D O C U M E N T 3 1 J A N U A R Y 1 9 2 2 5 7 8.) From this theorem by Gibbs therefore follows: No matter how long and far we run along with the group, we always see in it solely the waves with the small inclinations between , which correspond to one the slit breadth (or a part of it)—just the fictitious (dead!) waves far in advance of the head of the group have the large inclinations. 31. To Paul Ehrenfest [Berlin, between 19 and 22 January 1922][1] Dear Ehrenfest, Everyone is pelting me about my constant doggedness. Laue already had a proper duel with me at the colloquium.[2] But I’m sure of my point. I performed a fine, rigorous calculation that will surely convince you. It’s just too long for me to be able to tell you the essence so briefly. In a couple of weeks I’ll send you the cor- rection proofs.[3] The error in your consideration[4] lies in the following: You prove that a wave that enters at A after propagating has the same inclination for the wave normal at B as at A. This would be true—if the wave complex arrived at B. This, how- ever, is not what it does instead it arrives at C and its wave normal at C has the orientation AC.The whole problem can only be solved properly if one examines the course of the ray. I started from a strict solution and consider my proof secure— whatever a theoretical physicist may call secure. I am peni- tently remorseful of formerly committed blunders . . . . In any event I am curious what you’ll say about my proof. My love for it is naturally based for the most part on my having agonized over it. The Span- ish fellow scientist interested in the Au-ion affair is called Kuno Kocherthaler, Apartment 425, Madrid, 9 Lealtad Street. I laughed myself to tears about your mes- sage regarding him.[5] With Grommer I am trying to address the zero-point energy problem.[6] It’s not easy. I also have in mind to determine with Geiger and Bothe the quadratic Doppler effect by means of a few little tricks but it is difficult.[7] Prod K[amerlingh] Onnes about the superconductivity experiments.[8] Did the young American find fluctuations? I ought to have gone to Paris for New Year’s, but could not overcome my weariness of traveling.[9] Kramers recently visited me, an excel- lent fellow.[10] Are you still thinking about the spectral congress?[11] It really would be fine if Julius were to analyze the solar-center/solar-limb effect photographically/ photometrically by then, so that some clarity could be reached on this.[12] The main ε±