One can always tell which part of physics is advancing most rapidly by consulting a few numbers ofThe Physical Review. Judged by this criterion, the heroic period of the physics of the atom may be taken to be roughly the two decades from 1915 to 1935. (Such dates are never exact.) Prior to 1915 there had been much speculation about the electrical nature of the atom, and Rutherford's work on alpha particle scattering and Bohr's first work on a quantum theory of the atom came a little before 1915. By 1915, however, physicists had really caught the scent of an exciting new forward surge of discovery and were preparing to exploit it fully.