The Density necessary to produce the Nebulium Spectrum

Abstract

THE chief lines of the nebular spectrum have been identified by I. S. Bowen with forbidden transitions in the atoms of singly ionised nitrogen and singly and doubly ionised oxygen (NATURE, 120, 473; Oct. 1, 1927). The life of an atom in the metastable state is very long compared to that of the other excited states, and the transfer to another state is accomplished by collisions of the second kind. Bowen argues that in the nebula, the density being extremely low and hence a long interval of time between impacts, the atoms in metastable states would return to normal states spontaneously with the emission of radiation. From spectroscopic data he has computed the frequency and frequency differences for N II, O II, and O III, and finds agreements with the chief nebular lines. A. Fowler has since computed the frequency differences, using data not available to Bowen, and finds even better agreement with the nebular lines (NATURE, 120, 582, 617; 1927).