  The matter content of the Universe is strongly inhomogeneous on small scales.
Motivated by this fact, we consider a model of the Universe that has regularly
spaced discrete masses, rather than a continuous fluid. The optical properties
of such space-times can differ considerably from the continuous fluid case,
even if the 'average' dynamics are the same. We show that these differences
have consequences for cosmological parameter estimation, and that fitting to
recent supernovae observations gives a correction to the inferred value of
Omega_Lambda of ~10%.
