2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/2010The 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.Cosmological Physics with Gravitational Lensing, J.-P. Kneib, Y. Mellier, M. Moniez & J. Tran Thanh Van, eds., in press; 4 pages; 7 figuresAstrophysicstext