A method is described for reducing the width of a Debye-Scherrer line produced by diffraction from a polycrystalline medium, if this width is due to the spectral impurity of the primary characteristic radiation. In this method, a diverging polychromatic beam is allowed to fall on the plane surface of a single crystal. The beam diffracted by this crystal will diverge and will contain a bundle of rays whose wavelength range corresponds to the finite spectral width of the characteristic radiation. The polycrystalline sample is mounted normal to this bundle. It is shown that the different wave lengths diffracted by the sample can be brought to a narrow focal spot.