  We have used the Optical Redshift Survey (ORS; Santiago et al. 1995) to
construct the gravity field due to fluctuations in the galaxy density field out
to distances of 8000 km/s. At large scales where linear theory applies, the
comparison of this gravity field with the observed peculiar velocity field
offers a powerful cosmological probe, because the predicted flow field is
proportional to the parameter Omega^{0.6}/b, where Omega is the matter density
and b is the bias of the galaxy distribution. The more densely sampled ORS
gravity field, to excellent approximation, matches that of the earlier IRAS
1.2-Jy redshift survey (Fisher et al. 1995), provided beta is reduced by a
factor b(opt)/b(IRAS)=1.4. Apart from this scaling, the most significant
difference between the ORS and IRAS fields is induced by differing estimates of
the over-density of the Virgo cluster. Neither of these gravity fields is
consistent with the peculiar velocity field constructed from the full Mark III
(Willick et al. 1997) sample. We find that a simple but plausible non-linear
bias algorithm for the galaxy distribution relative to the mass has a
negligible effect on the derived fields. We conclude that the substitution of
optical for IRAS catalogues cannot alone resolve the discrepancies between the
IRAS gravity field and the Mark III peculiar velocity field.
