  The probability that high-redshift sources are gravitationally-lensed with
large image separations (i.e., greater than can be produced by galactic
deflectors) is determined by the cosmological population of group- and
cluster-sized halos. Thus the observed frequency of wide-separation lensed
quasars can be used to constrain not only the halo distribution, but also a
number of cosmological parameters. A calculation of the optical depth due to
collapsed, isothermal halos is a useful guide to the lens statistics, and
illustrates that the number of wide-separation lenses is a sensitive probe of
the mean density of the universe and the present day density variance whilst
being nearly independent of the cosmological constant.
