This classic work by the Nobel Laureate elaborates on the correspondence principle, discussing the theory's applications from a uniform point of view and considering the underlying assumptions in their relations to ordinary mechanics and electrodynamics. Bohr closely traces the analogy between quantum theory and ordinary theory of radiation. 1918-1922 editions.

Explains the phenomena that classical physics could not explain but quantum physics could, the photoelectric effect and line spectra.

For beginners and specialists in other fields: the Nobel Laureate's introduction to atomic spectra and their relationship to atomic structures, stressing basics in a physical, rather than mathematical, treatment. 80 illustrations.