The planetary nebulæ are beautiful objects; they are like planets with a round or oval disc, equable, slightly mottled and of enormous magnitude; one near γ Aquarii is twenty seconds, and another is twelve 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 uniformity 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 nebulæ, showed that their structure is utterly unlike anything else in creation,[24] 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 intensely 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 astounding conclusion, that planetary nebulæ 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 nebulæ when viewed with a very high telescopic power seem to consist of minute clustering stars, but the spectra of these two nebulæ have one bright line, the structure of both being of the same gaseous constitution.