ON THE ORIGIN OF THE PLANETARY SYSTEM. 187 



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In many nebulae small stars can be seen, as in figs. 

 15 and 16, from Sagittarius and Aurigo. More stars 

 are continually being discovered in them, the better 

 are the telescopes used in their analysis. Thus, before 

 the discovery of spectrum analysis, Sir W. Herschel's 

 former view might be regarded as the most probable, 

 that that which we see to be nebulae are only heaps of 



PIG. 15, FIG. 16. 



very fine stars, of other Milky Ways. Now, however, 

 spectrum analysis has shown a gas spectrum in many 

 nebulae which contains stars, while actual heaps of stars 

 show the continuous spectrum of ignited solid bodies. 

 Nebulae have in general three distinctly recognisable 

 lines, one of which, in the blue, belongs to hydrogen, 

 a second in bluish-green to nitrogen, 1 while the third, 

 between the two, is of unknown origin. Fig. 1 7 shows 



1 Or perhaps also to oxygen. The line occurs in the spectrum of 

 atmospheric air, and according to H. C. Vogel's observations was want- 

 ing in the spectrum of pure oxygen. 



