3 8 8 D O C U M E N T 4 1 0 N O V E M B E R 1 9 2 6 410. From Emil Rupp [Göttingen,] 8 November 1926 Dear Professor, Many thanks for your correction.[1] I was tempted to make such a statement be- cause I found no other way to justify the observed nonvanishing of the interferences of Hβ as a consequence of the thermal motion. I discussed the correc- tion thoroughly with Prof. Franck[2] at the proof stage. He thought I should omit section C altogether, since it has nothing to do with the other experiments anyway.[3] But because I must consider the observations to be correct, I would like to leave it standing in the form now corrected by you. I deem it certain that, in my earlier work,[4] Doppler components of the beam velocity were unconsciously compensated for during the turned-mirror experiment.[5] The only question is: Can the thermal motion in the canal & liquid air cooling be influenced by the electric field or the screen arrangement in such a way that the orthogonal components diminish? At first one would answer in the negative, whereas the result of the ex- periment does seem to speak for it. New experiments would probably have to be performed, which I want to postpone for the time being. At any rate, the grating experiment & turned-mirror experiment have nothing to do with this complication. One objectively feasible experiment that I want to take up sometime corre- sponds to the grating experiment, namely, subdividing a train of waves using a Kerr cell:[6] L = Hg resonance lamp that produces a very fine line at 2537Å. K = Kerr cell that is activated by short electric waves. A = absorption vessel with Hg. If K is voltage free, then the traversing light is absorbed completely in A if there is a high-frequency voltage at K, then a line broadening occurs the absorption in A diminishes. Then the reduced absorption at an increased frequency can be photo- graphically recorded at K. In cordially thanking you once again for your kind efforts, I remain very sincerely yours, E. Rupp