  For a wide variety of cosmological models characterized by the cosmic mass
density $\Omega_M$ and the normalized cosmological constant $\Omega_{\Lambda}$,
we derive an analytic expression for the estimate of magnification
cross-sections by an ensemble of isothermal spheres as models of galactic mass
distributions. This provides a simple approach to demonstrate how the lensing
probability by galaxies depends on the cosmological parameters. An immediate
consequence is that, while a non-zero cosmological constant indeed leads to a
significant increase of the lensing probability as it has been shown in the
literature, only a small fraction of sky to $z\sim 3$ can be moderately ($\mu
\sim 1.3$) lensed by galaxies even in a $\Lambda$-dominated flat universe.
Therefore, whether or not there is a nonzero cosmological constant, it is
unlikely that the overall quasar counts have been seriously contaminated by the
presence of galactic lenses.
