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\def\ltsima{$\; \buildrel < \over \sim \;$} \def\simlt{\lower.5ex\hbox{\ltsima}} % < over ~ \def\gtsima{$\; \buildrel > \over \sim \;$} \def\simgt{\lower.5ex\hbox{\gtsima}} % > over ~ \def\ea{et~al.\ }
Extensive monitoring of PKS~2155-304 with IUE in November 1991 revealed rapid large-amplitude variations in the ultraviolet flux of this BL Lac object. Many small, rapid flares are superimposed on a general doubling of the intensity. Spectral changes during the intensity variations are surprisingly small --- the whole spectrum rises and falls more or less uniformly. The ultraviolet and optical (FES) results are described by Urry \ea (1993, ApJ, 411, in press). Here we present these results in a new way, using a movie to convey the dramatic flaring and nearly imperceptible spectral variations. The movie runs at various speeds relative to real time, as well as backwards, the latter demonstrating the approximate time symmetry of the rapid flaring. The ultraviolet luminosity of this active galaxy is $\sim 10^{46}$~ergs~s$^{-1}$, so the observed 30\% changes over a period of 1 day correspond to $\Delta L / \Delta t \sim 5 \times 10^{40}$~ergs~s$^{-2}$, only a factor of 4 below the fiducial limit for Eddington-limited accretion with efficiency $\eta=0.1$. Given the estimated bolometric correction of $\sim10$, it is likely that relativistic beaming is important.
