8 5 2 A P P E N D I X C the other hand, one has a moving light source, as is the case with the canal ray particles, the interference image has to disappear, but it can be made visible again by a slight rotation of one of the two plates, so that the plates, which were originally positioned very precisely par- allel to each other, are at a small angle with respect to each other. This proposed experiment prompted a very lively discussion, in which participated, among others, Messrs. Ehrenfest, Zernike, Burgers, Van der Waals, and Vening Meinesz. Some were of the opinion that, if the proposed experiment were to show the expected result, this too could be explained from a classical point of view.