The statistics of wide-separation lensed quasars

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 h aloes in the universe, and hence constrain a number of cosmological paramet ers. The results agree with previous investigations in strongly ruling out the standard cold dark matter model but they are consistent with low-densit y universes in which the primordial fluctuation spectrum matches both clust er abundances and cosmic microwave background measurements. These conclusio ns are essentially independent of the cosmological constant, which is in st ark contrast to the statistics of galaxy lenses. The constraints presented here are nullified if clusters have core radii of greater than or similar t o 10 kpc, but are free of a number of potential systematic errors, owing to the homogeneity of the data.