COLOUR VISION. 277 



M. Exner states that the character of this structure is different according to 

 the colour of the light employed. When red light is used a veined structure 

 is seen ; when the light is green, the field appears covered with minute black 

 dots, and when the light is blue, spots are seen, of a larger size than the 

 dots in the green, and of a lighter colour. 



Whether these appearances present themselves to all eyes, and whether 

 they have for their physical cause any difference in the arrangement of the 

 nerves of the three systems in Helmholtz's theory I cannot say, but I am 

 sure that if these systems of nerves have a real existence, no method is more 

 likely to demonstrate their existence than that which M. Exner has followed. 



Colour Blindness. 



The most valuable evidence which we possess with respect to colour vision 

 is furnished to us by the colour-blind. A considerable number of persons in 

 every large community are unable to distinguish between certain pairs of colours 

 which to ordinary people appear in glaring contrast. Dr Dalton, the founder 

 of the atomic theory of chemistry, has given us an account of his own case. 



The true nature of this peculiarity of vision was first pointed out by Sir 

 John Herschel in a letter written to Dalton in 1832, but not known to the 

 world till the publication of Dalton's Life by Dr Henry. The defect consists 

 in the absence of one of the three primary sensations of colour. Colour-blind 

 vision depends on the variable intensities of two sensations instead of three. 

 The best description of colour-blind vision is that given by Professor Pole in 

 his account of his own case in the Phil. Trans., 1859. 



In all cases which have been examined with sufficient care, the absent 

 sensation appears to resemble that which we call red. The point P on the 

 chart of the spectrum represents the relation of the absent sensation to the 

 9olours of the spectrum, deduced from observations with the colour box furnished 

 by Professor Pole. 



If it were possible to exhibit the colour corresponding to this point on the 

 chart, it would be invisible, absolutely black, to Professor Pole. As it does 

 not lie within the range of the colours of the spectrum we cannot exhibit it ; 

 and, in fact, colour-blind people can perceive the extreme end of the spectrum 

 which we call red, though it appears to them much darker than to us, and 

 does not excite in them the sensation which we call red. In the diagram of 

 the intensities of the three sensations excited by different parts of the spectrum, 



