The blazar 3C 279 is well studied and shows frequent large continuum flares from radio to \gamma -ray wavelengths .
There have been a number of multi-wavelength observations of 3C 279 , and hence there are many ultraviolet data for this object available in the UV archives .
In this paper we present Ly \alpha emission line measurements for 3C 279 using all the archival IUE SWP spectra from 1988 to 1996 and all archival HST/FOS G190H spectra from 1992 to 1996 .

Individual archival IUE spectra of 3C 279 show weak Ly \alpha emission at \sim 1868 Å ( z = 0.536 ) , which is easily seen in the co-added data .
The Ly \alpha emission is observed in all the HST/FOS spectra .
The strength of Ly \alpha is nearly constant ( \sim 5 \times 10 ^ { -14 } erg cm ^ { -2 } s ^ { -1 } ) , while the 1750 Å continuum varies by a factor of \sim 50 , from \sim 0.6 to 31.6 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } Å ^ { -1 } .

The behavior of the Ly \alpha emission line flux and continuum flux is similar to that of the only other well observed blazar , 3C 273 , which shows constant line flux while the continuum varies by a factor of \sim 3 .
This near-constancy of emission line flux in the two best-studied blazars , suggests that the highly variable beamed continuum is not a significant source of photoionization for the gas .
Some other source , such as thermal emission from an accretion disk , must be providing a significant fraction of the photoionizing flux in these objects .
The large amplitude variability seen at \gamma -ray energies must be due to changes in the energetic electrons in the jet rather than changes in the external photon field .

Subject Headings : galaxies : active – quasars : individual ( 3C 279 ) – ultraviolet : observation – variability