  The distortion of images of faint background galaxies by (weak) gravitational
lensing can be used to measure the mass distribution of the deflector. The
image distortions can be used to define a weighted mean of the mass inside a
circular aperture, as was first suggested by Kaiser. The aperture mass can be
used to {\it detect} dark matter concentrations. Keeping in mind that
wide-field cameras will become increasingly available, this method can be used
to search for mass concentrations on wide-field images. To do this, the
aperture mass measure is generalized to account for different weighting
functions. For each such weighting function, a signal-to-noise ratio can be
calculated. For an assumed mass profile of the density concentrations, the
weighting function can be chosen such as to maximize the resulting
signal-to-noise ratio. Numerical simulations which adopt parameters
characteristic of 4-m class telescopes are then used to show that dark halos
with a velocity dispersion in excess of $\sim 600$\ts km/s can be reliably
detected as significant peaks in the signal-to-noise map. The effects of seeing
and an anisotropic PSF are then investigated and shown to be less important
than might be feared. It is thus suggested that the method of aperture mass
measures developed here can be used to obtain a mass-selected sample of dark
halos, in contrast to flux-selected samples. Shear fields around high-redshift
bright QSOs as detected by Fort et al. provide a first successful application
of this strategy. The simplicity of the method allows its routine application
to wide-field images of sufficient depth and image quality.
