  Improved analysis of ultraviolet and optical monitoring data on the Seyfert 1
galaxy NGC 3783 provides evidence for the existence of a supermassive,
(8.7+/-1.1)x10^6 M_sun, black hole in this galaxy. By using recalibrated
spectra from the International Ultraviolet Explorer satellite and ground-based
optical data, as well as refined techniques of reverberation mapping analysis,
we have reduced the statistical uncertainties in the response of the emission
lines to variations in the ionizing continuum. The different time lags in the
emission line responses indicate a stratification in the ionization structure
of the broad-line region and are consistent with the virial relationship
suggested by the analysis of similar active galaxies.
