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Published online by Cambridge University Press: 30 March 2016
Observations with the Einstein X-ray telescope show that quasars, as a class, are luminous X-ray emitters. By coupling our X-ray observations with quasar optical number counts, we can show that the quasars contribute significantly to the diffuse X-ray background. In fact, the X-ray data strongly suggest that somewhat above 20m the slope of the optical counts must flatten.