ABSTRACT
Some spectral lines turned out not to be single, but to be split; this meant that further quantum numbers were required to permit definite ellipticities, to describe the spin of electrons, and to cover effects of magnetism. But this old quantum theory still had an ‘ad hoc’ character; like the epicycles in ancient astronomy, it worked, but it was hard to believe that it could really represent truth about nature. Matter and energy seemed to have a dual character, with characteristics of waves and particles; and for Bohr this was as far as one could get. Theory was an instrument, a tool, to be judged pragmatically; and one knew which equation to apply in any situation. The fatal thing was to suppose that one could ask questions about what an electron was; we only see phenomena and construct models, and muddles are inevitable if we worry about how an electron gets from one orbit to another, or how a photon knows it is faced with a double or a single slit and modifies its behaviour accordingly.
