NEWTON AND THE COLOURS OF THE SPECTRUM 257 



the neighbouring colours being screened off, are the intermediate 

 colours visible. On p. 92 we find the first mention of indigo ; 

 on account of its importance for our purpose I reproduce the 

 entire passage together with its accompanying figure : 



Fzg, 4 . 



'* When I had caused the rectilinear line sides AF, GM, of 

 the Spectrum of Colours made by the Prism to be distinctly 

 defined, as in the fifth Experiment of the first book is described, 1 

 there were found in it all the homogeneal Colours in the same 

 order and situation one among another as in the Spectrum of 

 simple Light, described in the fourth Experiment of that Book. 

 For the Circles of which the Spectrum of compound Light PT 

 is composed, and which in the middle parts of the Spectrum 

 interfere and are intermixt with one another, are not intermixt 

 in their outmost parts where they touch those rectilinear sides 

 AF and GM. And therefore in those rectilinear sides when 

 distinctly defined, there is no new Colour generated by refrac- 

 tion . I observed also, that if anywhere between the two outmost 

 Circles TMF and PGA a right line, as 7S, was cross to the Spectrum 

 so as at both ends to fall perpendicularly upon its rectilinear 

 sides, there appeared one and the same Colour and degree of 

 Colour from one end of this line to the other. I delineated 

 therefore in a Paper the perimeter of the Spectrum FAPGMT, 

 and in trying the third Experiment of the first Book, I held the 

 paper so that the spectrum might fall upon this delineated 

 Figure, and agree with it exactly, whilst an Assistant whose 

 Eyes for distinguishing Colours were more critical than mine, 

 did by right lines a/3, 7S, e£, etc., drawn cross the Spectrum, 

 note the confines of the Colours, that is of the red Ma/3F, of the 

 orange a<y8(3, of the yellow 7e£S, of the green er)0£, of the blue 

 7}ik0, of the indico CkfiK, and of the violet \GA/i. And this 

 operation being divers times repeated both in the same and in 

 several Papers, I found that the Observations agreed well 

 enough with one another, and that the rectilinear sides MG 

 and FA were by the said cross lines divided after the manner of 

 a musical Chord. Let GM be produced to X, that MX may be 

 equal to GM, and conceive GX, XX, tX, 17X, eX, 7X, aX, MX, to 



1 I.e. the " ordinary camera " method is used. 



