We present optical (6500-9200 ˚ A) 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 se- quence 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 600 ˚ A absorption dip centered on 7700 ˚ A, which we identify with extremely strong (equivalent width several hundred ˚ A) absorption associated with the 7664,7698 ˚ A resonance doublet of K I. We find that model atmospheres which include the effects of molecular con- densation 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 dis- appearance 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.