3 5 8 D O C U M E N T S 3 6 1 , 3 6 2 A U G U S T 1 9 2 6 361. To Emil Rupp [Berlin,] 27 August 1926 Dear Mr. Rupp, Although I already submitted my paper in July,[1] I gave instructions to wait with the printing until you have submitted yours. Both papers are to appear directly one after the other in the Sitzungsberichte. The success of your previous experiments, however, can only be explained by means of an unconscious application of the turned-mirror experiment.[2] I am very happy about the detailed insight which we gained regarding elementary processes in optics. Mr. Frank’s objections are not justified[3] if the parallel-displacement law for ex- tended light sources does indeed hold.[4] The lens forms a sharp image in its rear focal plane[5] of the infinitely distant bundle of rays, in which the frequency is a function of position. The mirrors replace this image with two images the light coming from them creates a sharp interference pattern at infinity if both images are tilted with respect to each other according to the frequency distribution. Kind regards, A. Einstein Translators’ note: Based on a translation by Doris Lonk and Tilman Sauer 362. To Paul Ehrenfest [Berlin,] 28 August 1926 Dear Ehrenfest, I’ll gladly come after the scientists’ convention (right at the end of September).[1] I’m very much looking forward to our being together again, although I have but little brain power to bring along.— Cheer up about your daughter.[2] My Albert is doing very much worse. He is taking up with a girl who is very tiny, has goiter, a mother in a loony bin, and is ten years older than he. And nothing can be done about it.[3] Galinka is an incredible sorceress.[4] Such self- confidence. Such a thing reminds one of a hen that, hardly hatched out of the egg, is already pecking away at its seeds with absolute self-assurance. It’s a joy to see such a thing.— I consider Polianowska a kind of impostor you shouldn’t have let yourselves be duped by her.[5] I am confronting quantum mechanics with wondrous mistrust. I don’t under- stand the details of Dirac at all (Compton effect).[6] We’ll work at it together. Schrödinger is very captivating at first.[7] But the waves in the n-dimensional co-