8 D O C U M E N T S 3 , 4 A P R I L 1 9 2 3
3. From Paul Ehrenfest[1]
Leyden, 4 April 1923
Dear Einstein,
About a week ago I sent you a
postcard.[2]
As I still don’t have an answer at the
moment and reckon with the possibility that it or your answer are lost, I would like
to repeat the essentials of the content: I was very, very happy that you spontaneous-
ly spoke with my
wife[3]
about when you could perhaps come to visit us again. (She
probably told you that I did not dare to ask you!)— Well, I write you: It would be
best if you came right away. That means around about April 11— Reasons:
Throughout all of May neither Lorentz nor De
Sitter[4]
are here—both in En-
gland.— In June I have to—for money-making reasons—supervise many examina-
tions in Delft and this year also in Leyden. (One unexpressed reason: on April 11,
I have my 250th colloquium in
Leyden[5]
—luckily nobody here knows this—but it
certainly would be a fun thing to have you here on that evening.— Don’t laugh at
me!)[6]
— I thought: if you came now, your Leyden stay would still roughly fall
within the German Easter
vacation.[7]
— Whichever way—let me know what you
decide; or better yet, let
Ilse[8]
answer me; then I’ll know that there won’t be any
confusion!
Very warm regards to all of you from us—I wish your wife a speedy
recovery!!—
Yours,
P. Ehrenfest
4. From Max Planck[1]
Berlin, 6 April 1923
Dear Colleague,
The separate reprints of your new Academy article already arrived here a few
days
ago.[2]
That is why I presume that they have likewise already arrived for
you.— The paper treating the same problem, about which I spoke to you, originates
from Reichenbächer and can be found in Zeitschrift für Physik, vol. 13, p.
221.[3]
Cordially yours,
Planck.