  We report the detection of weak gravitational lensing of faint, distant
background galaxies by the rich, X-ray luminous cluster of galaxies MS1054-03
at z=0.83. This is the first measurement of weak lensing by a bona fide cluster
at such a high redshift. We detect tangential shear at the 5% - 10% level over
a range of radii 50'' < r < 250'' centered on the optical position of the
cluster. Two-dimensional mass reconstruction using galaxies with 21.5 < I <
25.5 shows a strong peak which coincides with the peak of the smoothed cluster
light distribution. Splitting this sample by magnitude (at I = 23.5) and color
(at R-I = 0.7), we find that the brighter and redder subsamples are only very
weakly distorted, indicating that the faint blue galaxies (FBG's), which
dominate the shear signal, are relatively more distant. The derived cluster
mass is quite sensitive to the N(z) for the FBG's. At one extreme, if all the
FBG's are at z_s = 3, then the mass within a $0.5h^{-1}$Mpc aperture is $(5.9
\pm 1.24)\times 10^{14}$\h1 $M_\odot$, and the mass-to-light ratio is $M/L_V =
350 \pm 70 h$ in solar units. For $z_s = 1.5$ the derived mass is $\sim$70\%
higher and $M/L \simeq 580 h$. If $N(z)$ follows the no evolution model (in
shape) then $M/L \simeq 800h$, and if all the FBG's lie at $z_s\la 1$ the
required $M/L$ exceeds $1600h$. These data provide clear evidence that large,
dense mass concentrations existed at early epochs; that they can be weighed
efficiently by weak lensing observations; and that most of the FBG's are at
high redshift.
