I N T R O D U C T I O N T O V O L U M E 1 5 l x x v turning one of the mirrors of the interferometer by a small angle (Doc. 277). Since Rupp had observed interference, Einstein now expected the wave theory to prevail, rather than the corpuscular theory, as he had hoped in his “Proposal” (Doc. 223). At the end of May 1926, Einstein was satisfied with the results of the grid ex- periments, so much so that he thought the turned-mirror experiment would add nothing to their corroboration (Doc. 299). Rupp, however, performed the experi- ment with various settings and observed interference (Doc. 306). Einstein reported to Lorentz and Mie that everything confirmed the wave theory (Docs. 292 and 310). He had already formulated his arguments in Doc. 278 (Einstein 1926v), and discussed his paper, and Rupp’s findings, at the Wednesday colloquium of 7 July 1926 (Doc. 315). Rupp’s paper not being ready, Einstein only presented his own paper to the Prussian Academy the next day, but requested that its publication be delayed until Rupp could complete his own. He was “happy with the explanation of the elementary processes of optics” (Doc. 361). On 21 October, Einstein submitted Rupp’s paper as well, after having corrected some of its theo- retical statements (Doc. 387, Einstein 1926w). Since Rupp’s original manuscript is not extant, one can reconstruct Einstein’s objections to the original only from his letters (Docs. 384, 389, and 395). He ex- plained to Rupp that the build-up of the “interference field” must be distinguished from the emission of energy. The experiments do not say anything about whether the production of an elementary interference field takes place before the excited atom emits quanta or not, only that this emission time is finite and comparable to the classical damping time. Einstein was convinced that atoms radiate the interfer- ence field in an excited state during the damping time, but “we cannot conclude that the atom goes from excited state to the non-excited state gradually” (Doc. 384). Rupp had apparently concluded in his manuscript that the experiments proved that the energy emission of excited atoms takes place only in waves, while Einstein was of the opinion that the “wave–quantum duality is increasingly com- ing to a head: energetically directed eventful process geometrically undirected uneventful process. When will one really understand all of this from a simple foundation so that one can gain some sense of its necessity?” he wrote to Mie (Doc. 292). The Einstein–Rupp correspondence is indeed peculiar. Whenever Einstein criticized Rupp’s experiments, within days Rupp would repeat them and obtain results entirely in accord with Einstein’s theoretical expectations. On many occasions, Rupp claimed to have “anticipated” Einstein’s suggestions (Doc. 313) and to have “unconsciously” adjusted his instrument’s settings so as to compen- sate for disturbances (Doc. 354).