DOCUMENTS 19, 20 MAY-JULY 1904 27 [4]See Einstein 1904 (Vol. 2, Doc. 5), in particular §5 where a proportionality is derived be- tween the inverse temperature and a quantity that is of the same order of magnitude as the wavelength at which the energy curve for black-body radiation for that temperature reaches its maximum. The numerical constant in this analog of Wien's displacement law is the product of Boltzmann's constant and the constant occurring in the Stefan-Boltzmann law. [5]Paul Habicht, who had registered his departure from Bern on 11 January 1904 (see Schriften-Kontrolle für Kantonsfremde Aufenthalter 1899-1902, SzBe-Ar, E 2.2.1.1. 113, "H," no. 42). [6]Habicht's father Johann Conrad (1842-1931), mother Susanna Elisabetha Habicht- Oechslin (1850-1908), and unmarried sister Emma Maria (1880-1954). 19. To Helene and Milivoj Savic [Bern,] Sonntag [15 May 1904][1] Meine Lieben! Gestern Morgen kriegten wir einen gesunden Buben.[2] Miza ist wohl[3] und läßt Euch herzlich grüßen. Gruß von Eurem Albert E. Der Junge heißt auch Albert. AKSX. [80 055]. The description of lot 71 in the Charles Hamilton Galleries dealer catalogue 127 (10 April 1980) states that the original is "penned on verso of an official Swiss postcard form, addressed in Serbo-Croat and bearing Berne and Belgrade postmarks." [1]Dated by the son's birth. [2]Hans Albert (1904-1973). [3]A month later Einstein-Maric wrote that she "was very weakened by the birth and greatly in need of good care" ("war durch die Geburt sehr geschwächt und bedurfte grosser Scho- nung"). See Mileva Einstein-Maric to Helene Savic, 14 June 1904, Milan Popovid, Belgrade. 20. To Mileva Einstein-Maric [Kandersteg,] Montag [25 July 1904] Liebes Weib. Ich habe bei herrlichem Wetter den tiefblauen Blausee gesehen und nun sind wir hier in Kandersteg bei wolkenlosem Himmel eingetroffen.[1] Herzliche Grüße an den Filius Rabatzel und Fuxl Dein Albert. AKS (MWalB, Albert Einstein Collection, no. 20). [71 261]. The verso is addressed to "Frau Mileva Einstein Kramgasse 49 Bern.," and postmarked "Kandersteg 26 VII 04." [1]The Blausee (900m) and the village of Kandersteg (1176m) are in the Bernese Oberland. Einstein may have been accompanied by Solovine, who was visiting Bern at the time (see Doc. 18 and the following two documents).