  We present the results of the deepest optically identified X-ray survey yet
made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of
1.6x10^-15 erg cm^-2 s^-1 (0.5--2.0 keV). Above a flux limit of 2x10^-15 erg
cm^-2 s^-1 we define a complete sample of 70 sources of which 59 are
identified. Some (5) other sources have tentative identifications and in a
further 4 the X-ray error-boxes are blank to R=23 mag. At the brighter flux
levels (>= 10^-14 erg cm^-2 s^-1) we confirm the results of previous less deep
X-ray surveys with 84% of the sources begin QSOs. At fainter fluxes, however,
the survey is dominated by a population of galaxies with narrow optical
emission lines (NELGs). In addition, a number of groups and clusters of
galaxies are found at intermediate fluxes. Most of these are poor systems of
low X-ray luminosity and are generally found at redshifts of > 0.3. Their
numbers are consistent with a zero evolutionary scenario, in contrast to the
situation for high luminosity clusters at the same redshift. We discuss the
significance of these results to the determination of the cosmic soft X-ray
background (XRB) and show that at 2x10^-15 erg cm^-2 s^-1, we have resolved
more than 50% of the background. We also briefly consider the probable
importance of NELG objects to the residual background and look at some of the
properties of these unusual objects.
