Key points are not available for this paper at this time.
The discovery by Walsh et al (1979) of the first bona fide gravitational lens, the doubly-imaged quasar, Q0957 + 561, happened at an opportune time, following several prescient theoretical papers, and just preceding the completion of radio and optical quasar surveys that have since yielded a dozen examples of this phenomenon. Interest in gravitational lenses stretches back over more than seventy years (Eddington 1919, Lodge 1919). Zwicky (1937a, b) appears to have been the first to realize that gravitational lensing ought to have a major impact on cosmology, specifically by "weighing" nebulae and providing crude telescopes to magnify lensed sources. The discovery of quasi-stellar "point" sources added two more possible uses of lenses, for distance measurement (Klimov 1963, 1964, Refsdal 1964b) and as probes of the stellar composition of lenses (Chang & Refsdal 1979), both of which may be just coming to fruition. These four topics constitute the primary theme of this review.
Blandford et al. (Tue,) studied this question.