  The absence of any wide-separation gravitational lenses in the Large Bright
Quasar Survey is used to place limits on the population of cluster-sized halos
in the universe, and hence constrain a number of cosmological parameters. The
results agree with previous investigations in strongly ruling out the standard
cold dark matter model but they are consistent with low-density universes in
which the primordial fluctuation spectrum matches both cluster abundances and
cosmic microwave background measurements. These conclusions are essentially
independent of the cosmological constant, which is in stark contrast to the
statistics of galaxy lenses. The constraints presented here are nullified if
clusters have core radii of more than 10 kpc, but are free of a number of
potential systematic errors, due to the homogeneity of the data.
