THE CONSTITUTION OF NEBULAE. 



137 



dark spaces ; the third is without any nucleus, but shows a well-defined 

 ring of light. 



The highest type of nebulae are certainly the stellar nebulae, in 

 which a tolerably well-defined bright star is surrounded by a com- 

 pletely rounded disk or faint atmosphere of light, which sometimes fades 

 away gradually into space, at other times terminates abruptly with 



Fig. 14. 



Fig. 15. 



PLANETARY ANNULAR NEBULA WITH Two STAE8. 



Planetary Nebula. 



a sharp edge. Figs. 16 and 17 exhibit the most striking of these very 

 remarkable stellar nebulae : the first is surrounded by a system of rings 

 like Saturn, with the thin edge turned toward us; the second is a veri- 

 table star of the eighth magnitude, and is not nebulous, but is sur- 

 rounded by a bright luminous atmosphere perfectly concentric. To 

 the right of the star is a small dark space, such as often occurs in 

 these nebulae, indicating, perhaps, an opening in the surrounding at- 

 mosphere. 



We have now passed in review all that is at present known of the 

 nebulae, so far as their appearance and form have been revealed by the 

 largest telescopes. The information as yet furnished by the spectro- 

 scope on this subject is certainly much less extensive, but is neverthe- 



Fig. 16. 



Fig. 17. 



Planetary Nebula. 



Stellar Nebula. 



less of the greatest importance, since the spectroscope has power to 

 reveal the nature and constitution of these remote heavenly bodies. 

 It must here again be remembered that the character of the spectrum 

 not only indicates what the substance is that emits the light, but also 



