478 DR WILLIAM POLE ON THE PRESENT STATE OF 



elaborate directions about mixing the colours, which he says may be done in an infinity 

 of wa}^s. And in chap. clxi. he states clearly what he considers the " simple colours " to 

 be. His words are : — 



Dei semplici colori il prime- e il bianco, benche i filosofi non amettano ne il bianco ne il nero nel 

 numero de' colori, perche uno e causa de' colori, l'alfcro e privatione. Ma perche il pittore non puo 

 far senza questi, noi li metteremo nel numero degl'altri ; e diremo il bianco in questo ordine essere il 

 primo ne' semplici; — il giallo il secundo ; il verde il terzo ; l'azzurro il quarto ; il rosso il quinto ; il 

 nero il sesto (Paris edition, 1651). 



It is also curious, that in the earliest investigation of my own case, the idea of four 

 primary colours spontaneously occurred to me as suggested by my colour impressions. 

 In my original paper, presented to the Royal Society in 1856, I inserted a note as 

 follows : — 



It occurs to me that if Helmholtz's new doctrine is correct, that blue and yellow combined 

 properly make white (their production of green being only accidental), it would be possible to 

 explain colour-blindness on the hypothesis that normal light was composed of four primary colours, 

 arranged in two complementary pairs, namely, blue and yellow, red and green, the last pair being 

 both invisible to the colour-blind. On this view, our light would be white, the same as to the normal- 

 eyed. I do not pretend to judge, however, whether such a theory would bear investigation. 



At the end of the Report on this Paper made to the Royal Society by Sir John 

 Herschel, he wrote the following remarks on the above passage : — 



On reperusing Mr Pole's paper, a note on p. 79, which had escaped my attention on the first 

 reading, attracted my notice. It appears that the composition of white by blue and yellow, together 

 with the idea of a tetrachromatism in the formation of our colour-scale,* has been suggested by M. 

 Helmholtz. I have not met with any other account of his researches, and am consequently unable 

 to say on what grounds this speculation rests ; but I presume that they must be of a nature similar 

 to those adduced above. 



Prismatic red and green do not make white, however, but yellow ; so that Mr Pole's explana- 

 tion of colour-blindness, in this note, which proceeds on the supposition that they make white, is 

 inapplicable. 



In deference to this opinion, not feeling emboldened to assert (as I now know I might 

 have done) that the hues of red and green invisible to me did make white, I altered the 

 statement, when the paper was printed in 1859, to the form that will be found there, 

 paragraph 17. 



I think the fact of the tetrachromic idea having suggested itself to a colour-bliDd 

 person, as a necessity for explaining his own case, ought to count for something in its 

 favour. 



And it must be added that Sir John did not exclude the possibility of this explana- 

 tion ; for in another part of the same document (see Proc. Roy. Soc, vol. x. p. 79), after 



* This appears to have teen an oversight, as the idea of tetrachromatism was not, to my knowledge, included in 

 Helmholtz's suggestions. 





