NEWTON AND THE COLOURS OF THE SPECTRUM 263 



used a circular aperture, and consequently obtained a spectrum 

 very impure in the middle, but pure towards its upper and 

 lower edges ; yet he says that if" a right line 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." In 

 order to test this I fitted an iris diaphragm to a spectroscope 

 and examined a continuous spectrum, adjusting the diaphragm 

 so as to give the spectrum the same dimensions as Newton's 

 had. At certain points the colour did not appear the same on 

 the same vertical, but I was not sure how far the result was 

 vitiated by internal reflection in the spectroscope. So I 

 focussed a carbon arc on a circular aperture, and by means of 

 a lens and large flint prism produced a very bright steady 

 spectrum about 8 inches long, and of the same shape and 

 degree of purity as Newton's. I saw then that in the middle 

 the yellow overflowed into the orange, and the orange into the 

 red. But others whom I asked gave contradictory answers. 

 Thus it appears that Newton's statement as to the hue being 

 the same on the one transversal is fairly true. 



I next attempted to reproduce his measurements on the 

 boundaries of the colours, using the same carbon arc as source, 

 and projecting the spectrum on a screen. His prism gave a 

 spectrum 2 long. I used in succession a crown prism giving 

 1J , and flint prisms giving 2°49' and 3 36', and in addition 

 to my own observations had measurements made by my best 

 " heptachromic." But in vain ; we could not get anything 

 like an agreement between his results and ours. Our yellow 

 was, for example, always much narrower than his, which 

 occupied ^ of the whole length of the spectrum. 



It is interesting to note that in his analogy between the 

 colours and intervals in the musical scale Newton gives the red 

 end of the spectrum the shortest wave-length. Also the scale 

 he uses does not agree with the diatonic scale ; the latter would, 

 however, fit the observations just as well. The essential 

 feature is that the widths of the orange and indigo strips are 

 the semitones, and the widths of the other five colours the tones. 



As is well known, Newton believed that the dispersion of 

 a medium was proportional to its refraction, and that chro- 

 matic aberration was an insuperable obstacle to the develop- 

 ment of the refracting telescope ; consequently he turned his 



