Lensing of Quasars
In 1979, two quasars only 6 seconds of arc
apart were discovered and they were found to have identical redshifts and spectra.
The adjacent image shows this "double quasar", which is designated QSO 0957+561.
The probability of this happening by accident is exceedingly small, and it was
postulated that the two bright blue objects
actually correspond to the same quasar that is lensed so
strongly by intervening mass that its image appears double as we view it from
Earth.
This interpretation was bolstered by the discovery of a galaxy (probably a giant elliptical)
just above and to the left of
the lower image (more yellow than the quasar and partially obscuring it),
and a surrounding cluster of galaxies that are candidates for the
intervening mass causing the lens effect.