218 RECENT PROGRESS OF THE THEORY OF VISION. 



covery of recent years, which has opened the extreme limits of 

 celestial space to chemical analysis. 



- There is an extremely interesting and not very uncommon 

 defect of sight which is known as colour-blindness. In this 

 condition the differences of colour are reduced to a still more 

 simple system than that described above ; namely, to combina- 

 tions of only two primary colours. Persons so affected are called 

 colour blind, because they confound certain hues which appear 

 very different to ordinary eyes. At the same time they distin- 

 guish other colours, and that quite as accurately, or even (as it 

 seems) rather more accurately, than ordinary people. They are 

 usually ' red-blind '; that is to say, there is no red in their 

 system of colours, and accordingly they see no difference which 

 is produced by the addition of red. All tints are for them 

 varieties of blue and green, or, as they call it, yellow. Accord- 

 ingly scarlet, flesh-colour, white, and bluish- green appear to 

 them to be identical, or at the utmost to differ in brightness. 

 The same applies to crimson, violet, and blue, and to red, 

 orange, yellow, and green. The scarlet flowers of the geranium 

 have for them exactly the same colours as its leaves. They 

 cannot distinguish between the red and the green signals of 

 trains. They cannot see the red end of the spectriTm at all. 

 Very full scarlet appears to them almost black, so that a red- 

 blind Scotch clergyman went to biay scarlet cloth for his gown, 

 thinking it was black. 1 



In this particular of discrimination of colours, we find 

 remarkable inequalities in different parts of the retina. In the 

 first place, all of us are red-blind in the outermost part of our 

 field of vision. A geranium-blossom when moved backwards 

 and forwards just within the field of sight, is only recognised as 

 a moving object. Its colour is not seen, so that if it is waved 

 in front of a mass of leaves of the same plant it cannot be dis- 

 tinguished from them in hue. In fact, all red colours appear 

 much darker when viewed indirectly. This red-blind part of 



1 A similar story is told of Dalton, the author of the Atomic Theory.' 

 He was a Quaker, and went to the Friends' Meeting, at Manchester, in a pair 

 of scarlet stockings, which some wag had put in place of his ordinary dark grey 

 ones. TB. 



