I N T R O D U C T I O N T O V O L U M E 1 4 x l i But already in January 1923, Arnold Sommerfeld, who was spending the winter at the University of Wisconsin in Madison, had discussed the topic in a long meet- ing with Compton.[5] In a letter of 21 January to Niels Bohr, congratulating him for being awarded the Nobel Prize, Sommerfeld reported about Compton’s remarkable discovery, “according to which one would have to drop the wave theory entirely in the case of Roentgen rays.” He was uncertain whether the results were correct, yet pointed to the possibility “that we might be expecting here an entirely new funda- mental viewpoint.”[6] Doubts about Compton’s experiment persisted for several months.[7] Later that year, Compton’s results were vigorously debated among a number of colleagues in the U.S., but that debate may not have been relevant for Einstein’s work as documented in this volume.[8] Einstein may have heard about Compton’s discovery from his Berlin colleagues after returning from Japan in late March 1923, although at the time he was still preoccupied with the idea of pushing the affine theory to its limits, and with an attempt to test one of its consequences empirically. He may also have discussed the Compton effect with Niels Bohr, whom he met sometime in July during his trip to Gothenburg (Docs. 76, 113). It is likely that Sommerfeld provided Einstein with significant information about the Compton effect while they were both guests at Hermann Anschütz-Kaempfe’s villa in Lautrach in August 1923 (Doc. 99 and Doc. 116, note 1). Walther Kossel from Kiel was also present, and he too engaged in discussions about the alleged new confirmation of the light quantum hypothesis. In fact, it may have been during these debates that Einstein and Kossel conceived of an experiment (Doc. 100). Lautrach Castle, where Einstein enjoyed his extended holiday very much, had been purchased by Anschütz-Kaempfe as a retreat for Munich professors, meant to serve as a conducive forum for scientific discussions.[9] Whether Kossel subsequently did engage in the proposed experiment is not known, but it is very likely that plans to put Compton’s findings to an independent experimental test in Berlin were conceived in Lautrach. Einstein mentions cursori- ly his “experimenting” in Kiel with Kossel (Docs. 18, 99, 100) and with Anschütz- Kaempfe’s physicist collaborator Karl Glitscher (Doc. 12). We know of at least two proposed experiments to test Compton’s hypothesis that were being discussed and pursued under Einstein’s guidance in Berlin after his re- turn from summer vacation in 1923. For the first of these experiments he again enlisted the help of Hermann Mark. In Compton’s original setup, the wavelength of molybdenum X-rays was measured using a calcite spectrometer. The shift in wavelength was determined both for the unscattered beam directly, and for a beam scattered off a graphite target at right