Abstract: An apparatus for measuring stress on the surface of a polycristalline body X-ray diffraction including an X-ray source assembled together with two position-sensitive radiation detectors of known type located symmetrically with respect to the primary X-ray beam emitted by said source. The diffracted radiation gives rise to gaseous discharges within said detectors, which deliver signals respectively indicating the location of the ionizing event along their collector electrodes. From these locations along the collector, it is possible to determine the interplanar spacing d of the crystal lattice, which varies with the stress it is subjected to.