268 Lord Eayleigh on the History of 



doctrine of three different kinds of luminous radiation was 

 regarded by every undulationist. The matter was not finally 

 set at rest until Helmholtz showed that Brewster's effects de- 

 pended upon errors of experiment not previously recognized. 



The following, from "W. Thomson*, is almost equally sig- 

 nificant : — 



" It is assumed in this communication that the undulatory 

 theory of radiant heat and light, according to which light is 

 merely radiant heat, of which the vibrations are performed in 

 periods between certain limits of duration, is true. ' The 

 chemical rays ' beyond the violet end of the spectrum consist 

 of undulations of which the full vibrations are executed in 

 periods shorter than those of the extreme visible violet light, 

 or than about the eight hundred million millionth of a second. 

 The periods of the vibrations of visible light lie between this 

 point and another, about double as great, corresponding to 

 the extreme visible red light. The vibrations of the obscure 

 radiant heat beyond the red end are executed in longer periods 

 than this ; the longest which has yet been experimentally 

 tested being about the eighty million millionth of a second." 



Again, in Lloyd's ' Wave Theory of Light '], we find the 

 following passage : — " It appears, then, that sensibility of the 

 eye is confined within much narrower limits than that of the 

 ear ; the ratio of the times of the extreme vibrations which 

 affect the eye being only that of 1*58 to 1, which is less than 

 the ratio of the times of vibration of a fundamental note and 

 its octave. There is no reason for supposing, however, that 

 the vibrations themselves are confined within these limits. In 

 fact, we know that there are invisible rays beyond the two 

 extremities of the spectrum, whose periods of vibration (and 

 lengths of wave) must fall without the limits now stated to 

 belong to the visible rays." 



I believe that it would be not too much to say that during 

 the decade 1850-1860 nearly all the leading workers in 

 physics, with the exception of Brewster, held the modern 

 view of radiation. It would be quite consistent with this 

 that many chemists, photographers, and workers in other 

 branches of science, who trusted to more or less antiquated 

 text-books for their information, should have clung to a belief 

 in the three entities. After 1860, and the discussions re- 

 specting the discoveries of Stewart and Kirchhoff, I should 

 have supposed that there were scarcely two opinions. 

 Stewart's ' Elementary Treatise on Heat ' was published in 



* " On the Mechanical Action of Kadiant Heat or Light ; " &c. Proc. 

 Roy. Soc. Edinb. Feb. 1852. 

 t Longmans, 1857, p. 16. 



