Abstract
A new optical technique for surveying large numbers of quasars for evidence of gravitational lensing is described. Using the Automated Plate Measuring Facility at Cambridge to scan broadband direct and objective-prism plates, hundreds of thousands of objects can be examined to find quasar candidates with multiple image structures. The technique is effective for separations greater than about two arcseconds, and allows the probability of detecting a lens of specified magnitude, component magnitude difference, and component separation to be precisely calculated. Tables are given that quantify the probability of detection of multiply imaged quasars. When combined with the large numbers of quasars surveyed, specific theoretical predictions of the frequency of lensing can be directly tested. The results of a survey of 2500 quasars with magnitudes in the range of 17-20 covering 130 sq deg of sky are described. Significant new limits can be put on the numbers of gravitationally lensed quasars with wide image separations. These limits enable some calculations of the distribution of mass in the universe to be undertaken. In particular, limits can be placed on the numbers of cosmic strings in the universe. 21 references.
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