298 DOCUMENT 269 JUNE 1911 In den meisten Fällen wird wohl die hierauf folgende nächste Näherung 2 a(1) = -V- 1 n v h2 + a2v 2Ŧ(0V0) ausreichen. Wenn die Erklärung ungenügend ist, erläutere ich die Sache gern ausführ- licher. Bei kleinen Teilchen gibt es mehr zu rechnen, aber das Resultat wird dafür sicherer. Bei ganz kleinen Teilchen, wenn ein Horizontalstrich unter Umständen mehrmals passiert wird , versagt die Rechenmethode. Der Fall dürfte aber nicht in Betracht kommen. Mit herzlichen Grüssen verbl. ich Ihr A. Einstein. ALS (Paul Böhi, Parpan, Switzerland). [76 053]. [1]The year is provided by the reference to Böhi 1911a. [2]In the spring of 1911 Paul Böhi (1883-1943), a doctoral student in medicine at the Uni- versity of Zurich, performed experiments on Brownian motion and diffusion in the laboratory of his dissertation adviser, Heinrich Zangger. His aim was to determine Avogadro's number. In the Brownian motion experiment, which is discussed here, mercury drops were observed while falling through a viscous medium (water), and their horizontal deflection from the ver- tical was registered at successive instants. Bohi's results are in Böhi 1911a and in his disser- tation, Böhi 1911b (a reprint of Böhi 1911a with an additional introduction), for which he re- ceived the doctorate in mid-December 1911 (see Verzeichnis d. Promotionen d. medizin. Fakultät 1833-1912, SzZU, AA 15:2, no. 1621). In his paper Böhi thanks Einstein for his as- sistance through correspondence (p. 212) the calculational method outlined in this letter is re- produced almost verbatim in the paper, with an acknowledgment of Einstein's authorship (pp. 187-188). [3]The word "gross" is deleted and "klein" interlineated in an unknown hand. In Böhi 1911a Einstein's version is followed. 269. From Ernest Solvay, with an Invitation to the Solvay Congress[1] Bruxelles, 43 Rue des Champs Elysees 9 Juin 1911. Tres honore Monsieur, Comme rapporteurs aux diverses questions figurant dans l'invitation ci- jointe, nous nous proposons de faire des demarches pour obtenir la repartition generale suivante: