188 ON THE OKIGIN OF THE PLANETARY SYSTEM. 



such a spectrum of a small but bright nebula in the 

 Dragon. Traces of other bright lines are seen along 

 with them, and sometimes also, as in fig. 17, traces 

 of a continuous spectrum ; all of which, however, are 

 too feeble to admit of accurate investigation. It must 

 be observed here that the light of very feeble objects 

 which give a continuous spectrum are distributed by 

 the spectroscope over a large surface, and are there- 

 fore greatly enfeebled or even extinguished, while the 



FIG. 17. 



undecomposable light of bright gas lines remains unde- 

 composed, and hence can still be seen. In any case, 

 the decomposition of the light of the nebulae shows 

 that by far the greater part of their luminous surface 

 is due to ignited gases, of which hydrogen forms a 

 prominent constituent. In the planetary masses, the 

 spherical or discoidal, it might be supposed that the 

 gaseous mass had attained a condition of equilibrium ; 

 but most other nebulas exhibit highly irregular forms, 

 which by no means correspond to such a condition. 

 As, however, their shape has either not at all altered, 

 or not appreciably, since they have been known and 

 observed, they must either have very little mass, or 



