Paul Dirac's relativistic quantum mechanics for electrons, published early in 1928, is on of the great landmarks i the history of cience. According to Norwood Russell Hanson, it has few predece s s in gr atness: "Theoretical physics has rarely witnessed such a owerful unification of concepts, data, theories and intuitions: Newton and Universal Gravitation; Maxwell and Electrodynamics; Einstein and Special Relativity; Bohr and the hydrogen atom; these are the high spots before Dirac. From a chaos of apparently unrelated facts and ideas, Newton in his way, and now Dirac in his, built a logically powerful and conceptually beautiful physical theory ..."! Despite the greatness of Dirac's theory it has never been subject to historical analysis. This may be due to a tacit assumption by most historians of physics that the period suitable for historical investigations ends about 1927, when quantum mechanics became conceptually complete.2 What lies beyond