186 ON THE ORIGIN OF THE PLANETARY SYSTEM. 



those whose light is sufficiently strong give for the 

 most part a coloured spectrum of fine bright lines, a& 

 they appear in the spectra of the ignited gases. The 

 nebulae are partly rounded structures, which are called 



FIG. 14. 



planetary nebulce (fig. 12) ; sometimes wholly irregular 

 in form, as the large nebula in Orion, represented in 

 fig. 13; they are partly annular, as in the figures in 

 fig. 14, from the Canes Venatici. They are for the 

 most part feebly luminous over their whole surface, 

 while the fixed stars only appear as luminous points. 



