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A number of important measurements of the velocity of light have been made in recent years. Four of these determinations are here outlined, with emphasis on the experimental techniques used. These include the work of Essen, and of Hansen and Bol, with microwave cavity resonators, Bergstrand's measurements with a modified Kerr-cell technique, and Aslakson's “Shoran” measurements. It is pointed out that these four determinations are consistent among themselves and yield a value very close to 299,790 km/sec for the velocity of light in vacuum. This is completely outside the probable error of the value 299,776±4 km/sec generally accepted by physicists in the years 1934–1949 on the basis of rotating mirror and Kerr-cell measurements. Some possible reasons for this discrepancy are discussed.
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