$\log~g~=~8.5$. We also integrated the spectra of the four white-dwarf primary spectrophotometric standards described by Bohlin et al. (1995) and the models of Wesemael et al. (1980). Both works are in good agreement (although not necessarily independent) with Koester’s models.

The quasar spectrum
-------------------

The quasar spectra used in Sect. 6 were derived from an average spectrum build from the composite spectra of Zheng et al. (1997) and Francis et al. (1991). The former was used from 310 to 2000 Å and the latter from 2000 to 6000 Å. The match between both spectra is reasonably good, as shown by Zheng et al. (1997, their Fig. 9). It was performed on the continuum windows identified by these authors between 1400 and 2200 Å. The absorption characteristics of the quasar spectra due to the intervening Ly$\alpha$ clouds were established, between Ly$\alpha$ and the Lyman break, on the basis of the concept of Oke & Korycansky (1982) and of the works presented by Irwin et al. (1991), Zuo & Lu (1993) and Warren et al. (1994). For the region below the Lyman break, three absorption models were computed. The first two were adapted from the works of Møller & Jakobsen (1990), Møller 