D O C U M E N T 2 6 2 A P R I L 1 9 2 6 2 7 5 In your exposition, I do not understand why you are permitted to use the last form in (4), because it does not agree with the condition [5] Best regards, your A. Einstein 262. To Emil Rupp [Berlin,] 23 April 1926 Dear Mr. Rupp, Further thinking on the subject has led to results that are important for our experiments.[1] It is about consequences of the wave theory, which always apply. 1) Also in our case, a light source that is extended in the direction of the ray acts as a planar light source. Hence, one can adjust the slit of the canal rays parallel to the ¢line of vision² direction of the light that is investigated without altering the phenomena, thereby enhancing the intensity considerably (for all experiments in which one need not cut out a sharply determined location of the canal ray bundle.[2] 2) A displacement of the light source (canal ray bundle) in the direction of the light path does not influence the phenomena. Hence, in all our experiments we can use the setup: [3] canal rays [4] canal ray slit [5] plane glass window This setup is, in its optical properties, equivalent to an infinitely thin bundle of canal rays that would be placed, e.g., in the plane E. Hence, one need not make any efforts to obtain a focused image of the canal ray bundle. 3) This simplifies our experiments about the tilted mirror (a) and the grating ex- periment (b) considerably: Σ ns const. = [3] [4] [5]