144 PROCEEDINGS OF THE AMERICAN ACADEMY 



the photosphere itself. Regarding the principal radiation of the sun 

 as coming from solid or liquid particles floating in a gaseous medium, 

 the cloud-like stratum thus formed is necessarily somewhat transpar- 

 ent. 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 gen- 

 eral theory of Kirchhoff with the observations and deductions of 

 Forbes, which, as we have seen, were a constant stumbling-block in 

 the way of accepting Kirchhoff's explanation. 



Lockyer seems to have accepted this theoiy, and to have defended 

 it in the earlier portion of his work ; t but in 1872, after Young's im- 

 portant observation of 1870 and its confirmation in 1871, he changed 

 his views, and regarded the layer just outside the photosphere as the 

 true seat of the selective absorption 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 precipitated from the vapors 

 of the photosphere, they cannot be at a higher temperature than the 

 circumambient gases ; on the contrary, on account of their greater 

 radiating power, they must be slightly cooler. But the fundamental 

 theory of absorption demands a lower temperature for the vapor pro- 

 ducing dark lines than that of the principal source of light behind it; 

 consequently this view of Faye cannot be accepted without great 

 modifications. 



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

 size two principles taught by the theory of absorption, to which all 

 hypotheses must be conformable. That Faye's fails in this is suflScient 

 cause for its rejection. 



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

 absorbed light must be at a higher temperature than that of the absorb- 

 ing medium. 



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

 of absorbed light cannot go without the spectral lines becoming bright. 



Of these, the first is familiar, and requires here neither proof nor 



* Comptes Rendus, Ix., 1865. 



t See " A Lecture delivered at the Royal Institution," May 28, 18G9, quoted 

 in Lockyer's Solar Physics, pp. 220, 221; also "The Rede Lecture," May 24, 

 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. 400. 



