Clusters of galaxies as gravitational lenses

It has been suggested (Karoji and Nottale, Nature 259:31 (1976)) that either light emitted by distant galaxies is red-shifted on passing through clusters of galaxies, or that distant sources appear more luminous when seen through intermediate clusters of galaxies, the clusters acting as gravitational lenses. The present authors examine the magnitude of such a lens effect and conclude that it is unlikely that the suggestion is valid. (U.K.)