Introduction

During the last two years a burst of results has come from radio and optical surveys of “galaxy lenses” (where the main deflector is a galaxy). Even if this kind of work were better known and had already been reviewed for this assembly a few years ago, I cannot pass up the main results which have presently emerged. This will be the first part of the presentation.

On the other hand, in September 1985 we pointed out a very strange blue ring-like structure on a Charge-Coupled Device (CCD) image of the cluster of galaxies Abell 370 (Soucail et al. (1987a)). After ups and downs and a persistent observational quest, this turned out to be the Einstein arcs discovery (Soucail et al. (1988); Lynds and Petrosian (1989)), an important observational step in this particular field since the first observation of the double Quasi-Stellar Object (QSO) 0957 + 561 in 1979 (Walsh et al. (1979)).

Following this discovery, new observational results have shown that many rich clusters of galaxies can produce numerous arclets: tangen-tially distorted images of an extremely faint galaxy population probably located at redshift larger than 1 (Fort et al. (1989); Tyson (1989)). This new class of gravitational lenses proves to be an important observational topic (Mellier (1989a); Fort (1989a)). This story will be the second part of this presentation.

The classification between galaxy lenses and cluster lenses is somewhat arbitrary.