  We perform a weak lensing and photometric study of the z=0.42 supercluster
MS0302+17 using deep I and V band images taken with the UH8K CCD mosaic camera
at the CFHT. We use archival ROSAT HRI data to estimate fluxes, gas masses and,
in one case, the binding mass of the three major clusters. We then use our CCD
data to determine the optical richness and luminosities of the clusters and to
map out the spatial distribution of the early type galaxies in the supercluster
and in other foreground and background structures. We measure the gravitational
shear from a sample of \simeq 30,000 faint background galaxies in the range 22
< m_I < 26 and find this correlates strongly with that predicted from the early
type galaxies if they trace the mass with M/L_B \simeq 250 h. We make
2-dimensional reconstructions of the mass surface density. These recover all of
the major concentrations of galaxies and indicate that most of the supercluster
mass, like the early type galaxies, is concentrated in the three X-ray
clusters, and we obtain mean mass-to-light ratios for the clusters of M/L_B
\simeq 260 h. Cross-correlation of the measured mass surface density with that
predicted from the early type galaxy distribution shows a strong peak at zero
lag (significant at the \simeq 9-sigma level), and that at separations \gsim
200 h^{-1}$kpc the early galaxies trace the mass very accurately. This
conclusion is supported by cross-correlation in Fourier space; we see little
evidence for any variation of M/L or `bias' with scale, and from the longest
wavelength modes with \lambda = 1.5-6 h^{-1}Mpc we find M/L \simeq (280 \pm
40)h, quite similar to that obtained for the cluster centers. We discuss the
implication of these results for the cosmological density parameter.
