2 9 2 D O C . 2 7 8 I N T E R F E R E N C E O F C A N A L R A Y L I G H T 278. “On the Interference Properties of Light Emitted by Canal Rays” [Einstein 1926v] Completed before 9 May 1926 Presented 8 July 1926 Published [after 21 October 1926] In: Preußische Akademie der Wissenschaften (Berlin). Physikalisch-mathematische Klasse. Sitzungsberichte (1926): 334–340. Hitherto I was of the opinion that experiments on the light of canal rays could yield results that do not agree with the findings of the classical wave theory.1) In the following I would like to present a simple consideration by which a failure of the classical wave theory seems virtually excluded within the domain under exam- ination. This consideration is of a certain interest also because it leads to a conve- nient prediction of the interference phenomena to be expected. Of essence in the consideration to be conveyed here is that it only avails itself of the wave theory in- sofar as its findings are recognized as confirmed by experiments. I presume the following statement, whose validity can scarcely be doubted: An extended, homogeneous source of light at rest can always be substituted optically by an equivalent resting light source that is shifted in parallel to it.[1] This statement naturally only claims validity insofar as the boundaries of the light source are un- noticeable. Its validity is expressed, e.g., in that the “interference phenomena on small wafers” are entirely independent of their distance from the light source of the interference apparatus. We now regard a homogeneous canal ray in a vacuum. It—observed from a co- ordinate system K' moving with the particles—is a light source at rest. In the ex- perimental arrangement first introduced by Wien[2] to analyze the dying out of the canal ray glow (owing to the decay along the canal ray), this light source is not ho- mogeneous, however but this is irrelevant to the interference properties of the emitted light. In accordance with the above statement, we can substitute this light source at rest with reference to K' with one shifted in parallel and at rest in K'. 1) Cf., e.g., my notice, “Proposal for an Experiment Pertaining to the Nature of the Elementary Process of Radiation Emission,” Naturwissenschaften 1926, issue 14.[3] [p. 334]
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