  We report on the detection of dark matter in the cluster Abell 2390 using the
weak gravitational distortion of background galaxies. We find that the cluster
light and total mass distributions are quite similar over an angular scale of
$\simeq 7^\prime \;(1 \Mpc$). The cluster galaxy and mass distributions are
centered on the cluster cD galaxy and exhibit elliptical isocontours in the
central $\simeq 2^\prime \; (280 \kpc)$. The major axis of the ellipticity is
aligned with the direction defined by the cluster cD and a ``straight arc''
located $\simeq 38^{\prime\prime}$ to the northwest. We determined the radial
mass-to-light profile for this cluster and found a constant value of $(320 \pm
90) h\; M_\odot/L_{\odot V}$, which is consistent with other published
determinations. We also compared our weak lensing azimuthally averaged radial
mass profile with a spherical mass model proposed by the CNOC group on the
basis of their detailed dynamical study of the cluster. We find good agreement
between the two profiles, although there are weak indications that the CNOC
density profile may be falling more steeply for $\theta\geq 3^\prime$
$(420\kpc)$.

Astrophysics