The structure of a typical double-ring planetary nebula NGC 2392

Monochromatic photographs in H, Nii] 6584 and Oiii] 5007 Å show many different details in the morphological structure of the Eskimo Nebula (NGC 2392). H and Nii] images show, in a first approximation, similar structure (elliptical inner ring and broken outer ring) whereas both rings in Oiii] are quite circular and regular in brightness. A photometric study using the method described by Louise (1974) gives various geometrical parameters of both rings which are practically the same for the observed lines. In other words, the classical stratification structure in planetary nebulae is not clearly observed in NGC 2392, in good agreement with previous observations (Wilson, 1950). This fact is probably a consequence of the peculiar structure in the geometry of the nebula. A model consisting of an inner toroid surrounded by a spherical shell is proposed to account for both photometric and spectroscopic observations.