  We present absolutely calibrated FUV spectra of the quasar 3C~273 covering
900- 1800 A. Our ~3 A resolution spectra were obtained with the Hopkins
Ultraviolet Telescope during the Astro-1 mission in December 1990 and during
the Astro-2 mission in March 1995. Both spectra exhibit a change in slope near
the Lyman limit in the quasar rest frame. At longer UV wavelengths, the
continuum has a power-law index of 0.5-0.7, while shortward of the Lyman limit
it is 1.2-1.7. The energy distribution in nuf_nu therefore peaks close to the
quasar Lyman limit. The short wavelength UV power law extrapolates well to
match the soft X-ray excess seen in simultaneous observations with BBXRT and
nearly simultaneous ROSAT observations. The general shape of the broad-band
spectrum of 3C 273 is consistent with that of an optically thick accretion disk
whose emergent spectrum has been Comptonized by a hot medium. Our UV spectrum
is well described by a Schwarzschild black hole of 7e8 Msun accreting matter at
a rate of 13 Msun/yr through a disk inclined at 60 degrees. Superposed on the
intrinsic disk spectrum is an empirically determined Lyman edge of optical
depth 0.5. The Comptonizing medium has a Compton parameter y~1, an optical
depth to electron scattering of unity, and a temperature of 4e8 K. This overall
shape is the same as that found by Zheng et al. and Laor et al. in their UV and
X-ray composite spectra for quasars, giving physical validity to the composite
spectrum approach. When combined with those results, we find that the generic
ionizing continuum shape for quasars is a power law of energy index 1.7-2.2,
extending from the Lyman limit to ~1 keV. The observational gap in the extreme
ultraviolet for these combined data describing the quasar continuum shape is
now only half a decade in frequency.
