  A new measure of reddening (E$_{(B-V)}$$\sim$0.00) has been obtained from the
comparison between the observed and the theoretical intensity decrement for 20
emission lines of the $\ion{He}{ii}$ Fowler (n$\to$3) series. This value has
been confirmed by the STIS and IUE continuum distribution, and by the value of
n$_H$ from the damped profile of the IS H Ly-$\alpha$ line. We have obtained
very accurate measurements for about thirty Bowen lines of $\ion{O}{iii}$ and a
precise determination of the efficiency in the O1 and O3 excitation channels
(18 % and 0.7 %, respectively). The relative $\ion{O}{iii}$ intensities are in
good agreement with the predictions by Froese Fischer (1994). A detailed study
of the decays from all levels involved in the Bowen mechanism has lead to the
detection of two new $\ion{O}{iii}$ Bowen lines near $\lambda$ 2190. High
resolution IUE data have shown a nearly linear decline with time, from 1978 to
1995, in the efficiency of the O1 and O3 processes, with a steeper slope for
the O3 channel. A detailed study of the $\ion{N}{iii}$ $\lambda$ 4640 lines and
of their excitation mechanism has shown that, recombination and continuum
fluorescence being ruled out, line fluorescence remains the only viable
mechanism to pump the 3d $^2D_{5/2}$ and 3d $^2D_{3/2}$ levels of
$\ion{N}{iii}$. We point out the important role of multiple scattering in the
resonance lines of $\ion{O}{iii}$ and $\ion{N}{iii}$ near $\lambda$ 374 and
show that the observed $\ion{N}{iii}$ line ratios and intensities can be
explained in terms of line fluorescence by the three resonance lines of
$\ion{O}{iii}$ at $\lambda$$\lambda$ 374.432, 374.162 and 374.073 under
optically thick conditions.
