Bohr's preface frames the three essays as stages in applying quantum theory to atomic structure, noting that the theory does not attempt an 'explanation' in the usual sense but rather establishes connections between unexplained facts. The first essay, from a 1913 address, already contains the germ of the correspondence principle later developed in the second essay. Readers will encounter a consistent emphasis on formal relationships over mechanistic description.

From Hydrogen to Helium: A Surprising Similarity

Bohr illustrates how the theory predicts that the spectrum of ionized helium should resemble that of hydrogen, with a formula differing only by a factor of four. He calls this 'intimate connection between the properties of two elements' an immediate expression of the nuclear atom's simple structure. The excerpt shows Bohr using specific numerical relations to argue from theory to observation, noting that the helium spectrum was later obtained from pure helium. This section demonstrates his method of deriving testable predictions from quantum postulates.

Moseley's X-Ray Spectra and Atomic Number

Bohr presents Moseley's discovery that X-ray spectra vary simply with atomic number, which he interprets as confirming that atomic number equals the number of electrons. He reproduces Moseley's formula for the strongest line in the most penetrating group, writing it with a constant from the hydrogen spectrum. Bohr emphasizes the 'great significance' of this discovery while noting the difficulty in understanding the similarity between hydrogen and X-ray spectra. The discussion ties experimental results directly to the theoretical framework of stationary states.

Sommerfeld's Fine Structure and Quantum Numbers

Bohr introduces Sommerfeld's theory of fine structure, where relativistic mass variation causes the electron orbit to precess, requiring two quantum numbers. He distinguishes the 'principal quantum number' from a second number, noting that the former determines energy to a close approximation. This section shows Bohr incorporating refinements while maintaining the core idea of stationary states characterized by integers. The excerpt reveals how the theory evolved from a single quantum number to a more complex scheme without abandoning its foundational principles.

The Correspondence Principle as a Guiding Idea

Bohr's preface describes the correspondence principle as a formal link between classical electrodynamics and quantum theory, first appearing in the deduction of the hydrogen spectrum constant. The second essay's second part surveys results from this development, attempting to elucidate problems using the principle. Readers see Bohr treating the principle not as a derivation but as a heuristic that connects otherwise disparate conceptions. The excerpts consistently avoid claiming full explanation, instead emphasizing the establishment of formal connections.

These essays reward attention to how Bohr moves between specific formulas and broad interpretive principles. The recurring phrase 'does not attempt an explanation' signals his epistemological caution. Readers should note how each essay builds on the previous one, with later sections referencing earlier results. The correspondence principle emerges gradually rather than being announced fully formed.