The Origin of the Nebulium Spectrum

Abstract

IN his letter to NATURE of Oct. 1, p. 473. Mr. I. S. Bowen has made the important suggestion that several of the chief lines in the spectra of gaseous nebulæ may be due to what spectroscopists have called ‘forbidden’ combinations of terms in the spectra of ionised nitrogen (N II), ionised oxygen (O II), and doubly-ionised oxygen (O III). It is, of course, no longer permissible to suppose the existence of hypothetical elements to account for the long-standing mysteries of nebular spectra, and we must accordingly regard the nebular lines as being produced by known elements under conditions of excitation which have not yet been imitated in the laboratory. It has seemed natural enough to look to the lighter elements, and those who, like myself, have given special attention to the spectra of these elements under widely varied conditions, have doubtless kept in view the possibility of finding some indications of nebular lines in the course of their observations. Extensive experiments, however, have failed to reveal any traces of them.