Electron densities for six planetary nebulae and HM SGE derived from the C 3 lambda 1907/1909 ratio

Electron densities for IC 418, NGC 6572, IC 1297, NGC 3242, NGC 6818, NGC 3211, and HM Sge derived from high dispersion IUE C III spectrograms are consistently higher than those derived from either surface brightness measurements of forbidden line intensity ratios in the visible. The nebulae were selected for a range of excitation classes from 3 to 9. Line splitting due to expansion velocities is observed for three objects. The great width of the lambda 1909 C III line in HM Sge suggests large expansion velocities.