  We use a sample of 332 Hubble Space Telescope spectra of 184 QSOs with z >
0.33 to study the typical ultraviolet spectral properties of QSOs, with
emphasis on the ionizing continuum. Our sample is nearly twice as large as that
of Zheng et al. (1997) and provides much better spectral coverage in the
extreme ultraviolet (EUV). The overall composite continuum can be described by
a power law with index alpha_EUV = -1.76 +/- 0.12 (f_nu ~ nu^alpha) between 500
and 1200 Angstroms. The corresponding results for subsamples of radio-quiet and
radio-loud QSOs are alpha_EUV = -1.57 +/- 0.17 and alpha_EUV = -1.96 +/- 0.12,
respectively. We also derive alpha_EUV for as many individual objects in our
sample as possible, totaling 39 radio-quiet and 40 radio-loud QSOs. The typical
individually measured values of alpha_EUV are in good agreement with the
composites. We find no evidence for evolution of alpha_EUV with redshift for
either radio-loud or radio-quiet QSOs. However, we do find marginal evidence
for a trend towards harder EUV spectra with increasing luminosity for
radio-loud objects. An extrapolation of our radio-quiet QSO spectrum is
consistent with existing X-ray data, suggesting that the ionizing continuum may
be represented by a single power law. The resulting spectrum is roughly in
agreement with models of the intergalactic medium photoionized by the
integrated radiation from QSOs.
