D O C U M E N T S 3 9 1 , 3 9 2 O C T O B E R 1 9 2 6 3 7 9 391. From Emil Rupp [Göttingen,] 22 October 1926 Dear Professor, I thank you most sincerely for your corrections.[1] I have meanwhile also made the corresponding ones in my copy. One point does pose difficulties for me: Ac- cording to the setup of the experiments, the minimum time for the generation of the interference field is identical to Wien’s glow duration if the latter corresponds to the retention time, the interference field would then have to form during the reten- tion period.[2] I would like to present one more remark against to G. Joos (Physikalische Zeitschrift, p. 401),[3] who assumes a spectroscopically detectable splitting of the original frequency in the grating experiment. A splitting is only possible for quantumlike processes, though a light wave cannot be “modulated” along its path through space. Rather, a line broadening would occur instead. I intend to conduct the experiment with a grating & an echelon grating spectrograph. Respectfully yours, sincerely, E. Rupp 392. Foreword to Emil Julius Gumbel, Vom Russland der Gegenwart [Einstein 1927b] Dated 23 October 1926 Published 1927 In: Emil Julius Gumbel. Vom Russland der Gegenwart. Berlin: E. Laub, 1927, p. 5. Dear Mr. Gumbel, I read your memoir about Russia, with, indeed, sincere and joyful admiration. You made remarkably objective and wide-ranging observations, as not many could.[1] This work ought to be published by a good publishing house. With kind regards, your A. Einstein