Despite Einsteins doubts about the observability of gravitational lenses the Swiss-born F. Zwicky pointed out in an article "Nebulae as Gravitational Lenses" in the journal Physical Review (1937) that this effect should become observable if galaxies are considered as lenses.

In 1979 Walsh, Carswell und Weyman observed a pair of quasars, which are only 6.1 arcseconds apart. Both quasars have similar spectra and the same redshift. This was the first observation of a gravitational lens.

Nowadays one not only knows hundreds of lens effects, but gravitational lensing serves as tool in astrophysics.

The picture shows luminous arcs, which are produced by the galaxy cluster CL0024*1654 acting as a lens.