1 6 4 D O C U M E N T 9 1 O C T O B E R 1 9 2 5 William H. Bragg (1862–1948) was Professor of Chemistry and director of the Royal Institution of Great Britain. Henri A. Abraham (1868–1944) was Professor and director of the Physics Laboratory at the École normale supérieure, Paris. The third assembly of the Conseil international de recherches had been held in Brussels on 7– 9 July (see Schröder-Gudehus 1966, p. 242). Max Planck (1858–1947) was Professor of Physics at the University of Berlin and secretary of the Prussian Academy of Sciences. The Locarno Treaties were negotiated between 5 and 16 October 1925 in Locarno, Switzerland, and signed in London on 1 December 1925. They consisted of five distinct documents: a Treaty of Mutual Guarantee—the “Rhineland Pact”—which was a multilateral agreement consisting of two parts: a reciprocal treaty of nonaggression between the powers situated on the Rhine (Germany, France, and Belgium) who agreed not to attack, invade, or resort to war against each other and a treaty of mutual assistance which stipulated that the United Kingdom, France, Germany, Belgium, and Italy guaranteed to observe the demilitarization of the Rhineland, to defend the existing borders between France and Germany and between Belgium and Germany, and to provide military assistance to any of the signatories who fell victim to a violation of the treaty. The four other treaties were arbi- tration agreements between Germany and its neighbors France, Belgium, Poland, and Czechoslova- kia (see Vincent 1997, pp. 286–287). The IIIC. Robert Eisler. Jacob E. de Vos van Steenwijk (1889–1978). 91. To Arthur S. Eddington [Berlin,] 20. X. 25. Verehrter Herr Kollege! Ich bitte Sie sehr, mir Ihre Arbeit über das Plancksche Strahlungsgesetz zu senden, damit ich über dieselbe im Colloquium referieren kann. Das Resultat über den Sirius-Begleiter hat mich sehr interessiert und ebenso Ihr schönes Buch über Relativitätstheorie. Im Sommer habe ich einen neuen Versuch publiziert, den Zusammenhang von Gravitation und Elektrizität zu erklären. Leider bin ich schon wieder in der Lage, skeptisch darüber denken zu müssen. Ich sende Ihnen aber trotzdem die Arbeit. Es grüsst Sie herzlich Ihr ganz ergebener A. Einstein AKS (UkCF). [73 948]. The postcard is addressed “Herrn Prof. Dr. Eddington Observatory Cam- bridge” and postmarked “Berlin–Wilmersdorf 21 10 25 11–12V[ormittags].” An offprint of Eddington 1925b. See also Einstein’s comment on this paper in Doc. 114. In Eddington 1924 the density of Sirius B, the white dwarf companion of Sirius, is calculated to be 53,000 g/cm3, which would give rise to a gravitational redshift of 20 km/sec, about thirty times that of the Sun. The results of observation at Mount Wilson (Adams 1925) had confirmed the calcu- lations. Eddington 1925a. Einstein 1925t (Doc. 17).