  Observations of clusters of galaxies that gravitationally lens faint
background galaxies can probe the amount and the equation of state, $\5$, of
the dark energy (quintessence) in the universe. Provided that the mass profile
and the mass normalization of the cluster are determined, it is possible to
constrain the cosmological parameters that enter the lensing equations by means
of the angular diameter distances, by locating (either by observations of giant
arcs and magnification bias effect) the critical lines corresponding to known
redshift source populations of galaxies. This method can help to distinguish
between accelerating and decelerating models of the universe. Furthermore,
since the position of critical lines is affected, especially in low-matter
density universes, by the properties of quintessence, the observations of a
suitable number of lensing clusters at intermediate redshifts can determine the
equation of state. A very preliminary application of the method to the cluster
CL 0024+1654 seems to support a flat accelerating universe dominated by dark
energy.
