THE technical application of metals involves a detailed knowledge of their behaviour under external stresses. In the usual method of testing, measurements are made of the changes in external dimensions of a test piece when stresses of known magnitude are applied, as in the determination of the elementary load extension diagram. It is nowadays practicable, however, by use of modern X-ray diffraction technique, to obtain simultaneously with the external load-extension diagram the analogous diagram for the atomic lattice itself, in which direct measurements are made of the displacements of the atoms from their normal positions under stress and of their recovery when the applied stress is removed.