2 4 8 D O C U M E N T 2 4 0 M A R C H 1 9 2 6 240. To Emil Rupp [Berlin,] 31 March 1926 Dear Colleague, I received your interesting letter only yesterday[1] because I was out of town for a couple of days.[2] Thank you very much for your kind remarks, which naturally interested me very much. I was, of course, very interested in the experiment to produce cut-up light waves by means of the Kerr effect. It would definitely be in- teresting it is just not quite as clean as the other one, because a transfer of energy onto the light could be caused by the variable electric field, roughly as by a moving mirror. Nonetheless, this experiment might perhaps also offer proof, although it does also need special theoretical examination for its interpretation. I have also been thinking a lot about the problem of the interferences of canal ray light and arrived at interesting results that you could easily check. My main preference would be if we could then publish the findings together in the Academy’s proceedings. It is, however, possible that your boss could hold this against you, because he has a quite rabid aversion to me.[3] If you think that it would not be advisable for you to copublish with me, then we could both publish separately, e.g., both in the Prussian Academy’s proceedings or in the Zeitschr. für Physik but in a way that the two articles appear directly after each other in the same issue. Now I would like to give you a brief outline of the new considerations. 1) The simplest case is the one in which there is no lens between canal ray and the interference apparatus. In the following I shall always draw the interference ap- paratus as a plane-parallel plate, but I mean the Michelson apparatus. In this case, given ideal homogeneity in the canal ray beam, there are interfer- ences with arbitrary path differences. That is because each direction of the incident rays corresponds to one—and only one—color of emitted light. It is unlikely that this case can be realized, by reason of intensity problems. 2) Two lenses are interposed that correspond to a telescope set at ∞. [4] canal ray [7] F[=] focal length of L [5] 2f [8] Interf. apparatus [6] f = focal length of l [9] telescope set to [8] [4] [6] [7] [9] [5] ∞