56 ON COLOUR. Paet I. 



tinguishing red from green, which those whose perception of 

 colour is imperfect cannot do. To give an eye for colour is 

 no more possible, as I have before said, than to give an ear 

 for sound ; and though both may be improved by study, if 

 possessed, so both may be impaired by bad habit. No effort 

 will create a natural gift, as no rules will correct the defective 

 vision called " colour-blindness," which confounds a colour with 

 its accidental one. And so common is this defect in England *, 

 that one man in every seven hundred and fifty is said to be 

 colour-blind, i. e. unable to distinguish a certain colour from 

 another, as red from green. And the fact of these two being 

 so often confounded, makes the custom of having red and 

 green lights for opposite signals on board our steamers and 

 on railway lines reprehensible and dangerous. For by those 

 who have defective vision no two colours are so generally 

 confounded as red and green, and to such a degree that a 

 soldier's red coat and the grass of a field, and strawberries (or 

 cherries) and their leaves, appear to them to be of the same 

 colour. Nor is it always the accidental that is mistaken for 

 its complementary colour : some confound orange with grass- 

 green, and yellow with light-green ; and others see " indigo 

 and Prussian blue as black," and pink as pale blue. But 

 black and white, which are accidental to each other, are not 

 confounded, f 



* Women are supposed to have this defect in a minor degree than men. 



•f In the Proceedings of the Royal Society, vol. viii. p. 172, Mr. Pole refers 

 to Dr. Wilson of Edinburgh, who says there are three kinds of colour-blindness. 

 " 1. Inability to discern any colour but black and white. This is very rare. 

 2. Inability to discriminate between the nice distinctions of colour, so common 

 as to be apparently rather the rule than the exception. 3. Inability to dis- 

 tinguish between any of the colours most marked to normal eyes, and its most 

 complete form is what is called dichromic vision, being total blindness to one 

 of the three primary colours. In this last, according to the symptoms exhibited 

 in different cases : 1st, blue and yellow are perfectly distinguished : 2nd, almost 

 all colour-blind persons think they see red, but it is frequently confounded with 

 green (the most common mistake), black, orange, yellow, brown, blue, and 



