Excitation of Nebular Lines

Abstract

THE note which was added in proof by Menzel to his letter in NATURE regarding the effect of collisions on the intensities of nebular lines¹ prompts me to make the following remarks. In presenting the results to which he refers to the San Diego meeting of the Astronomical Society of the Pacific last June, I pointed out that a probable reason for the increase in intensity with pressure of the forbidden transitions in nitrogen is that the rate of production of the metastable states increases more rapidly with pressure than does the rate of de-excitation. It is quite probable that the process of excitation of the metastable nitrogen molecules and atoms in the afterglow is a much more complex process than the production of excited states in nebulæ.