  From deep color-magnitude arrays made from V and I images taken with the
Hubble Space Telescope's WFPC2 camera we have determined luminosity functions
(LFs) down to a level that corresponds to about 0.13 solar masses, for the
low-metal-abundance globular clusters M15, M30, M92, and NGC 6397. Because of
the similarity of the metallicities of these clusters, differences in their
luminosity functions directly trace differences in their mass functions. The
LFs of M15, M30, and M92 agree closely over the entire observed range, whereas
that of NGC 6397 drops away sharply at the faintest magnitudes. We suggest that
the deficiency of low-mass stars in NGC 6397 is due to tidal shocks, to
ejection through internal relaxation, or to a combination of the two. With the
presently available mass-luminosity relations, we find that even in M15, M30,
and M92 the mass functions probably do not rise so fast as to make the low-mass
end dominant.
