2 4 2 D O C U M E N T 2 3 3 M A R C H 1 9 2 6 We are receiving letters from all parts of Germany, giving witness to the fact that a powerful movement for European cooperation is under way among us. Every German who cares about the advancement of our country and the fate of Europe must join the new association. Friends of the European idea, please contact the office of the Association for European Understanding (Berlin NW 7, Doro- theenstr. 53).[8] 233. From Emil Rupp [Heidelberg,] 23 March 1926 Dear Privy Councillor, Many thanks indeed for your kind letter of 20 March 1926,[1] & with regard to your inquiry I would like to convey to you the following: The occurrence of null positions as a result of Doppler components of the canal rays, for which, of course, the aperture angle of the lens does come into consider- ation, caused problems for my investigation for a long time. The diaphragms you surmised, with whose help the “effective” α can be strongly reduced, are indeed there they are the diaphragms b b in front of the interferometer mirrors in Fig. 1, p. 5.[2] At first, I used arrangement I in the sketch (enclosed sheet). There the dis- tance l of the slit center from the end of the canal is not determined precisely enough, though, & the α is not quickly smaller than . The beam path of the final arrangement is the following, II: The distributor plate P is omitted. The light of the Doppler components, which are determined by the of lenses L1 & L2, does not reach the interferometer. That is because the diaphragms b b, 4–5 mm wide slits, select from the light beam a beam parallel to the ray at the angle ϕ, whereas vertical to the beam the entire aperture angle is exploited. As cm the ∠ϕ = = for an ideal beam path. This angle evidently becomes even smaller if the two diaphragms b b are not exactly set at the center & if the beam path from lens L2 is slightly divergent. I could not establish exactly how in my investigation the effective α is composed, mainly be- [p. 10] 1- 10 ----- α1&α2 f2 40 = 4 400 -------- - 1 100 -------- -
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