  We use recent low-mass star models, which reproduce accurately the observed
sequences of various globular clusters, to convert the observed luminosity
functions into bolometric luminosity functions and mass functions down to the
bottom of the main sequence. These mass functions are well describedby a slowly
rising power-law $dN/dm\propto m^{-\alpha}$, with $0.5\wig < \alpha \wig <
1.5$, down to $\sim 0.1 \msol$, independently of the metallicity, suggesting a
rather universal behaviour of the cluster initial mass functions. We predict
luminosity functions in the NICMOS filters in the stellar and in the brown
dwarf domains for different mass functions and metallicities.
  We apply these calculations to the determination, slope and normalization, of
the mass function of the Galactic halo (spheroid and dark halo). The spheroid
mass function is well described by the afore-mentioned power-law function with
$\alpha \sim 1.7\pm 0.2$ down to 0.1 $\msol$, although a slowly decreasing mass
function below $\sim 0.15 \msol$ can not be excluded with the data presently
available. Comparison with the Hubble Deep Field star counts is consistent with
such a mass function and excludes a significant stellar population in the dark
halo.
  Consistent analysis with recent microlensing experiments towards the LMC
shows that the spheroid and the dark-halo stellar+brown dwarf populations
represent at most $\sim$ 1% of the Galactic dark matter density. This clearly
excludes brown dwarfs and low-mass stars as significant dark matter candidates.
