1. Interferometric apparatus comprising: (a) a rotary O.P.D. scanning assembly mounting a mirror pair formed by a beam splitter and a folding mirror, said beam splitter having a semi-reflective beam-splitting layer one face of which is the optical input surface in accurately parallel face-to-face relationship with said folding mirror and the other the output surface; (b) two stationary reversing mirrors co-operating with said mirror pair for defining two interferometric arms, each arm receiving one of the two beams resulting from the splitting of an input beam by the beam splitter, for rotationally scanning successive O.P.D. values between the two beams up to a predetermined maximum, and for recombining the beams at said output surface so that they emerge therefrom as a single beam directed along an output path; (c) an output mirror mounted at right angles to the output surface of said beam splitting layer, said output mirror being borne by the scanning assembly and positioned to receive the output beam; and (d) pivotal means arranged to ensure that the scanning assembly rotates around an axis substantially coincident with the line of intersection of the prolongation planes of the semi-reflective beam-splitting layer and the face of the output mirror, respectively.