transition were observed at high resolution, the conclusions given here are supported fully by the results of our low resolution BD spectroscopic survey (M03) where two objects of every spectral type from M6 to L5 was included.

As shown in Table 3, these lines arise from absorption from an energy level at 3.14 eV. Interestingly, the Na I line at 1.268 $\mu$m in order 60 is already absent in the M9, and careful inspection shows that a somewhat broadened Mn I line at 1.290 $\mu$m in order 59 persists through L0. The Na I line is excited from a high energy state at 3.6 eV whereas the Mn I line comes from a state at only 2.1 eV. Thus, the sequence in which the lines disappear is at least qualitatively consistent with thermal excitation. But the abrupt loss of Al I lines at the classical M-L boundary is too great to be explained by Boltzmann factors alone. For example, for a temperature change from 2850 K from the M6 to about 2400 K for the M9 say, the population of excited atoms in the upper level would drop by 51% and the equivalent width of the line might change from 