To decrease the precision requirements placed to the scanning blocks of interferometric spectrometers used in Fourier spectroscopy, use is made of a scanning block (4) which is made either of two flat mirrors which are parallel and connected to each other or two trapezoid Dove prisms (2) which are optically and mechanically interconnected along the shorter base of the trapezoid forming thus a uniform optical element so that the hypotenuse faces of both Dove prisms operate as flat parallel mirrors, while the scanning block is installed in the interferometer so that both shoulders of the interferometer pass through it and the block has the possibility of swinging movement to and fro around its pivot (o) which is parallel to the surface of the mirrors, whereas movement of the scanning block round the pivot brings along increase of optical path in one shoulder of the interferometer and decrease in the other.
