Page:The wonders of optics (1869).djvu/267

 partly consist of gaseous matter in a luminous condition is evidenced by their showing a series of bright lines in the spectroscope, exactly like those produced by terrestrial gases. Their light is therefore not emitted by a solid or liquid incandescent body, but by a glowing gas. The lines mentioned by Messrs. Huggins and Miller showed that the nebula in the sword-handle of Orion consists of hydrogen and nitrogen in a state of luminous incandescence. Not the slightest trace of a continuous spectrum can be detected in the light emanating from this body; consequently, according to present hypotheses, it contains no solid matter at all. A number of other nebulæ have given similar results.

There are numerous star clusters which, unlike the true nebulæ, give continuous spectra when their light is submitted to the action of the prism. Of these may be specially mentioned the great clusters in Andromeda and Hercules, which give continuous spectra, interrupted by dark bands on the red and orange. The light thrown by these experiments upon the nebular hypotheses of Sir William Herschel, who considered that true nebulæ consisted of the primordial gaseous matter out of which suns and stars have been elaborated, is very great, and will be appreciated even by those whose knowledge of astronomy is small.

Spectral analysis has also been the means of our witnessing a celestial conflagration, and understanding the cause of this marvellous event. It is well known to most people that from time to time stars have suddenly burst upon us, and have almost as suddenly disappeared. The theories advanced to account for these singular celestial visitors, have been more numerous than satisfactory. In May 1866, a star of the second magnitude suddenly burst forth in the Northern Crown, and was almost immediately noticed by Mr. Huggins who brought every power of prism and telescope to bear