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402 APPENDICES 



The turn of mind and character of a man depend much 

 on his physical constitution. Dalton in one respect was 

 singular, he was colour blind ; that is to say, his perception of 

 colour was abnormal ; thus, for example, he could not distin- 

 guish the red berries on a green tree, and hence his out- 

 look was different from that of other men. This peculiarity, 

 which is now well known, has sometimes been called Daltonism 

 from the fact that Dalton was the first to investigate it. It is 

 curious that he was twenty-six years old before he noticed 

 this abnormality ; and yet it is not so strange when we think 

 that thousands of persons must have suffered the same lack 

 of perception all their lives and yet remained in ignorance of 

 their defect. It was not till October, 1794, that the first 

 scientific communication on the subject was made by Dalton 

 under the title of Extraordinary Facts relating to the Vision 

 of Colours. The fact of his colour-blindness was first brought 

 home to him in an amusing fashion. He bought a pair 

 of stockings for his mother, which he believed were of a 

 drab colour, but on receiving the gift she said, " Thou hast 

 bought me good hose, John, but what made thee fancy 

 such a bright colour? Why, I can never show myself at 

 Meeting in them." Her son, disconcerted, replied that to his 

 eyes these stockings were of a dark bluish-drab, a very 

 proper go-to-meeting colour. " Why, they're red as a cherry, 

 John," said his mother, applying for confirmation to her 

 other son, who agreed with John in the matter. On consult- 

 ing her neighbours the verdict was unanimously given in her 

 favour ; " Verra fine stuff but uncommon scarletty." Thus 

 the peculiarity of both brothers' vision was ascertained. 



The cause of colour-blindness is not yet definitely known. 

 It is, however, certain that it is not due to any defect in the 

 lens or aqueous humour of the eye, and Dalton's eye, after 

 death, was found in this respect to be normal. It is now 

 known that a very considerable number of persons, as many 

 in some places as one in fifty-five, are found to be colour- 

 blind. It is evident that the safety of travellers, both by sea 

 and by land, is jeopardised if the conduct of the train or the 

 steering of the ship is in the hands of men whose eyesight is 

 thus defective, and the testing of persons thus employed as 

 to their perception of colour is now universally enforced. 



My friend, Sir William Abney, has written a book on 

 colour-vision, in which the whole subject is ably discussed. 



Dalton's habits were, as I have said, of the simplest, most 

 methodical, and uniform kind. He lived for his science. Every 

 day, and all day long, he spent in his laboratory, except 



