  We revise the physical conditions in the O++ zone of the planetary nebula NGC
6543, obtaining two different estimates of the electron temperature (Te) and
one estimate of the electron density (Ne). The electron temperature is computed
by means of two independent methods, the nebular-to-auroral ratio [O III]
5007/4363, and the diagnostic diagram that combines 5007 to the [O III]
infrared lines 52m and 88m. The optical and infrared fluxes have been obtained
from archive HST/WFPC2 images and ISO LWS spectra respectively, and the
continuum intensity in the optical has been measured from narrow-slit spectra
obtained with the Isaac Newton Telescope at La Palma. The measured continuum
intensity is higher than predicted by recombination theory under the hypothesis
that all the Ly-alpha photons either escape or are destroyed. This fact can be
explained in terms of an enhancement of the 2-photon continuum due to Ly-alpha
conversion, a process that depends strongly on the local structure of the
nebula. Alternative possibilities, outside the framework of recombination
theory, have also been considered: e.g., the optical tail in the X-ray emission
of a very hot plasma, and dust scattering of stellar radiation, but these
hypotheses are not supported by quantitative estimates. While the electron
temperature and density derived from the diagnostic diagram agree with the most
recent determination, the temperature derived from 5007/4363,, Te(O++)_opt, is
somewhat smaller than previously published values. We discuss several technical
issues that contribute to the overall uncertainty in our results, focusing on
the instrumental effects that might bias the [O III] 4363 intensity. We also
discuss the effects of the collisional de-excitation of the O++ ground terms on
the relation between Te(O++)_opt and Te(O++)_IR.
