5 1 2 D O C U M E N T 3 3 0 O C T O B E R 1 9 2 4 our joint very cordial invitation to you to spend such time with us this winter as you can afford to give. Even if you should have to shorten your stay to six weeks we should be delighted to have you for that length of time and would arrange to pro- vide the same stipend. With very cordial regards from Mrs. Millikan[9] and myself, and all the rest of your friends here, such as Epstein, Bateman, Tolman, Hale, Adams, St. John, Loria,[10] etc., I am Very cordially yours, R. A. Millikan TLS. [17 291]. The letter is addressed “Professor A. Einstein, University of Berlin, Berlin, Germany” and is written on letterhead “California Institute of Technology Pasadena Norman Bridge Laboratory of Physics.” [1]Hendrik A. Lorentz and Paul Ehrenfest had visited Caltech in 1922 and 1924, respectively (see Paul Ehrenfest to Einstein, 4 February 1922 [Vol. 13, Doc. 45], note 5 and Doc. 136). [2]Louis A. Bauer Samuel J. Barnett (1873–1956) was an American physicist and research asso- ciate at Caltech. [3]Ira S. Bowen (1898–1973) was collaborating with Millikan on spectroscopic research. [4]Millikan and Bowen had discovered an important analogy between X-ray spectra and optical spectra, which, however, in some cases seemed to contradict the usual Bohr-Sommerfeld theory. This is the difficulty Millikan is referring to the relativistic mass-velocity dependence he mentions here had been used by Arnold Sommerfeld in his explanation of the fine structure of spectral lines. The difficulty was eventually resolved through the introduction of the electron spin quantum number. See Bowen and Millikan 1924 and Millikan and Bowen 1924, 1925 see also the discussion in Mehra and Rechenberg 1982, pp. 662–665. [5]Albert A. Michelson. [6]John C. Merriam (1869–1945) was an American paleontologist George E. Hale Walter S. Adams (1876–1956) was an American astronomer and director of the Mount Wilson Observatory Charles E. St. John. [7]Paul S. Epstein. [8]See Doc. 298. [9]Greta E. Millikan (1878–1953). [10]Harry Bateman (1882–1946) was a British mathematician and Professor of Theoretical Physics, Mathematics and Aeronautics at Caltech Richard C. Tolman (1881–1948) was Professor of Physical Chemistry and Mathematical Physics at Caltech Stanislaw Loria (1883–1958) was a Polish physicist and a research associate in physics at Caltech. 330. To Jost Winteler[1] [Lucerne, 3 October 1924] Lieber Herr Professor Hier bin ich mit Michele und Anna[2] froh beisammen und wir plaudern zusam- men schier ohne die Jahre zu spüren, die uns die Mutter Zeit auf den Buckel