1 4 2 D O C . 1 6 9 N O V E M B E R 1 9 1 9 as Kroo does, it already makes a difference whether one wants to treat the electron as having a surface charge or a space charge!! I followed the matter through up to and including the terms v4/c4.[10] Agreement with the experimental data, as Kroo presents them, became a little better but not as nice as I had hoped. But yesterday evening I got hold of the new issue of the Physical Review, Oct. 1919, where Duane and Kang-fuh Hu report their latest determination of the -line of rhodium the wavelength corresponds exactly, yes, exactly with my calculation![11] So I must be getting somewhat better experimental data than Kroo used last year. (And what seems to be emerging is that even in the innermost core of atoms the theory of elec- trons does not lose its validity. Bohr’s quantum rule seems all the more puzzling.) We would be extraordinarily pleased, and de Sitter no less than I, if you really did find occasion during your next trip to Zurich to take a detour to Arosa. There are so many problems about which I would like to be advised by you. And it cer- tainly is beautiful up here! I do understand, though, that you will be very busy and that we must not ask you too insistently, but if you would like to escape the hubbub of Zurich for a while, then do think of us exiles among the snowy peaks![12] I think now I ought not to touch upon the issues raised by thoughts contained in your letter. Accept my cordial greetings, also on behalf of my wife[13] and on behalf of de Sitter. The latter must stay in bed, unfortunately, but the doctors are satisfied with his progress. It goes very much to his heart that he cannot support the interests of his observatory from here.[14] I remain yours sincerely, A. D. Fokker. P.S. Please do not forget the promised offprints! 169. From Max Planck Grunewald, 18 November 1919 Dear Colleague, You are right about the peculiar dual feelings in such experiences as the one just granted me.[1] But what consoled me highly in this case upon receiving your lines is the cheerfully asserted confidence that fair-mindedness is most firmly grounded especially among those most qualified to judge and that makes me truly deeply happy and grateful. I am very much looking forward to our time together in Rostock.[2] Most cordially yours, Planck. Ka1