8 0 D O C U M E N T 7 2 S E P T E M B E R 1 9 2 5 72. From Paul Epstein California Institute of Technology, Pasadena, 19 September 1925 Dear Professor, Many thanks for your kind letter. The fact that you must dispense with your trip is, however, a great disappointment for us[1] since we had already counted on the great scientific and personal benefits that your presence would have bestowed on us as a sure thing. However, we cannot disregard the validity of your reasons and must simply accept the consequences. One of these consequences is that we are now sending a young man by the name of Dr. Robertson to Europe. Robertson re- ceived his PhD this spring from our school with a thesis on relativity theory.[2] He was awarded a national fellowship and intended to spend another year in Pasadena to enjoy the privilege of your advice. After receipt of your letter of cancellation, we had his fellowship converted into an international fellowship and, on my advice, he is now going to Göttingen, since he is actually a mathematician and not a phys- icist. He will attempt to connect with you by visiting Berlin on occasion, and I be- lieve that the effort of explaining new theories to him will be worthwhile because he will understand everything. He is, in fact, a man of exceptional, precocious mathematical ability and is extraordinarily suited for working out difficult special cases. You ask about the new experiments concerning ether wind which have been started here.[3] Neither Millikan[4] nor I deserve any credit for it, as the idea origi- nates entirely with the laboratory assistant involved named Kennedy.[5] It deals with the following modification of Michelson’s experiment. A beam of light is split by a Nichol prism into an ordinary and an extraordinary beam. One is guided north- south, the other west-east. Then, they are reunited. The delay of one light path by one wavelength corresponds to a rotation of the polarization plane by 360°. It seems that this method is extraordinarily sensitive. However, Michelson thinks[6] that due to secondary effects, the sensitivity will be the same as that of his appara- tus. Anyway, it is an interesting variation. Miller has been on Mount Wilson for some time and I had the opportunity to speak with him extensively.[7] The longer I ponder the conversation, the more cer- tain my impression becomes that the business is humbug, because he obviously absolutely does not understand his results. E.g.: He believes and he tells everyone that his effect is a regular function of the time of day. In the albeit very limited nu- merical data that he has communicated to me, I can, however, detect nothing of the