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‎Observation of a Line in the Galactic Radio Spectrum. - [DISCOVERY OF THE HYDROGEN LINE]‎

‎London, Macmillan & Co., 1951. Royal8vo. Bound in contemporary full cloth with black lettering to spine. In ""Nature"", Vol. 168, 1951. Bookplates to front free end-paper. Fine and clean. Pp. 356-8. [Entire volume: LXI, (1), 1132 pp.].‎

‎First announcement of the important discovery of the hydrogen line, 21 centimeter line or HI line which brought about a revolution in the study of galactic structure. After 1952 the first maps of the neutral hydrogen in the Galaxy were made and revealed, for the first time, the spiral structure of the Milky Way.""[Ewen & Purcell] had seen the signal months before, but waited for corroboration by Dutch and Australian astronomers before publishing: Muller and Oort's paper in the pages following Ewen and Purcell's report includes the text of a cable sent by Pawsey from Australia. Oort had already realized the significance of the discovery - that detection of this spectral line, produced by transitions between hyperfine levels of the ground-state hydrogen atom, would permit measurements of velocities by the Doppler effect. The 21-cm line put radio astronomy on the map, and brought about a revolution in the study of galactic structure."" (Nature Physics Portal).‎