THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



brushing these off. When this is done the retina loses its green 

 fluorescence while the brushed-off outer hmbs, when massed together, 

 are seen to exhibit it. It was this experiment which lead ewald and 

 KUHNE to postulate a final invisible product, visual white or leucop- 

 sin, in the bleaching of visual purple. 



KUHNE also found that visual purple could be obtained in solution 

 by treating retinae with bile. He was led to try this because of the 

 solvent action of bile on blood corpuscles, the medulla of nerves and 

 lecithin. 'As to the way in which it works on the rods of the retina, 

 that once seen will never be forgotten. A fresh frog's retina placed in 

 a drop of bile under a cover-sHp, immediately breaks out into most 

 wonderful movements, at the edge the rods shoot out like rockets, 

 and where the bile comes in contact with the separated freely move- 

 able outer limbs, these may be seen to curl up like worms, and then 

 suddenly, with a jerk, to stretch out straight again and shoot forward 

 lengthwise; it is just at this moment that the longitudinal striation 

 becomes first visible, then the whole column of the superimposed row 

 of the laminae may be seen, and finally the whole vanishes. It often 

 is as if a roll of coins was shot out of a tube, or Uke a cartridge of 

 grape shot.* 



Solutions of a carmine red colour were prepared by treating frog's 

 retinae with 5 per cent cholate solution. With such preparations 

 KUHNE satisfied himself that the sequence of colour changes 

 which followed exposure to Hght was the same as in the intact 

 retina. 



Many qualitative experiments were carried out by kuhne. The 

 main part of his investigations are described in his monograph 'On 

 the photochemistry of the retina and on visual purple,' an excellent 

 Enghsh translation of which was published in 1878. The passages 

 quoted in the present section were taken from this translation and 

 are typical of kuhne's vividly descriptive style. 



CLASSIFICATION OF THE VISUAL PIGMENTS 



In 1896 KOTTGEN and abelsdorff pubhshed the results of their 

 quantitative investigations into the Hght-absorbing properties of the 

 visual pigments. kDhne's qualitative observations had already 

 pointed to the occurrence of differently coloured 'visual purples' 

 in diff'erent species, kottgen and abelsdorff selected their 

 material from four classes of vertebrates: mammals (4), birds 



27 



