SECT. TV. SPECTRA OF VARIOUS NEBULA. 159 



seconds in diameter. Sir John Herschel computed that 

 if these objects be as far from us as the nearest of the 

 fixed stars, their magnitude, on the lowest estimation, 

 would fill the orbit of Uranus. He discovered twenty- 

 eight or twenty-nine of them, some of a beautiful blue 

 tint, in the southern hemisphere; and from the uni- 

 formity of the discs in both hemispheres, and their 

 apparent want of condensation, he presumed that they 

 may be hollow shells emitting a feeble light from their 

 surfaces only. The spectrum analysis of that light, by 

 Mr. Huggins, in six of the planetary nebulae, showed 

 that their structure is utterly unlike anything else in 

 creation, 6 for instead of an ordinary spectrum he 

 found, to his infinite surprise, that the spectra of 

 the feeble light of these bodies consist only of three 

 bright lines, such as those which proceed from an in- 

 tensely heated gas, and that the lines exhibited some of 

 those of the hydrogen and nitrogen spectra and an 

 unknown gaseous substance : whence he draws the as- 

 tounding conclusion, that planetary nebulae are probably 

 composed of hydrogen, nitrogen, and some unknown 

 gas, without any solid nucleus whatever. 



The annular nebula in Lyra, which is probably nearest 

 to the earth, and the dumb-bell nebula, gave a spectrum 

 indicating matter in a gaseous form. The annular nebula 

 appears to be a hollow elliptical ring of nebulous matter 

 of enormous magnitude. The interior opening of the 

 ring is not entirely dark, but filled with a faint hazy 

 light, like fine gauze stretched over a hoop. The dumb- 

 bell nebula in the constellation Vulpecula is like an 

 hour-glass of bright matter surrounded by a thin hazy 

 atmosphere, which gives the whole the form of an oblate 

 spheroid. Both of these nebulae when viewed with a 

 very high telescopic power seem to consist of minute 



6 Since this observation, Mr. Huggins discovered that two small comets 

 give an analogous spectrum. 



