The origin of the nebular lines and the structure of the planetary nebulae.

Identification of nebular lines: Eight of the strongest nebula lines are classified as due to electron jumps from metastable states in N II, O II and O III. Several of the weaker lines are identified with recently discovered lines in the spectrum of highly ionized oxygen and nitrogen. Behavior of lines in nebulae: The lines thus identified are shown to behave in various nebulae in a way consistent with the foregoing classifications. A similar study of the few lines yet unknown makes it possible to estimate the stage of ionization from which they arise. Structure of the planetary nebulae: On the basis of the foregoing identifications, the relative sizes and intensities of the monochromatic images of the planetary nebulae are explained by an extension and modification of the ideas developed by Zanstra for hydrogen in the diffuse nebulae.