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A measure of the red shift has been obtained on radio source Cygnus A by means of the hydrogen4ine absorption effect in the continuous microwave spectrum of the source The measured microwave red shift agrees in magnitude with the optical red shift, and, within the limits imposed by the experiment, X/X is constant over an interval of 500000 to tin the electromagnetic spectrum.
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Lilley et al. (1956) studied this question.
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