x x x i v I N T R O D U C T I O N T O V O L U M E 1 3 and Correspondence will now appear together and, as in the past, in chronological order in consecutive volumes. The editors believe that this new format will facili- tate the contextual reading and historical understanding of the documents. Einstein’s scientific preoccupations within two major fields of physics of the time, atomic and quantum physics and theory of relativity, began to center on three specific issues. In the atomic physics of the emerging quantum theory, Einstein focused his at- tention on understanding the radiative and energetic processes of the individual atom. Thus, his canal ray experiment was designed to decide whether light emis- sion by an excited atom occurs in a continuous emission of electromagnetic radia- tion, according to the classical wave-theoretic conception, or else instantaneously, as a monochromatically emitted light quantum, as postulated by the quantum the- ory. Similar concerns seem to have been the motivation for further experiments that he designed or planned to execute with experimental physicists such as Peter Pringsheim or Pieter Zeeman. In Einstein’s and Paul Ehrenfest’s analysis of the puzzling results of the Stern- Gerlach experiment, the authors focused on ways to account for its outcome by studying the possible mechanisms of alignment of the individual silver atoms in the magnetic field. And, again in a similar vein, Einstein tried to advance the under- standing of superconductivity by teasing out the consequences of the assumption that the superconducting current is established by electrons that move radiation- free on Bohr orbits. In relativity theory, Einstein continued to be concerned with its cosmological implications and with the extent to which Mach’s principle would be vindicated in special solutions. He also began to investigate the possibilities and restrictions that general relativity implied for a unified theory of the gravitational and electromag- netic fields. This latter concern is at the center of a joint publication with Jakob Grommer about the impossibility of having static, spherically symmetric, hence particle-like solutions in Kaluza’s five-dimensional approach. Similarly, he studied Arthur S. Eddington’s recent work, in which Eddington had begun to reinterpret relativity as being based solely on the concept of the so-called affine connection, a mathematical concept whose importance for the theory of general relativity had been brought to light by Tullio Levi-Civita and Hermann Weyl. While Einstein’s original formulation of the theory was based on the concept of a metric, Levi-Civita and Weyl had shown that the concept of the affine connection is independent of the metric. In particular, the Riemann curvature tensor can be expressed only in terms of the linear connection, and hence Eddington argued that in the general theory of relativity it is a logically prior concept to the metric which is based upon it. Pon- dering this approach during long periods of leisure on the return voyage from