2 4 0 D O C U M E N T 2 3 1 goes to the slit S, then the spectral width generated by the canal ray velocity is not solely determined by the angle at which slit S appears from the center of the lens but also by the aperture angle α of the lens. The relative spectral width due to the Doppler effect is then[4] . As not smaller than , because otherwise the intensity would become too low. From this it would follow that beats of a length of wavelengths would exist, that is, with a distance between the null positions of about ½ cm.[5] For a very small path difference[6] , this would lead to a vanishing of the interference, as can be easily seen. It remains a mystery to me how you were able to avoid this phenomenon. You probably had a diaphragm that reduced the effective α considerably. I would be very thankful if you would comment on this. With friendly regards, A. Einstein v c -- sinα v c -- 10 3– sinα , = 1- 10 ----- 104 α2 α1) – (