D O C . 3 9 1 A P R I L 1 9 2 0 3 2 7 Finally I would still like to point out to you the following things which drew my attention upon translation: Page 30. It has been brought to my attention that the expression for the kinetic energy was somewhat confusing and that it might be better to speak here of total energy. The k[inetic] e[nergy] would then be . What do you think, Professor? It seems you did this on purpose and the first few lines of p. 31 also speak for this interpretation. Would you suggest a change? p. 30, 4th line from the bottom: replace “theory” with “energy.” p. 33, section 16: 3rd line: “to be answered” is unnecessary, since comp. beginning of the sentence. p. 36, “FitzGerald” is spelled with a “t”. p. 37, section 17: 9th line: Wouldn’t it be better to speak of “that” [jenem] point in- stead of “every” [jedem] here? One can best judge this after reading the whole sentence.[7] I hope you will excuse this long letter. In closing still, kind regards from yours very truly, Robert W. Lawson. 391. From Julio Rey Pastor[1] Leipzig, 22 April 1920 Highly esteemed Professor, As I informed you during my visit, I communicated to the Institut d’estudis Cat- alans and the Junta para ampliacion de estudios in Madrid (the two most important institutes for higher culture and scientific research in Spain) that it was not impos- sible that upon invitation you could grant us the honor of your visit.[2] The Diputació de Cataluña telegraphed me that I should immediately extend this invitation, before you receive official notice, to hold a few lectures at the Institut d’estudis in Barcelona.[3] The time, topic, and number of lectures are left entirely to you to decide freely, if you would be so kind as to want to accept this invitation. mc2 1 ν2 c2 ----- – ------------------ mc2 1 ν2⎞ c2⎠ -----⎟ – ⎝ ⎜ ⎛ 1 2 -–-- 1–