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The possibility of using the presence of an intervening galaxy to obtain information about the distance of a source of radiation is examined. The probability that photons received from a distant object have passed near enough to a galaxy to be significantly modified is computed and found to be appreciable for cosmological redshifts >1. Various effects of the galactic matter on the beam are calculated, using the observed properties of n%arby galaxies Included are Faraday rotation, 21-cm absorption, total optical scattering and absorption, and optical absorption lines. If these features are not present in the expected fraction of large redshift sources, then either (a) galaxies of moderate redshift (assumed cosmological) are much different than those nearby or (b) the sources are not at cosmological distances.
Robert V. Wagoner (Fri,) studied this question.