  The aim of our observations is to investigate the intranight variability
properties and the spectral variability of BL Lacertae. 799 optical multi-band
observations were intensively made with the Beijing-Arizona-Taiwan-Connecticut
(BATC) 60/90cm Schmidt telescope during the outburst composed of two subsequent
flares in 2005. The second flare, whose rising phase lasted at least 44 days,
was observed with amplitudes of more than 1.1 mag in three BATC optical bands.
In general, on intranight timescale the amplitude of variability and the
variation rate are larger at the shorter wavelength, and the variation rate is
comparable in the rising and decaying phases within each band. A possible time
lag between the light curves in bands e and m, around 11.6 minutes, was
obtained. Based on the analysis of the colour index variation with the source
brightness, the variability of BL Lacertae can be interpreted as having two
components: a "strongly-chromatic" intranight component and a
"mildly-chromatic" internight component, which may be the results of both
intrinsic physical mechanism and geometric effects.
