  We present optical (6500-9200A) spectroscopy of eight cool dwarfs detected in
a 231 square degree ``Mini-survey'' of the Deep NEar Infrared Survey (DENIS)
data. We are able to confirm that the spectral types derived from the
Mini-survey infrared spectroscopy are meaningful. We provide a spectral
sequence which extends beyond the M-dwarf range and into the proposed ``L''
class of dwarfs. The dominant spectral features in the optical for these L-type
dwarfs are resonance lines of Cs I and molecular band heads of CrH and FeH. The
other dominant feature in these L-type spectra is a broad 600A absorption dip
centered on 7700A, which we identify with extremely strong (equivalent width
$\sim$ several hundred A) absorption associated with the 7664,7698A resonance
doublet of K I. We find that model atmospheres which include the effects of
molecular condensation without dust opacity (to simulate rapid gravitational
settling of dust grains) produce significantly better agreement with observed
optical spectra for L-type dwarfs, than models including dust opacity. This
suggests gravitational settling of dust grains plays an important role in
L-dwarf photospheres. The extreme strength of the K I resonance doublet, and
disappearance of TiO and VO, and the consequent dominance of CrH and FeH in
L-dwarf spectra offer considerable prospects as sensitive effective temperature
diagnostics, even at low spectral resolution.
