intermediate pigment which they called visual 

 yellow (or xanthopsin) and, finally, a colourless substance called 

 visual white (or leucopsin). The most effective light for bleaching 

 visual purple was stated to be that of a yellow-green colour but for 

 the conversion of visual yellow to visual white, Hght of a shorter 

 wavelength, i.e. Ught strongly absorbed by visual yellow, was 

 necessary. 



When an unbleached retina is examined in ultra violet light it 

 shines with a faint bluish fluorescence. After bleaching the retina to 

 the colourless condition, the fluorescence is stronger and of a greenish 

 colour. The new fluorescence is confined to the bacillary layer and, 

 in fact to the outer limbs of the rods as may be demonstrated by 



26 



