CHAPTER VII. 123

COMETS. Life and work of Olbers—His repulsion theory—Life and work of Encke—His comet—Biela’s comet—Faye’s comet—Return of Halley’s comet—Donati’s comet—Comet of 1861—Spectroscopic study of comets—Theory of Brédikhine—Spectra of comets—Comets of 1880 and 1882—The capture theory—Cometary photography