the Doctrine of Radiant Energy. 269 



1866, and was widely used in schools and colleges. In 

 book II. ell. 11., lie elaborately discusses the whole question, 

 summing up in favour of the view that " radiant light and 

 heat are only varieties of the same physical agent, and that 

 when once the spectrum of a luminous object has been 

 obtained, the separation of the different rays from one 

 another is physically complete ; so that, if we take any region 

 of the visible spectrum, its illuminating and heating effect 

 are caused by precisely the same rays." What there was 

 further for Draper or any one else to say in 1872 I am at a 

 loss to comprehend^. 



To pass on to another point. I have followed the excellent 

 advice to read W. Herschel's original memoirs ; but I must 

 confess that the impression produced upon my mind is 

 different in some respects from that expressed by Prof. 

 Langley. It seems to me that Herschel fully established the 

 diversity of radiant heat. In the first memoir f a paragraph 

 is headed " Radiant Heat is of different Refrangihility ,'' the 

 question being fully discussed ; and from the following- 

 memoir (p. 291) it is evident that this proposition extends to 

 invisible radiation. " The four last experiments prove that 

 the maximum of the heating power is vested .among the in- 

 visible rays ; and is probably not less than half an inch beyond 

 the last visible ones, when projected in the manner before 

 mentioned. The same experiments also show that the sun's 

 invisible rays, in their less refrangible state, and considerably 

 beyond the maximum, still exert a heating power fully equal 

 to that of red-coloured light. ..." Can it then be said of 

 De la Eoche that he, in 1811, before anyone else, "derives 

 the just and most important, as well as the then most novel 

 conception, that radiant heat is of different kinds " ? It was 

 doubtless a most important step when De la Roche and 

 Melloni exhibited the diversity of radiant heat hy means of 

 selective absorption ; but I do not see how we can regard 

 them as the discoverers of the fact. 



It would take too long to establish by quotations, but it is 

 pretty evident that in his two. earlier papers | Herschel leaned 

 to the view that Kght was not " essentially different from 

 radiant heat.''' Why then, after laying hands upon the truth, 

 did he let it go, and decide that light and heat are not 

 occasioned by the same rays ? 



* I have limited myself to citations from Englisli writers, but I have 

 no reason to think that the course of opinion was different in France and 

 Germany. 



t Phil. Trans. 1800, p. 255. 



X See pp. 272, 291, 292. 



