CHAP. XII.] COLOUR ONCE MORE. 379 



the difficulty? Why should the conditions of thinking 

 correctly be inviolable in the sense of not preventing you 

 from thinking incorrectly, provided they are inviolable in 

 the sense of ensuring that you take the consequences if you 

 do ? The laws of projection in geometry are inviolable, but 

 nobody ever thought it a paradox that it is possible for a 

 picture to be out of drawing in spite of them, nor is it a 

 paradox that in unfamiliar classes of cases a rigorously 

 accurate piece of perspective looks out of drawing. Perhaps 

 you meant, for I suppose the report in Nature is incomplete, 

 that it was a difficulty to say in what sense mathematical 

 propositions could be said to be certain, considering that one 

 may make mistakes about them. Perhaps something else, 

 which for the above reason or others, is hidden from me. 



... By the way, I hope it is true that you are to pro- 

 fess experimental physics at Cambridge, or what I hope 

 comes to the same thing, that you are a candidate. 



To C. J. MONRO, Esq. 



Glenlair, Dalbeattie, 15th March 1871. 



I have been so busy writing a sermon on Colour, and 

 Tyndalising my imagination up to the lecture point, that 

 along with other business I have had no leisure to write 

 to any one. 



I think a good deal may be learned from the names 

 of colours, not about colours, of course, but about names ; 

 and I think it is remarkable that the rhematic instinct 

 has been so much more active, at least in modern times, on. 

 the less refrangible side of primary green (A, = 510 X 10~ 9 

 inches). 



I am not up in ancient colours, but my recollection of 

 the interpretations of the lexicographers is of considerable 

 confusion of hues between red and yellow, and rather more 

 discrimination on the blue side. Qu. If this is true, has 

 the red sensation become better developed since those days ? 

 Benson has a new book, Chapman & Hall, 1871, called 

 Manual of Colour. 



