248 RECENT PROGRESS OF THE THEORY OP VISION. 



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 spectrum at all. Very full scarlet appears to them 

 almost black, so that a red-blind Scotch clergyman went 

 to buy scarlet cloth for his gown, thinking it was black.* 



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 distinguished 

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

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

 retina is most extensive on the inner or nasal side of the 

 field of vision ; and according to recent researches of 

 Woinow, there is at the furthest limit of the visible field 

 a narrow zone in which all distinction of colours ceases 

 and there only remain differences of brightness. In this 

 outermost circle everything appears white, grey, or black. 

 Probably those nervous fibres which convey impressions 

 of green light are alone present in this part of the retma. 



In the second place, as I have already mentioned, the 

 middle of the retina, just around the central pit, is 

 coloured yellow. This makes all blue light appear some- 

 what darker in the centre of the field of sight. The 

 effect is particularly striking with mixtures of red and 

 greenish-blue, which appear white when looked at directly, 

 but acquire a blue tint when viewed at a slight distance 



' 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. 



