4 2 4 D O C U M E N T 2 4 9 A P R I L 1 9 2 6 TLS. [10 134]. [1]Circularly polarized lines are observed in the case of the longitudinal Zeeman effect (observa- tion along the field lines). The paragraph is in Ehrenfest’s hand. [2]Ehrenfest has drawn an arrow in red pencil in the left margin pointing at the number II. [3]Ehrenfest’s scientific notebook ENB:1, 30 (NL-LeRM, Ehrenfest Archive) contains several entries from February and March 1926 on the problem of the light particle with spin, which Ehrenfest dubs “Roton-Photon” (see, e.g., entry 6331). [4]By 1920, Arthur Compton had already considered the possibility that the electron spins around its axis with an angular momentum of . The resulting magnetic moment could explain ferro- magnetism (see Compton 1920). Five years later, George Uhlenbeck and Samuel Goudsmit intro- duced the hypothesis of electron spin in the framework of atomic theory and spectroscopy (see Uhlenbeck and Goudsmit 1925). [5]At this point in the original text, Ehrenfest indicates a note he has appended at the foot of the page: “Aus Brief von Frenkel weiß ich dass er mit Pauli etwas sehr Schönes darüber gemacht hat. Wir hatten uns in Leiden damit geplagt aber das Nicht-Punktförmig-Sein des Elektrons störte uns in Über- legung.” Jakov Frenkel (1894–1952) was Professor of Theoretical Physics at the Leningrad Polytech- nical Institute. Wolfgang Pauli. See Frenkel 1926, which was written while Frenkel visited Pauli in Hamburg. [6]Ehrenfest has offset the paragraph in the left margin with three vertical lines in red pencil. [7]The sentence is underlined in red pencil. [8]Kornel Lanczos (1893–1974) was Assistant Professor of Physics at the University of Frankfurt am Main. See, e.g., Lanczos 1926a, 1926b. [9]Abram F. Ioffe (1880–1960), Professor of Physics at the Leningrad Polytechnical Institute and at the University of Leningrad. [10]The words “oder…werden?” are underlined in red pencil. [11]Tatiana Ehrenfest-Afanassjewa. [12]Leonard Ornstein. [13]Tatiana Ehrenfest. [14]Einstein 1926p (Doc. 223). [15]In 1922, Einstein had devised an experiment to decide whether light had wavelike or corpuscu- lar character. The experiment was performed and seemed to prove the corpuscular hypothesis until Ehrenfest pointed out an error in Einstein’s reasoning that made the experiment inconclusive (see Vol. 13, Introduction, pp. xxxvi–xxxviii, for an account). [16]Hans Albert Einstein Margot Einstein. [17]Elsa Einstein. [18]The remainder of the text is in Ehrenfest’s hand. [19]Anna (Galinka) Ehrenfest. 249. To Hans Reichenbach [Berlin, 8 April 1926] Lieber Herr Reichenbach Sie haben vollständig recht. Es ist verkehrt zu glauben, dass die „Geometri- sierung” etwas Wesentliches bedeutet.[1] Es ist mir eine Art Eselsbrücke zur Auffindung numerischer Gesetze. Ob man mit einer Theorie „geometrische“ Vorstellungen verbindet, ist unwesentliche Privatsache.[2] Das Wesentliche bei Weyl liegt darin, dass er die Formeln neben der Invarianz bezüglich Transfor- h 2 π⁄