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 AY L I G H T 2 9 3 Viewed from the coordinate system “at rest,” K, this means that we can shift a canal ray parallel to itself, without our being able to ascertain this from the light emitted by it. Furthermore, however: In its optical effects, a canal ray can be conceived as substituted by an infinitely remote one of the same character and the same velocity. This statement puts us in a position to predict easily the interference phenomena of the light emitted by a canal ray, because an infinitely faraway canal ray, with ref- erence to the light transmitted to the optical system located at a finite distance, can obviously be substituted with a system of continuously distributed light sources of suitable color at rest. We suppose the observed canal ray K to be parallel to the Y-axis of a system of coordinates. We imagine it as substituted by one cutting the negative X-axis at in- finity and restrict ourselves to the propagation directions that are nearly parallel to the X-Y plane. If is the eigenfrequency of the canal ray particles, then the light emitted at the angle α with respect to the X-axis has to first approximate the fre- quency .[4] We are allowed to calculate as if the light sources belonging to α were resting at infinity and had the frequency ν. Further, we may regard the intensity of the radiation as independent of α if we still confine ourselves to small angles α, which we want to do. Thereby, every problem of diffraction is reduced to one with light sources at rest. Some problems of the kind will be discussed in outline in the following. The inter- ference apparatus will be formed out of two semi-reflective parallel planes whose distance is . The observation is conducted by eye or in combination with a tele- scope adjusted to infinity. Formally, this comes down to our examining the exci- tation in a plane set at infinity perpendicular to the X-axis, without there being an optical apparatus behind the interference apparatus. Case 1. There is noth- ing interposed be- tween the canal ray and the interference apparatus that could deflect the light rays. The phase difference between the two rays at highest intensity is for the , [p. 335] ν0 ν ν0© 1 v c -- sinα· + ¹ § = d 2 -- - x ∞) = ( Canal ray Interference apparatus α∠ dcosα λ0© 1 v c -- sinα· – ¹ § -----------------------------------