**3.2 Strong Gravitational Lenses**

A number of strong (elliptical galaxy) gravitational lens systems are
known that may offer the potential of constraining the value of
_{m} and _{} through modeling of the
lens properties. This method is less sensitive to _{} than
the statistics of lensing, and again it is sensitive to a number of
possible systematic effects: possible perturbations by cluster
potentials, uncertainties in the underlying properties of the lensing
galaxies, and model-dependent corrections due to evolution. The
objects are faint and the errors in the luminosities and velocity
dispersions are potentially very significant. A recent analysis of 7
strong lenses has been undertaken by
Im et
al. (1996). Their
current results yield _{} =
0.64^{+0.15}_{-0.26}
(i.e., this measurement sits almost at the end of the range
*excluded* by
Kochanek (1996)
at 95% confidence. Im et al.
exclude _{m} = 1.0 at
97% confidence.