(*) Helge Kragh, Centre for technology and culture, University of Oslo, 0371 Oslo, Norway. I thank the Niels Bohr Archive, Copenhagen, for permission to study and quote from the correspondence of March, Heisenberg, Bohr, and Schrôdinger. For constructive criticism of draft versions I am grateful to Abraham Pais, Olivier Darrigol and Karl von Meyenn. Special thanks to Bruno Carazza for his clarification of the physical problems related to space-time discretization. I am grateful to the University of Parma and the Danish Research Council of the humanities for partial support. Rev. Hist. ScL, 1995, XLVIII/4, 401-434