The paper describes an experimental equipment (known as the `N.P.L. Microwave Mekometer') using elliptical polarization modulation of a light beam at 9.4 Gc/s (9400 Mc/s) and intended for the accurate measurement of distances of the order of 50 m. The beam is returned from a distant reflector to the modulator at which the ellipticity is either increased or cancelled. A knowledge of the speed of light and the modulation frequency serves for the derivation of the modulation wavelength; the distance from reflector to modulator is then obtained in terms of the number of modulation wavelengths in it.