THE COMPOSITION OF THE STARS AND NEBULA, 51 



Thus we can make the stars teach us their own 

 composition with light which started from its source in 

 some cases before we were born light older than our 

 Association itself. 



Until 1864, the true nature of the unresolved nebulse 

 was a matter of doubt. In that year, however, Huggins 

 turned his spectroscope on to a nebula, and made the 

 unexpected discovery that the spectra of some of these 

 bodies are discontinuous that is to say, consist of bright 

 lines only, indicating that ' in place of an incandescent 

 solid or liquid body we must probably regard these 

 objects, or at least their photo- surfaces, as enormous 

 masses of luminous gas or vapour. For it is from 

 matter in a gaseous state only that such light as that of 

 the nebulae is known to be emitted.' So far as obser- 

 vation has yet gone, nebula) may be divided into two 

 classes : some giving a continuous spectrum, others one 

 consisting of bright lines. These latter all appear to 

 give essentially the same spectrum, consisting of a few 

 bright lines. Two of them, in Mr. Huggins' opinion, 

 indicate the presence of hydrogen : one of them agrees 

 in position with a line characteristic of nitrogen. 



But spectrum analysis has even more than this to 

 tell us. The old methods of observation could deter- 

 mine the movements of the stars so far only as they 

 were transverse to us ; they afforded no means of 

 measuring motion either directly towards or away from 

 us. Now Doppler suggested in 1841 that the colors 

 of the stars would assist us in this respect, because they 

 would be affected by their motion to and from the earth, 

 just as the sound of a steam- whistle is raised or lowered 

 as it approaches or recedes from us. Everyone has ob- 



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