168 S. P. Langley on the Selective Absorption of Solar Energy. 



London, the spectro-bolometer by W. Grunow, of New York ; 

 and both have been very satisfactory.) 



Sir William Herschel, in 1800*, showed that heat extended 

 below the visible spectrum. He found that about one half the 

 spectrum consisted of obscure, and one half of luminous heat. 

 Seebeck and Melloni in various memoirs showed that the dis- 

 position of the heat depended on the substance of the prism, 

 and that this was due in part to its absorption. 



In 1840 Sir John Herschelf gave a thermograph of the 

 invisible spectrum, indicating unequal absorption below the red. 



Dr. J. W. Draper, in 1842$, observed three wide bands 

 in this region, which he called a, /3, 7. In 1846 MM. 

 Foucault and Fizeau appear to have observed the same lines. 

 Dr. Draper § states that prior researches lead him to believe 

 that the hottest part of the normal spectrum will be found in 

 the yellow. 



Dr. J. Miiller || gives a construction showing how we may, 

 from the distorted prismatic spectrum, obtain the true or 

 normal dispersion. Dr. Miiller conjectures that the wave- 

 length of the extremest infra-red ray is about 1 M, 8 ; and from 

 his diagram it appears that nearly two thirds of the heat is 

 below the visible portion. 



Tyndall Tf gives the position of the maximum of heat in the 

 prismatic spectrum, and estimates the invisible radiation of the 

 sun to be twice the visible. 



In 1871 Lamansky"** gave a drawing showing three gaps 

 in the continuity of the infra-red curve as observed by the 

 thermopile. Lamansky repeats the usual statement that these 

 infra-red rays are strongly absorbed by the atmosphere. 



In 1879 M. Moutontfj in a valuable memoir, speaks of 

 there being four known bands in the infra-red whose wave- 

 lengths are M '85, 0^99, F'23, F'48, and gives F'77 and 2^-14 

 as wave-lengths he supposes himself to have identified. 



If our charts be correct, there is no considerable band at 

 l/*'48; and 2^*14, which he marks as the termination of the 

 spectrum, is in fact the hottest point in its neighbourhood. 



It seems probable, however, that he had perceived by his 

 ingenious method the existence of the band whose wave-length 



* Phil. Trans. 1800. t Ibid. 1840. 



X Phil. Mag. May 1843. § Ibid. 1847. 



|| Poggendorffs Annalen, vol. cv. 51 PHil. Trans. 1866. 



** Monatsbericht Konigl. Akad. Wissenschaft. JBe?'lin, 1871: Phil. Mag. 

 1872,7ol. xliii. 

 tt Comptes Rendus, vol. lxxxix. p. 291, and vol. lxxxiii. p. 1190. 



