Niels Bohr's studies on the quantum theory of the atom over the years 1912–1923 are discussed. The sources of his work on these questions are analyzed. The beginning of his quantum studies in 1912 is described. His famous paper on the theory of the hydrogen atom and on the origin of spectra is analyzed in detail. The subsequent development of his ideas regarding atomic structure is discussed, with an emphasis on his postulates regarding stationary states and radiative transitions and on the development of the correspondence principle. How well he understood the difficulties of the model theory and how he strived for a profound understanding of quantum phenomena are shown.
It is widely known that Paul Ehrenfest formulated and applied his adiabatic hypothesis in the early 1910s. Niels Bohr, in his first attempt to construct a quantum theory in 1916, used it for fundamental purposes in a paper which eventually did not reach the press. He decided not to publish it after having received the new results by Sommerfeld in Munich. Two years later, Bohr published "On the quantum theory of line-spectra." There, the adiabatic hypothesis played an important role, although it appeared with another name: the principle of mechanical transformability. In the subsequent variations of his theory, Bohr never suppressed this principle completely. We discuss the role of Ehrenfest's principle in the works of Bohr, paying special attention to its relation to the correspondence principle. We will also consider how Ehrenfest faced Bohr's uses of his more celebrated contribution to quantum theory, as well as his own participation in the spreading of Bohr's ideas.