On the Continuous Radiation found in some Celestial Spectra beyond the Limit of the Balmer Series of Hydrogen

Abstract

IN the account of his observations of the eclipse of January 22, 1898, published in the Philosophical Transactions, A 197, pages 389 and 399, Mr. Evershed directed attention to a curious continuous spectrum emitted by the solar chromosphere and prominences. This spectrum begins near the limit or head of the Balmer series of hydrogen lines, and extends with gradually decreasing intensity in the direction of shorter wave-lengths. In describing the phenomenon Evershed referred to the early observation by Huggins of an absorption in the corresponding region in the spectra of Vega and other stars having especially strong hydrogen lines (stars of Class A),¹ and advanced the opinion that the spectrum, like the Balmer series which it so curiously supplements, is due to hydrogen. The grounds for this view were afterward strengthened through the discovery by Wood of a continuous spectrum occurring beyond the limit of the sodium series of dark lines, under conditions of laboratory experimentation that favoured the development of the higher members of the sodium series.² More recently an emission spectrum, apparently identical in character with the one observed by Evershed in the chromosphere, has been found to be characteristic of the planetary nebulæ.³ The spectrum seems also to occur in the diffuse nebula N.G.C. 1499,⁴ and has been a conspicuous feature in the radiation of the. novæ. It may therefore be regarded as rather a commonplace phenomenon pertaining to the spectra of celestial objects which appear to exist under conditions of strong thermal or electrical excitation. For the purposes of this note I shall use the term outlying spectrum in referring to it in order to distinguish it from the general continuous spectrum of more uniform distribution which is found even in the gaseous or “bright-line” nebulæ.