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.
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