60 



On the Spectra of the Great Nebula in Orion. [May 2, 



solid bodies had been converted into a gas of a very high temperature. 

 They would take the same place, if we assume with Sir William 

 Thomson* the coming together of two or more cool solid masses by the 

 velocity due to their mutual gravitation alone. 



I pointed out in 1864f that the gaseous nature of these bodies 

 would afford an explanation of the appearance of flat disks without 

 condensation which many of them present. The light emitted by the 

 portions of the gas further from us would be in part or wholly 

 absorbed by the gas through which it would have to pass, in this way 

 giving to us the appearance of a luminous surface only. 



In some of these bodies there is also a, very faint continuous 

 spectrum, which if we had more light might be found to consist, in 

 great part at least, of closely adjacent bright lines. Such is probably 

 the nature, in part, of the apparently continuous spectrum of the 

 nebula with which this paper deals chiefly, the Great Kebula in 

 Orion. 



In other gaseous nebulae strong condensations are seen, and a 

 stronger "continuous" spectrum. When we come to nebulee of 

 which the nebula in Andromeda may be taken as representative, the 

 strong bright line spectrum is absent, and we have what for con- 

 venience I called in my original observations of these bodies a 

 " continuous " spectrum, though I was careful to point out that it was 

 probably " crossed by bright or dark lines." 



Out of about sixty nebulae and close clusters observed by me 

 up to 1866, I found a proportion of about one-third, namely, nineteen, 

 to present the spectrum of bright lines. % 



The stage of evolution which the nebula in Andromeda represents is 

 no longer a matter of hypothesis. The splendid photograph recently 

 taken by Mr. Roberts § of this nebula shows a planetary system at a 

 somewhat advanced stage of evolution ; already several planets have 

 been thrown off, and the central gaseous mass has condensed to a 

 moderate size as compared with the dimensions it must have possessed 

 before any planets had been formed. 



[Mr. Maunder permits me to add that he does not consider the 

 measures and estimations of the motions of the nebula taken in 1884 

 and 1887 of any weight, but he attaches great importance to the 

 direct comparisons of March, 1884, which show that the nebula has 

 but very little, if any, sensible motion in the line of sight. — May 16.] 



* ' Eoy. Instit. Proc.,' vol. 12, pp. 15, 16. 



f ' Phil. Trans.,' 1864, p. 442. 



% < Phil. Trans.,' 1866, p. 383. 



§ ' Monthly Notices R.A.S.,' vol. 65, p. 49. 



[The diagrams have been made with care, but the positions of the lines must be 

 taken from the tables of wave-lengths. — May 13.] 



