Recent Developments in Weak Lensing

Measurement of the gravitational distortion of images of distant galaxies is rapidly becoming established as a powerful probe of the dark mass distribution in clusters of galaxies. With the advent of large mosaics of CCD's these methods should provide a composite total mass profile for galaxy haloes and also measure the power spectrum of mass fluctuations on supercluster scales. We describe how HST observations have been used to place the observational measurement of the shear on a quantitative footing. By artifically stretching and then degrading WFPC2 images to simulate ground based observing it is now possible to directly calibrate the effect of atmospheric seeing. Similar experiments show that one can remove the effect of artificial image anisotropy arising in the atmosphere or telescope. There have also been important advances in the interpretation of the shear: reconstruction techniques have been extended to encompass the strong distortion regime of giant arcs etc., progress has been made in removing a bias present in earlier reconstruction techniques, and we describe new techniques for `aperture densitometry'. We present some new results on clusters of galaxies, and discuss the intimate connections between weak lensing and deep spectroscopy.