Use of the Ewald sphere in aligning crystal pairs to produce X-ray moiré fringes

One crystal plate can be superposed upon another of the same material to produce observable X-ray moiré fringes in the simultaneously diffracted beam transmitted through the plates. During alignment it is necessary to determine, and then minimize, the small difference vector between the acting reciprocal lattice vectors. By rigid rotation of the crystal pair about the mean reciprocal lattice vector the Ewald sphere is caused to make various angles with the difference vector. This vector can then be fully determined by measuring its component normal to the Ewald sphere at two or more settings of the sphere.