ABSTRACT
Franck was an ardent admirer of Bohr and believed in him as the highest authority in physics. He asked Frank whether he finished his experiment or not. He have written first to Bohr, and he has not answered yet. It was the time before the establishment of quantum mechanics, and he was trying, with his collaborators, to find weak points and contradictions in Bohr's semi-classical theory of atoms. The author made first step from the bright realm of classical mechanics into the still dark and unexplored underworld of the new quantum mechanics. He made the second step with the question: can one not find, by similar systematic guesswork, the interaction between two electronic systems in terms of 'transition quantities'? Indeed, by a proper re-interpretation of the classical perturbation theory, the corresponding quantum formula could be constructed. It was later fully confirmed by quantum mechanics.
