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Published online by Cambridge University Press: 25 May 2016
There are now several examples of distant radio sources being gravitationally lensed by foreground galaxies (e.g. PKS1830–211, Jauncey et. al. 1991). In such cases we know that the line of sight must be passing through a substantial amount of ionized material, so it is pertinent to ask whether these free electrons influence the observed properties of the source in any significant way.