2 8 2 D O C U M E N T 2 6 9 A P R I L 1 9 2 6 grating you described were moving at canal ray velocity in its plane, orthogonal to the grating slits? One can surely say: If the convex lens labeled by you as G and the interference apparatus were not moving along with it, one would have to observe the coherence of the light emitted by ¢an atom² the source. Therefore, according to the principle of relativity, coherence must also be detectable in your experiment, if only the grating were at rest whereas lens G and the interference apparatus were in motion with the canal ray particles at the same velocity. But, if incoherence resulted in the experiment actually to be performed in which lens G and the interference ap- paratus were also at rest, in my opinion that would only mean that the light, emitted by the source in noticeably different directions, isn’t coherent anymore. Therefore, the light would have to shoot out of the atom as “needle radiation” in a single di- rection, but the needle ray in itself would be coherent from the outset. It seems to me that the latest experiment by Bothe does in fact prove the character of light as “needle radiation.”[5] Might there be a possibility to determine the aperture of the narrow radiation cone ejected out of an atom in an elementary process, perhaps by strongly varying the velocity of the canal rays in the experiment that you are plan- ning? On Friday the 30th of this month I shall be in Berlin, and I shall also be coming to the meeting of the Physical Society. Perhaps I might see you on that occasion and hear your view on this subject? With best regards, sincerely yours, Gustav Mie 269. From Hendrik A. Lorentz Haarlem, 28 April 1926 Dear Colleague, Thank you very much for both of your letters.[1] After being somewhat unsettled by the first, I was very glad to gather from the second that the prospects of a reconciliation between the Germans and the others are gradually improving, after all. Now I am going to wait and see what Mr. Painlevé does.[2] Also, I must say to you that I learned with great joy that you are now ready to be a member of the Solvay Institute’s scientific committee.
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