We have all seen this picture by now and I think most of us probably know that it shows gravitational lensing.
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Asked by Mandl to publish on the subject of gravitational lensing, Einstein at first declined, citing the fact that the phenomenon was unobservable. But Mandl persevered, and eventually, Einstein agreed to a brief publication in the journal Science:
In this brief note, Einstein also presented formulae for the optical properties of a gravitational lens. They are the exact same formulae which he had derived some 24 years earlier, right down to the formula for the magnification factor. Mandl receives an honourable mention as the person who had asked Einstein to derive and publish these results.
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A brief history of gravitational lensing « Einstein-Online
Eventually we discovered quasars which turned out to be galaxies and we were able to find evidence of lensing.
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The first gravitational lens was found in 1979 by Dennis Walsh, Robert F. Carswell and Ray J. Weymann, who identified the double quasar Q0957+561 as a double image of one and the same distant quasar, produced by a gravitational lens.
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Einstein initially thought of double images, but reality is more complex with Einstein crosses (4 images) and rings also being possible.