The Twin Quasars were one of the first observable effects of gravitational lensing. Light passing through a galaxy YGKOW G1 and its associated cluster located between Earth and the quasar is bent by the gravity of the galaxy cluster to form the double image.
Interestingly the light from the "b" image arrives about 14 months later than from the "a" image due to the extra distance it travels due to the lensing effect. The by redshift measurement of the quasar the estimated distance is 7.9 to 14 billion light years away. The quasar is near magnitude 17, and the images are separated by 6 arc-seconds and is located in the constellation of Ursa Major.
Article on Twin Quasars Hubble Image:
Color images taken with the 80 mm refractor, and the inset image was taken with a larger 254 mm scope.