; 2) \[O[iii]{}\] $\lambda$5007/H$\beta > 6.0$ and \[N[ii]{}\] $\lambda$ 6584/H$\alpha > 1.0$; 3) \[O[iii]{}\] $\lambda$5007/H$\beta > 6.0$ and \[N[ii]{}\] $\lambda$ 6584/H$\alpha
< 1.0$. These line ratios are not very sensitive to the reddening, but are subject to uncertainties related to the slit position and size relative to the nebula. A fourth group of miscellaneous PNe (see below) was also observed as time permitted in the program.

The angular sizes of relatively young PNe are expected to be quite small at distances outside our local neighborhood $\sim 1$ kpc, which has been explored in CO by earlier observations. For example, the well known PN NGC 6720 (the Ring Nebula) which is used as an intermediate benchmark below, has an angular diameter of $76\arcsec$ at a distance of 450 pc (O’Dell et al. 2002), and would have an angular diameter of $\la 11\arcsec$ at distances $\ga 3$ kpc. More compact PNe would have even smaller angular diameters, and such objects are well suited to line searches made with single observations centered on the nebula. The average surface brightness of the PNe in the Balmer lines or the radio continuum is independent of distance, and is a useful guide to the degree of 