A monochromator to be used in an X-ray device having an X-ray source is formed by a crystal for spectral restriction of X-rays produced by the X-ray source. The monochromator includes a positioning device that can move the crystal so that it changes the spectral composition of the X-radiation. The crystal can be moved so that it changes the angle between an X-ray path and the crystal, or so that the crystal is removed out of X-ray path or returned into it.