D O C U M E N T 2 9 2 M A Y 1 9 2 6 3 0 9 of the lines and of the gaps is 0.01 cm each? But even then it would not come out right for the finer grid. The fact that the canal ray does not produce the effect in a larger distance from the entry point could only result from a diffusion of magnitudes and directions of the velocities by means of interaction with molecules at rest. But you explicitly de- nied this possibility in your Annalen paper.[3] The widening of the ray through a divergence that existed from the beginning could not explain a washing-out of the effect along the ray. Incidentally, the appearance of intensity at rest indicates the existence of diffusion, as you yourself emphasize in your letter. I am not able to refute the concerns 1) and 2) on the basis of your data. I would like to thank you very much for keeping me up to date on such a regular basis re- garding the experiments. Best regards, A. Einstein Translators’ note: Based on a translation by Doris Lonk and Tilman Sauer. 292. To Gustav Mie [Berlin,] 22 May 1926 Dear Colleague, Unfortunately, I was prevented from coming to that meeting of the Physical Society.[1] I was very sorry about that. The business by Heisenberg and Schröding- er greatly impresses me as well.[2] Doing away with strict causality need not be fi- nal, for Heisenberg’s theory does not claim to be a complete theory at all, but is rather just a mathematical version of the correspondence principle. I must say that, mathematically, the matrix functions do not appeal to me in the least. But all of this will later perhaps assume another countenance. As pertains to the canal rays, it is already as good as certain that everything pro- ceeds entirely classically and that particularly the generation of the undulatory field happens entirely as if the excited atom were carrying Hertz oscillators.[3] The wave-quantum duality is increasingly coming to a head: energetically directed eventful process geometrically undirected uneventful process. When will one re- ally understand all of this from a simple foundation so that one can gain some sense of its necessity? Numerical mastery alone does not sanctify. Kind regards, your A. Einstein