180 



On Bright Lines in the Spectra of Nebula, [Apr. 26, 



There are not found in the spectra of different nebulae the differences 

 of relative brightness of the bright lines and of the continuous spectrum 

 which would be expected on Mr. Stone's hypothesis. 



The star-clusters which are just within the resolving-power of the 

 largest telescopes do not give, even faintly, a spectrum of bright lines. 



The same bright lines appear to be common to all the nebulae which 

 give a bright-line spectrum. On Mr. Stone's view, differences in the 

 constitution of the enclosing atmospheres of different star-groups would 

 be probable. 



On this point I may be permitted, perhaps, to add the following 

 sentences from my paper " On the Spectra of some of the jVebulae"* : — 



" It is indeed possible that suns endowed with these peculiar conditions 

 of luminosity (giving bright-line spectra) may exist, and that these bodies 

 are clusters of such suns. There are, however, some considerations, 

 especially in the case of planetary nebulae, which are scarcely in accord- 

 ance with the opinion that they are clusters of suns. Sir John Herschel 

 remarks of one of this class, in reference to the absence of central con- 

 densation : — ' Such an appearance would not be presented by a globular 

 space uniformly filled with stars or luminous matter, which structure 

 would necessarily give rise to an apparent increase of brightness towards 

 the centre, in proportion to the thickness traversed by the visual ray. 

 We might therefore be inclined to conclude its real constitution to be 

 either that of a hollow spherical shell, or of a flat disk presented to us 

 (by a highly improbable coincidence) in a plane precisely perpendicular 

 to the visual ray'f. 



" This absence of condensation admits of explanation without recourse 

 to the supposition of a shell or flat disk, if we consider them to be masses 

 of glowing gas. For supposing, as we probably must do, that the whole 

 mass of the gas is luminous, yet it would follow, by the law which results 

 from the investigations of Kirchhoff, that the light emitted by the por- 

 tions of gas beyond the surface visible to us would be in great measure, 

 if not wholly, absorbed by the portion of gas through which it would 

 have to pass ; and for this reason there would be presented to us a 

 luminous surface only"+. 



It appears, therefore, that the results of observation do not accord well 

 with Mr. Stone's theory. 



But the theory itself appears open to grave objections. It is obvious 

 (and was strongly insisted upon by Prof. Stokes in remarks made when 

 the paper was read) that in a star-cluster in which the stars are sur- 

 rounded by self-luminous atmospheres, the proportion between the sum 

 total of the light from the stars and the light from the atmospheres will be 

 independent of the distance of the cluster from us. Unless, then, we sup- 



* Phil. Trans. 1864, p. 443. 



t ' Outlines of Astronomy,' 7th edit. p. 646. 



% See also Sir William Herschel, Phil. Trans. 1811, pp. 314, 315. 



