Data from a survey of 51 planetary nebulae (PN) observed with the image tube scanner on Shane Telescope at Lick Observatory are combined with those for a comparable number of objects previously reported. For nearly all of the PN included in the later program, it was possible to obtain adequately accurate plasma diagnostics and line intensities to derive ionic concentrations for He, N, O, Ne, S, Cl, and Ar. To get ionization correction factors we calculated theoretical nebular models to fit the excitation level and the intensities of individual important lines. Final model parameters include the stellar radius, emergent flux, Fv(*), from Husfeld et al. (1984, Astron. Astrophys., 134, 139), nebular size, the optical depth at the hydrogenic Lyman limit, and chemical abundances. Many PN do not appear to be optically very thick in the Lyman continuum.