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GRAVITATIONAL LENSES IN ASTROPHYSICS

The idea that there should be gravitational lenses in astronomy has been around for a very long time,’ and searches have been conducted with varying degrees of vigor for most of that time. The emergence upon the astronomical scene of quasars, with their very small angular sizes and very great distances, made the idea more attractive than ever, and it has seemed likely for some time that samples of gravitationally lensed quasar images would eventually be found.* It would appear that that has now happened; we have not one case but two, the geometries of which are very different. In this paper, we will review the evidence that we have, in fact, found the sought-for phenomena and discuss some astrophysical uses to which they might be put.