D O C U M E N T 2 9 7 J U N E 1 9 2 6 3 1 5 The figure in Ann. p. 17 [5] shows that at a greater distance from the canal termi- nal (wider diaphragm), light-emission disturbances occur also in “decaying glow” of Hg lines (as opposed to hydrogen), partly caused by atoms at rest, partly by ones running into each other. I think that by varying the diaphragm-opening it will be possible to attribute the disappearance of the interferences at greater distances from the canal terminal to such disturbances. Despite the improved decaying glow, the minima in experiment a) are scarcely lower than in earlier experiments. I would like to conclude from this that the non- disappearance of the interferences at the minima locations is largely attributable to causes connected with the apparatus, which, however, I am unable to control when working with such low luminous intensities. Intensities as low as these can also easily lead to physiological errors. For that reason I only recorded ones that are im- mediately perceptible to a properly rested eye as minima. Extended searching pro- duces inaccuracies or even illusions. A finer grating substantially reduces the intensity, & the minima become less distinct. Perhaps this indistinctness is partly also physiologically based, in any event I would prefer not to make any indications in experiment c) wherever I can only obtain them through a lengthy process of trial and error. The use of even finer gratings would therefore not help bring about any useful results. Please judge as you see fit how these series of measurements agree with the the- ory in anticipation of your valued letter, I remain sincerely yours, E. Rupp 297. Recommendation for Hans Reichenbach [Berlin,] June 1926 Mr. Reichenbach is, in my opinion, one of the most talented and profound epis- temologists of our time. He has a thorough knowledge of modern physics and the history of philosophical thought. His scientific production is of great lucidity and incisiveness. His critique is matter-of-fact, poignant, and original. I value him highly for his fine mind and as a person. A. Einstein
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