SIMBAD references

Except for a very few planetary nebulae, it is usually assumed that intrinsic extinction in such sources is comparatively small, and may be ignored. Such an assumption underpins all previous estimates of nebular distance employing the so-called ``extinction" method. We here suggest that extinction may in fact be appreciable for sources having radii R<0.1pc, and rise to C∼2 where R=8x10^–3pc. Such trends are shown to be in reasonable agreement with a simple model of nebular extinction.