the Constitution of the Sun. 95 



thus formed is necessarily somewhat transparent. According 

 to his views, these particles are the sources of the continuous 

 spectrum, and the medium in which they float is the locus of 

 the selective absorption*. Thus he attempts to reconcile the 

 general theory of KirchhofF with the observations and deduc- 

 tions of Forbes, which, as we have seen, were a constant 

 stumblingblock in the way of accepting KirchhofF's expla- 

 nation. 



Lockyer seems to have accepted this theory, and to have 

 defended it in the earlier portion of his workf; but in 1872, 

 after Young's important observation of 1870 and its confirma- 

 tion in 1871, he changed his views, and regarded the layer just 

 outside the photosphere as the true seat of the selective absorp- 

 tion producing the Fraunhofer lines J. I supposed in 1873 

 that my observations then published could be explained on 

 Faye's hypothesis. 



There is, however, a fatal objection to the explanation as 

 given by this theory. If the luminous particles are precipi- 

 tated from the vapours of the photosphere, they cannot be at a 

 higher temperature than the circumambient gases; on the con- 

 trary, on account of their greater radiating power they must be 

 slightly cooler. But the fundamental theory of absorption 

 demands a lower temperature for the vapour producing dark 

 lines than that of the principal source of light behind it; con- 

 sequently this view of Faye cannot be accepted without great 

 modifications. 



Before advancing any theory of my own, it may be well to 

 emphasize two principles taught by the theory of absorption, 

 to which all hypotheses must be conformable. That Faye's 

 fails in this is sufficient cause for its rejection. 



1st. To produce dark lines in a spectrum by absorption, the 

 source of absorbed light must be at a higher temperature than 

 that of the absorbing medium. 



2nd. There is an inferior limit of brightness below which the 

 course of absorbed light cannot go without the spectral lines 

 becoming bright. 



Of these, the first is familiar and requires here neither proof 

 nor comment; the second, though not less evident, is less 

 familiar because less important. As we shall make use of it, 

 however, it may be well to enforce it by reference to common 

 experience. Were it not true, it would be impossible to see 



* Comptes Rendus, vol. lx. 1865. 



t See " A Lecture delivered at the Royal Institution, May 28th, 1869," 

 quoted in Lockyer's ' Solar Physics,' pp* 220, 221 ; also ' The Rede Lec- 

 ture,' May 24th, 1871, quoted in ' Solar Physics,' pp. 317, 318. 



X See revised report of two lectures delivered at Newcastle-upon-Tyne 

 in October 1872, < Solar Physics/ p. 409. 



