The Spectrum and Composition of the Gaseous Nebulae

I. The primary source of nebular luminosity is the emission of radiation on the re- turn of electrons after ionization by extreme ultra-violet starlight. Additional luminos- ity is produced by the following secondary mechanisms: i. Ionization by secondary radiaticm.-Additional atoms are ionized by extreme ultra- violet quanta emitted by the primary mechanism. 2. Excitation by secondary radiation-Line absorption of secondary quanta may ex- cite an atom of the same kii~d as the primary emitting atom, or any other atom that has absorption lines which coincide with lines of the emitting atom. This mechanism, made possible by certain chance coincidences between lines of He ii, 0 iii, and N iii, explains the presence with a high intensity of a selected group of 0 iii and N in lines while all other lines of these elements are missing. 3. Excitation by secondary electrons ejected by the primary mechanism.-This is prac- tically the sole source of excitation of the forbidden lines. II. A study of nebular line intensities in the light of the foregoing processes indi- cates that H is the most abundant element and He is the second. N, 0, Ne, S-and possibly C and A-are present but are very much rarer. The lines of these heavier ele- ments are strong, not because the elements are very abundant but because they are able to make use of large sources of energy that are not available to the predominent H and He. Lines of F, Na, Si, F, Cl, K, and Ca are missing