S. P. Langley—Selective Absorption of Solar Energy. 183 
Sir William Herschel, in 1800,* showed that heat extended 
below the visible spe ectrum. He found that about one-half the 
spectrum consisted of obscure Ty 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 Herschel* gave a thermograph of the in- 
visible spectrum indicating unequal absorption below the red. 
DY. J. Draper, in 1842,* observed three wide bands in 
this region which he called a, 8,7. In 1846, Messrs. Foucault 
and Fizeau appear to have observed the same lines. Dr. 
Draper‘ states that prior researches lead him to believe that 
ah 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 nor- 
mal dispersion. Dr. Miiller conjectures Me the wave-length 
of the extremest infra-red ray is about 1:8, and from his dia- 
gram it appears that nearly two-thirds of ob heat is below the 
visible portion 
ndall° 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 
i continuity of the infra-red curve as observed by the ther 
i 
there ei four known bands in the infra-red whose wave- 
lengths are 0:85, 0-99, 14-28, 1°48, and gives Ve ‘77 and 
2"-14 as wave-lengths he supposes himself to have identified. 
If our charts be correct there is no considerable band at 
1"-48, and 2“-14 which he marks as the termination of the spec- 
trum is in fact the hottest point in its neighborhood. 
It seems probable, however, that he had perceived by his in- 
genious method the existence of the band whose wave-length 
On our charts is marked i”:37, and in doing so had reached the 
furthest band then certainly observed. 
Captain Abney,” in 1880, mapped by photography the infra- 
red prismatic spectrum as far as wave-length 1”-075 with a pre- 
cision and completeness till then schrolly unknown, besides 
1 Phil. ence 1800. * Phil. Trans, 1840. * Phil. Mag., May, 1843. 
4 Phil. Mag., 1857. > Poggendorf’s Annalen, vol. cv. Phil. Trans., 1866. 
* Monatsbericht Konig. Acad. Wissenschaft., ’ Berlin, ‘tae. Phil. Mag., 1872, 
mptes Rendus, vol. Ixxxix, p. 298. 
: Comptes Rendus, vol. Ixxxviii, p. 1190. Phil. Trans., 1880. 
