Gravitational screens and superluminal separation in quasars

The differential bending of radio waves by a suitably placed ''gravitational screen'' such as an intervening galaxy can lead to large magnification of the separation velocity of the components in the nuclear region of a quasar. It is suggested tha the apparent superluminal separation of such components observed in some quasars by the VLBI techniques could be due to velocity magnifications of this type. The astrophysical feasibility of this explanation is critically examined, and an optical search for objects of large mass-to-light ratio en route to quasars showing superluminal separation is advocated