Deep two-color imaging of 40 clusters of galaxies with the NOT and the University of Hawaii 2.24-m telescope is used to measure the weak gravitational shear acting on faint background galaxies .
From this , maps of the projected cluster mass distribution are constructed , and the cluster masses are measured within circular apertures of up to \sim 3 h ^ { -1 } Mpc .
The results are used to derive the cluster mass function at z \sim 0.2 .
The average mass-to-light ratio of the clusters indicate a low-density Universe with \Omega _ { 0 } \simeq 0.25 .
Additional mass concentrations are found along the lines of sight to the clusters , some of which are most likely associated with clusters at z > 0.5 .
In addition , the data are used to measure weak gravitational lensing due to large-scale structures along the line of sight .
The future prospects for this kind of research with the NOT are briefly discussed .