2 9 4 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 in the case that the canal rays and the reflective planes are set exactly perpendicular to the X-axis. For a sufficiently small α, this is equal to . The motion of the canal ray particles hence simply has as a consequence a shift of the interference pattern by the angle . This offers a convenient method for mea- suring the canal ray velocity. The case is dealt with equally simply where an optical system that is equivalent to a telescope adjusted at ∞ and enlarges the angle z times is interposed between the canal ray and the interference apparatus. In this case, the angle shift in the in- terference pattern is 1/z times larger than in the case just considered. Case 2. A lens or a system of lenses with focal length f is interposed between the canal ray and the interference apparatus. The lens or system of lenses generates an image, out of the substitute canal ray imagined to be at infinity, that can be substituted by light sources at rest, and is per- pendicular to the X-axis. To the ordinate y of this image belongs the wavelength , where , therefore the wavelength . The effective- ness of both mirrors can be taken into account by imagining this light source to be duplicated by the reflection the second image formed this way[5] would be imag- ined to be at the abscissa distance –d from the first, in such a way that two points of each of the light sources with the same y are coherent. Both images act as coher- ent light sources.[6] If both of the light sources were monochromatic, all of their matching pairs of points would yield the same interference pattern at infinity. For this to happen, it would be necessary that the points of all the pairs, measured in wavelengths, were at the same distance. As this is not the case, no clear interference can form at infinity. Complete interference is produced by tilting the image formed by reflection off the interference mirrors by β[7] against the other, according to the scheme: d λ0 ----- 1 1§ 2© -- - α v· c¹ -- – 2 – [p. 336] + v c -- λ0© 1 v c -- α· – ¹ § α y f -- = λ0© 1 v c -- y· f¹ -- – § second image formed by interference mirror image of canal ray at infinity