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Published online by Cambridge University Press: 14 August 2015
A search was made for neutral hydrogen in the following galactic star clusters — the Pleiades, NGC 1502, the Trapezium-Orion cluster, NGC 2244, and NGC 6910 (Davies and Tovmassian 1963). Observations were made with a six-channel receiver having a 5 kc/s bandwidth; it was used with the 250-foot radio telescope which has a beamwidth of 12′ by 18′. The first stage of the receiver was a parametric amplifier with an excess temperature of 200°K. Simultaneous measurements with a 5 Mc/s bandwidth showed the existence of ionized hydrogen associated with the clusters.