Abstract: In an interferometer for measuring displacements of a movable structural component, comprising a measurement interferometer arrangement (1), a partial beam traverses a measurement branch of variable optical length and is guided by an adjustable reflector system (11) connected to the movable structural component. The measurement interferometer arrangement (1) is arranged, together with the adjustable reflector system (11) including the whole measurement branch, in a sealed housing (3) which also contains a wavelength measuring device (2) for measuring the wavelength of the partial beam which travels through the measurement branch in a gaseous medium. A ventilator device (14) produces an air current in the housing (3), so that identical conditions, in particular identical temperature, pressure and humidity, always prevail in the medium in the measurement branch and in the spatially separate wavelength measuring device (2).