Electron Densities in Planetary Nebulae.

Measurements are given of [0 ii] X 3 729/X 3726 intensity ratios in planetary nebulae. These observations show that large-scale density fluctuations of large amplitude do not occur in these planetaries. There is a systematic relation between surface brightness and electron density, of the form that would be expected if all planetaries were objects of similar mass and temperature, observed at different stages in their expansion from small, dense objects to large, low-density nebulae. The measured mean electron density of NGC 3587, together with estimates of its brightness by Minkowski and Aller, show that its mass is only about 0.02 Mo The measurements of electron densities in NGC 6720, together with measurements of its brightness by Liller, show that the small-scale density fluctuations in it are not so extreme as in the diffuse nebula NGC 1976 or in the planetary nebula NGC 7027.