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Gravitational lenses provide an opportunity to observe galaxies at very large redshifts, and can be used to constrain cosmological parameters and to provide information about the formation and evolution of cosmic structure. Because the very luminous far-infrared sources IRAS F10214+4724 and the Cloverleaf QSO are gravitationally lensed, we investigate the effects of lensing by galaxies along the line of sight on the source counts of distant galaxies in the millimetre and submillimetre wavebands, and in some circumstances we predict that the majority of detected sources could be strongly lensed. We then investigate the prospects for observing a statistical sample of lenses using existing and proposed millimetre/submillimetre-wave telescopes, and find that a deep survey with a large telescope or telescope array could reasonably detect about 100 strongly lensed sources at very large redshifts.
A. W. Blain (Wed,) studied this question.