D O C U M E N T 4 7 F E B R U A R Y 1 9 2 2 6 9 asked you in my postcard whether I had read correctly “9 Scultur Street”.[3] You didn’t answer this question—please do so finally.– As I already informed you, we now managed to raise the concentration of the Au ions up to 6.96 × 10–3, thanks to the coprocessing of a local preparation of 1.00 × 10–3 and one from Methu[en] 1.08 × 10–3. I received the report by Villars just now that he manufactured for us a preparation of approx. 0.38 × 10–3,[4] all in our units. But, you see, the concentration is so low that it’s perhaps better to wait with it until you come here again and then ask Villars to send it he’ll have a higher concentra- tion by then, all right. Please immediately choose when you’ll be coming to Leyden.—Our Easter hol- idays will be from 8 April incl. until 24 April incl. Before Easter, Lorentz is hardly likely to be back yet.[5] In March my wife will possibly be away too.[6] In this short period I cannot arrange anything either for “redsh[ift].”[7] – After Easter the weather ought to be very nice here.– We could set one part of the eventual discussions (e.g., redsh.) within the period (vacation!) from 19 April until 24 April.– Choose quickly and reliably!! and answer me immediately. Regards to all of you together, yours, P. Ehrenfest Leyden Holland[8] 46. From Joan Voûte[1] Batavia, Weltevreden 11 February 1922 Java [Not selected for translation.] 47. To Paul Ehrenfest [Berlin, 12 February 1922] Dear Ehrenfest, In the same post I am sending you the correction proof of my notice about the ray path in dispersive media.[1] It’s nice how simply the business comes out. Your note about the high concentration of Au ions is very welcome, considering that the state of our examination here makes such a high concentration appear very desirable.[2] The address of our Madrid colleague is: 9 Lealtad (Kuno Kochertha- ler), Madrid.[3] It would be preferable if we could shift the congress to the end of September.[4] I was with you for so long just recently.[5] I’m glad that your wife can now also go away for once.[6] I can’t grasp her opinion that the quantum difficulty