Observational aspects of gravitational lensing

Various systematic optical surveys of multiply lensed extragalactic objects presently being conducted are reviewed to describe gravitational lens candidates and the effects of gravitational lensing. Optical surveys of highly luminous quasars, quasars, and distant 3C radio sources are employed to discuss these objects' candidacy as gravitationally lensed objects. Some gravitational lens candidates are then described in detail, including 0957 + 561, 1115 + 080, 2237 + 0305, and 2345 + 007. An extensive list of observational evidence for gravitational lensing is then given, including statistical evidence and gravitational lensing in relation to AGN segregation. The examination of gravitational lensing candidates is shown to help determine how significantly this lensing perturbs the view of the distant universe and induces an apparent segregation among different classes of extragalactic objects.