D O C U M E N T 4 2 J A N U A R Y 1 9 2 2 1 2 3 ALS. [6 171]. [1]Breit (1899–1981) was an American National Research Council Fellow at the University of Ley- den and assistant to Paul Ehrenfest. [2]Einstein returned from his visit to Leyden on 27 November 1921. [3]Balthasar van der Pol (1889–1953) was a physicist at the Natuurkundig Laboratorium of N. V. Philips Gloeilampenfabrieken in Eindhoven.
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