  We present 6300--10100 {\AA} spectra for a sample of 13 T dwarfs observed
using LRIS mounted on the Keck I 10m Telescope. A variety of features are
identified and analyzed, including pressure-broadened K I and Na I doublets;
narrow Cs I and Rb I lines; weak CaH, CrH, and FeH bands; strong H$_2$O
absorption; and a possible weak CH$_4$ band. H$\alpha$ emission is detected in
three of the T dwarfs, strong in the previously reported active T dwarf 2MASS
1237+6526 and weak in SDSS 1254$-$0122 and 2MASS 1047+2124. None of the T
dwarfs exhibit Li I absorption. Guided by the evolution of optical spectral
features with near-infrared spectral type, we derive a parallel optical
classification scheme, focusing on spectral types T5 to T8, anchored to select
spectral standards. We find general agreement between optical and near-infrared
types for nearly all of the T dwarfs so far observed, including two
earlier-type T dwarfs, within our classification uncertainties ($\sim$1
subtype). These results suggest that competing gravity and temperature effects
compensate for each other over the 0.6--2.5 $\micron$ spectral region. We
identify one possible means of disentangling these effects by comparing the
strength of the K I red wing to the 9250 {\AA} H$_2$O band. One of our objects,
2MASS 0937+2931, exhibits a peculiar spectrum, with a substantial red slope and
relatively strong FeH absorption, both consequences of a metal-deficient
atmosphere. Based on its near-infrared properties and substantial space motion,
this object may be a thick disk or halo brown dwarf.
