32 The Perception of Colour. 



was necessary to dispose of this preliminary cliiBculty, since 

 the result of growing knowledge of the structure and 

 functions of the organ of vision has been to connect colour 

 impressions specially with the cones. 



If it be granted that retina-purple plays an important part 

 in the act of vision, as has been shown, we are in a position 

 for considering facts and arguments in favour of its im- 

 portance in the perception of colour. The first point in 

 favour of that view is the fact that light of different colours 

 acts differently on it. An experiment of Kiihne's shows this 

 in a very unmistakable manner. He arranged frogs' retinas 

 on a screen, and exposed them simultaneously to the 

 whole length of the solar spectrum. He found the bleach- 

 ing process begin with, and pass successively through 

 greenish -yellow, yellowish-green, bluish-green, greenish-blue, 

 blue, indigo, and violet ; later, through pure yellow and 

 orange ; much later, through ultraviolet; and finally, through 

 red. He found that the human retina is bleached by blue 

 to violet in twelve minutes, by green in twenty-five minutes, 

 and by red only in about eight hours. He further found 

 that the various stages in the transformation of the pig- 

 ment, from red through orange to yellow, as well as the 

 ultimate disappearance of all colour, are passed through 

 with varying rapidity. Green light rapidly brings about 

 the change to yellow, but complete decomposition is then 

 slower ; while with violet light the change to yellow is made 

 very slowly, but from that point the advance to complete 

 transparency is rapid. Whether the transformations of the 

 retina-purple differ in kind as well as in the rapidity of 

 their production, under the influence of light of different 

 colours, has not been determined, very little being yet known 

 with regard to its chemical constitution ; and even less is 

 known of the nature and function of the green colour found 

 in certain rods in the retina of the frog, though it also varies 

 under the action of different kinds of monochromatic light. 

 It is established that the photo-chemical changes in the 

 retina are not the same under the stimulus of different 

 colours, and it is therefore fair matter of hypothesis that 

 the sensation of colour is produced by the action of 

 different modifications of the retina -purple or other 

 pigments on the fibres of the optic nerve. Absolute 

 demonstration of this mode of production of sen- 

 sations of colour is, for obvious reasons, difficult, per- 

 haps impossible of attainment • but its claim to acceptance 



