  We investigate the cosmological test recently proposed by B. Fort, Y. Mellier
and M. Dantel-Fort (FMD), where the observed location of the critical line in
gravitational lensing is used to determine the cosmological parameters,
$\Omega$ and $\lambda$. Applying this method to the cluster of galaxies
Cl0024+1654, FMD obtained a constraint on the cosmological constant, $\lambda >
0.6$, assuming the spatially flat universe. It plays a crucial role in this
method that the angular diameter distance-redshift relation depends on the
cosmological models through the cosmological parameters. First, using the
angular diameter distance in the Friedmann-Lemaitre- Robertson-Walker universe,
we show that one can hardly determine $\Omega$ by this method without the
assumption of the spatially flat universe. We also investigate the effect of
inhomogeneities of the universe by using the Dyer-Roeder angular diameter
distance. It is shown that the effect of inhomogeneities can become too large
to be ignored, particularly for a high density universe. As a result, this
method cannot be taken as a clear cosmological test to determine $\Omega$ and
$\lambda$, though it may provide a bound on $\Omega$ and $\lambda$. Moreover,
we mention the uncertainty of the determination of the velocity dispersion,
which is regarded as one of the most serious problems in this test.
