280 Mr. J. C. M c Connel on Di fraction- Colours, 



Lord Rayleigh to depend on " the light being heterogeneous, 

 the source of finite area, and the obstacles in motion." 



2. Curves on the Colour Diagram. 



In a very interesting paper (Trans. Roy. Soc. Edinb. July 

 1886) Lord Rayleigh has set out a curve representing the 

 series of colours of thin plates on Maxwell's form of Newton's 

 diagram. Before such a calculation had been made, it would 

 have been impossible to predict from theory, except in the 

 very roughest manner, the nature of these colours, though the 

 exact composition of the light in terms of wave-lengths were 

 thoroughly known. The reading of this paper made me 

 anxious to obtain a more complete theory of the splendid 

 colours of iridescent clouds, and I have incidentally deter- 

 mined some of the colours of various diffraction-patterns. 

 This led to the discovery of a serious blunder which I 

 made in my former paper on iridescent clouds, in supposing 

 that the central band in the diffraction-pattern of a slit was 

 colourless. I was following high authority, for Verdet says 

 (Legons oV Optique Physique, § 70), " on apercoit au centre du 

 phenomene une bande blanche et brillante, qui est situ^e sur 

 la direction normale a la fente diffringente." As soon as 

 attention is called to the matter, it is obvious that the edge 

 must be reddish, since the breadth of the band in homogeneous 

 light is proportional to the wave-length ; and, as a matter of 

 fact, this red fringe is the finest red of the whole series. The 

 centre of the band is a pale though bright blue. But this 

 depends on the introduction of the factor Ar 1 in the expression 

 for the secondary vibration, the necessity for which was not 

 recognized in Verdet's time. I do not remember seeing the 

 coloration of the central band distinctly pointed out, though 

 it is implicitly contained in a statement of Verdet in the very 

 section I have quoted. He says that, when white light is 

 used, the red bands correspond to the absence of the brightest 

 part (i. e. the yellow) of the spectrum. This is not quite 

 correct, for my results show that they correspond to the 

 absence of the blue-green. 



As the three corners of his diagram, Maxwell (" Theory of 

 Compound Colours," Phil. Trans. March 1860) selected equal 

 widths on his prismatic spectrum, near the points marked on 

 his scale by 24, 44, and 68. Between any colour whatever 

 and these three a match can be made by altering the propor- 

 tions ; either a combination of three matching the remaining 

 one, or a combination of two matching a combination of the 

 other two. Thus, for example, unit width at any point of his 

 prismatic spectrum could be expressed as the sum of multiples 



