Like elements in the two figures are denoted by like references, except that the photodetector 5 in FIG. 1 is to be regarded as an interferometer output device 5 in FIG. 2, in that in the latter figure it could be a photodetector, as in the case of FIG. 1, but it could also be something else, e.g. a screen for viewing the interferometric fringes, depending on the application for which the interfer