Among Professor Bohr's numerous writings (some 115 publications), three appearing as books in the English language may be mentioned here as embodying his principal thoughts: The Theory of Spectra and Atomic Constitution, University Press, Cambridge, 1922/2nd. ed., 1924; Atomic Theory and the Description of Nature, University Press, Cambridge, 1934/reprint 1961;The Unity of Knowledge, Doubleday & Co., New York, 1955.