Abstract: Method and apparatus for measuring variations in the distance between a point of reference and the surface of an object. In the method light emitted from a substantially monochromatic light source is reflected from the surface of the body and in an interferometer is combined with a reference beam with a known path and from the same light source and by means of a light sensitive device is converted into an electric signal. The electric signal is converted into an output signal in a digitized signal processing device comprising for a continuous processing component of the first derivative of the curve of the electric signal, which output signal is an expression for the variation in distance being measured. In order in a simple way to determine the sign or polarity of variation of the distance without reducing the wide field of measurement obtained by the method, the length of the path of the reference beam is substantially momentarily changed with a value corresponding to 1/4 wavelength of the emitted light.