INF 



THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



OCTOBER 1905. 



XLYIII. 1 he Origin of the Prismatic Colours. 

 By Lord Bayleigh, 0.31., F.R.S.* 



rf^HE fact that by the aid of a spectroscope interferences 

 JL may he observed with light originally white used to he 

 regarded as a proof of the existence of periodicities in the 

 original radiation ; but it seems now to be generally agreed 

 that these periodicities are due to the spectroscope. When a 

 pulse strikes a grating, it is obvious that the periodicity and 

 its variation in different directions are the work of the 

 grating. The assertion that Newton's experiments prove the 

 colours to be already existent in white light, is usually made 

 in too unqualified a form. 



When a prism, which has no periodicities of figure, is sub- 

 stituted for a grating, the modus operandi is much less obvious. 

 This question has been especially considered by Schuster 

 (Phil. Mag. xxxvii. p. 509, 1894 : vii. p. 1, 1904), and 

 quite recently Ames has given an " Elementary Discussion 

 o£ the Action of a Prism upon White Light " ('Astrophysical 

 Journal/ July 1905). The aim of the present note is merely 

 to illustrate the matter further. 



I commence by remarking that, so far as I see, there is 

 nothing faulty or specially obscure in the traditional treat- 

 ment founded upon the consideration of simple, and accord- 

 ingly infinite, trains of waves. By Fourier's theorem any 

 arbitrary disturbance may be thus compounded ; and the 

 method suffices to answer any question that may be raised, 

 so long at least as we are content to take for granted the 



* Communicated by the Author. 

 Phil. Mag. S. 6. Vol. 10. No. 58. Oct. 1905. 2 F 



