204* Newton's Letters, Hypothesis and Experiments 



the weakest, blues and violets ; the middle with green, and a 

 confusion of all, with white; much after the manner that in 

 the sense of hearing nature makes use of aerial vibrations of 

 several bignesses, to generate sounds of divers tones ; for the 

 analogy of nature is to be observed. And further, as the har- 

 mony and discord of sounds proceed from the proportions of 

 the aerial vibrations, so may the harmony of some colours, as 

 of a golden and blue, and the discord of others, as of red and 

 blue, proceed from the proportions of the aetherial. And 

 possibly colour may be distinguished into its principal de- 

 grees: red, orange, yellow, green, blue, indigo, and deep 

 violet, on the same ground that sound within an eighth is 

 graduated into tones. For, some years past, the prismatic co- 

 lours, being in a well-darkened room, cast perpendicularly 

 upon a paper about two-and-twenty foot distant from the 

 prism, I desired a friend to draw with a pencil lines across 

 the image or pillar of colours, where every one of the seven 

 aforenamed colours was most full and brisk, and also where 

 he judged the truest confines of them to be, whilst I held the 

 paper so that the said image might fall within a certain com- 

 pass marked on it. And this I did, partly because my own 

 eyes are not very critical in distinguishing colours, partly be- 

 cause another to whom I had not communicated my thoughts 

 about this matter could have nothing but his eyes to deter- 

 mine his fancy in making those marks. This observation we 

 repeated divers times, both in the same and divers days, to 

 see how the marks on several papers would agree ; and com- 

 paring the observations, though the just confines of the co- 

 lours are hard to be assigned, because they passed into one 

 another by insensible gradation; yet the differences of the 

 observations were but little, especially towards the red end ; 

 and taking means between those differences that were, the 

 length of the image (reckoned not by the distance of the 

 verges of the semicircular ends, but by the distance of the 

 centres of those semicircles, or length of the straight sides, as 

 it ought to be) was divided in about the same proportion that 

 a string is between the end and the middle to sound the tones 

 in an eighth. You will understand me best by viewing the 

 annexed figure, in which AB and CD represent the straight 

 sides about ten inches long, A P C and B T D the semicir- 

 cular ends, X and y the centres of those semicircles, X Z the 

 length of a musical string double to Xj/, and divided between 

 X and y so as to sound the tones expressed at the side (that 

 is X H the half, X G and G I the third part, y K the fifth 

 part, y M the eighth part, and G E the ninth part of X y) ; 

 and the intervals between these divisions express the spaces 



