On the Decomposition of Light, 163 



the spaces of each colour, and found that they had a certain 

 rtlation with the tones of a musical gamut *. This same 

 division, expre?std in cyphers, serves Newton afterwards for 

 establishing different calculations, and particularly the me- 

 thod by which he determined the course of the shades of 

 coloured rings : he returns, besides, in several places of his 

 work, to speak of the confines of this and that colour, and 

 directs his experiments accordingly. We cannot suppose, 

 therefore, that he was not persuaded himself of this funda- 

 mental proposition, or that he hesitated to announce it por 

 sitively. 



With respect lo the truth of the proposition itself, it is 

 easy to ascertain it in several ways. 



1st, By repeating Newton's experiment, which is the se- 

 venth of the second book of the first part of his Optics, it 

 is merely sufficient, without searching for the utmost ex- 

 actitude, to procure in a dark chamber a solar spectrum not 

 too confined in its breadth. We there see distinctly enough, 

 if not all the sections of the colours, at least some of them, 

 particularly in the region of the most refrangible rays. If 

 we also arrange an apparatus for giving a very long and very 

 narrow spectrum by means of a small lens placed before the 

 prism ; then, by receiving the image upon a card rather 

 nearer the lens than its focus, we shall there find the blue 

 in particular, terminated at each end by a straight line, very 

 clearly defined. 



2d, 'I'he disk of the moon, when full, and far from the 

 horizon, observed with a simple prism, displays upon her 

 lengthened image several circles which indicate separations 

 of colours perfectly unblended. 



3d, Examine also with the prism the coloured fringes of 

 a white rectangular body, a little large, and well illuminated, 

 you will remark on one side a red stripe falling into the 

 yellow abruptly, which, in its turn, loses itself in the white; 

 and, on the other hand, some blue, distinctly enough be* 

 tween the white and a violet stripe : from this it wouM fol- 

 low that there are only four kinds of colours. 



* Optics, book i. part 'J. problem 1. 



L 2 4th, Lastly, 



