2 9 0 D O C U M E N T S 2 7 5 , 2 7 6 M A Y 1 9 2 6 275. From Emil Rupp [Heidelberg,] 1 May 1926 Dear Professor, My last communication to you contains a meaningless fact about the image plane of K & the diaphragm size B at the grating.[1] The fact that I obtained clear findings anyway agrees with your argument that the grating’s location is irrelevant to the interference capacity of canal rays.[2] I have now introduced the following arrangement & have taken measurements with it: The grating is at a 2.4 cm distance from the canal ray K. B is a diaphragm of 4 mm width between the plane // plate P at the tube & the grating G for selecting that portion of the canal rays capable of interference. L1 & L2 as earlier. This arrangement is much more strongly luminous, thus I am going to be able to estimate well the interference sharpness as a function of the path difference. As I was able to establish, the findings relayed to you still remain standing. I am hoping to be able to send you soon a curve plotting interference definition-path difference. Respectfully yours, E. Rupp[3] 276. To Sigmund Freud[1] Zionist Organisation, 77, Gt. Russell Street WC [1, London,] 4 May 1926 Professor Einstein and Dr. Weizmann,[2] presidents of the Board of Governors of the Hebrew University in Jerusalem, send on its behalf to their colleague, the pio- neer of a new science and developer of new areas of human knowledge, heartfelt good wishes on his 70th birthday. The undersigned joins most heartily in these good wishes. Eder[3]