 The guiding principle behind Bohr’s and later Heisenberg’s work in the development of a consistent theory of atoms was the correspondence rule. The full rule states that a transition between stationary states is allowed if, and only if, there is a corresponding harmonic component in the classical motion (CW Vol. 3, p. 479). Bohr furthermore realized that according to his theory of the hydrogen atom, the frequencies of radiation due to the electron’s transition between stationary states with high quantum numbers, i.e. states far from the ground state, coincide approximately with the results of classical electrodynamics. Hence in the search for a theory of quantum mechanics it became a methodological requirement to Bohr that any further theory of the atom should predict values in domains of high quantum numbers that should be a close approximation to the values of classical physics. The correspondence rule was a heuristic principle meant to make sure that in areas where the influence of Planck’s constant could be neglected the numerical values predicted by such a theory should be the same as if they were predicted by classical radiation theory.