D O C U M E N T 2 6 8 A P R I L 1 9 2 6 2 8 1 [4] canal ray [5] interf[erence] app[aratus] [6] lattice with mm spacing [7] telescope set to To an emission direction R corresponds a point on the telescope’s focal plane. Rays emitted by a particle in direction R reach, or do not reach (alternately) the tele- scope for a suitable relation between particle velocity and path difference, the interference would have to cancel out, which however I do not believe. The diffrac- tion at the grating has a disturbing effect, but it is not so strong as to disturb the conclusiveness of the experiment. With kind regards, your A. Einstein 268. From Gustav Mie[1] Freiburg i[m] B[reisgau], Hebelstr. 33, 27 April 1926 Dear Colleague, My most obliging thanks to you for sending me all your papers of the last few years! Much of it is, in fact, not yet known to me, and I am now going to get down to studying them thoroughly. Over the past few weeks I have been familiarizing myself with the theory of Heisenberg, Born, and Jordan [2] at many passages, par- ticularly regarding black-body radiation,[3] I unfortunately had to realize that it is not easy for a much-harassed institute director to stay up to date in every respect and it was only now that I thoroughly read some of the older papers. The beauty of the new quantum mechanics impressed me greatly, although it initially took me some real effort to thoroughly embrace the standpoint that we should abandon the principle of causality in order to replace it from the outset with statistics. But I must say that I have now also reached the conviction that, at least for the foreseeable fu- ture, we cannot advance further otherwise. A short while ago I read your note in Naturwissenschaften,[4] from which I see that it really is possible to conduct successful experiments on the optical coherence of light originating from various parts of a canal ray beam. This is highly interest- ing, and I wish you and your collaborators speedy success with that fundamental experiment. Please allow me, though, to make a minor comment about the theory, even on the risk that I tell you something already long known to you! What kind of result might be expected if the light source were at rest and if, on the contrary, the 1 10 ----- - ∞