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CHAPTER XII.^[1]

COMETS IN THE SPECTROSCOPE.

The application of the Spectroscope to Comets.—Photography as applied to the Spectra of Comets.—Historical Survey of the progress made.—Four varieties of carbon Spectra.—Three Comets which have yielded special results.—Conclusions of Hasselberg.—The great Comets of 1881 and 1882.—Schäberle'a Comet.—Wells 's Comet.—Instruments of a special kind needed for the Spectra of Comets.—Frost's Dictum.—Borelly's Comet of 1890 (i.).—Brooks's Comet of 1890 (ii.).—Swift's Comet of 1892 (i.): Holmes's Comet of 1892 (iii.).—Rordame's Comet of 1893 (ii.).—Perrine-Griggs's Comet of 1902 (ii.).—Brooks's Comet of 1904 (i.).—Daniel's Comet of 1907 (iv.).—Morehouse's Comet of 1908 (iii.).—One of the most remarkable on record.—Summary of the present state of our knowledge.—Importance of Photography in the study of Comets.—Newall's Theory as to cometary radiation.

The spectroscope has not been employed in the study of comets on the same scale as it has been used for the purposes of solar and stellar research, and consequently the harvest of knowledge obtained by it has not been so great as one could have wished. Until quite recently most cometary spectra observations were made visually, thus excluding the violet and ultra-violet radiations. Prior to 1902 all attempts to photograph the spectra had been made with long-focus telescopes and spectroscopes, primarily designed for stellar work, a procedure now very generally recognised as inadequate except in the case of an exceptionally bright comet.

A comet is a diffuse object, and is not, usually, very bright. To obtain a spectrum of such an object it is essential that the light be condensed as much as possible; therefore long- focus telescopes and spectroscopes which spread out the light are unsuitable for cometary work. For this reason short-focus prismatic cameras have recently been employed, and

1. This Chapter is the joint work of Mr. E. W. Maunder and Mr. W. E. Rolston, two experts in spectroscopic work.