IN the past five years, neutron diffraction analysis has been carried out with wide neutron beams and specimens of several cubic centimetres in volume, in order to obtain a manageable diffracted intensity. Such an expedient has many disadvantages. In the powder method it leads to bad resolution, and incoherent scattering in the sample produces a background which is sometimes larger than the significant counting-rate. This is particularly serious for the study of hydrogenous substances, which give large disordered scattering, and it has been widely concluded that deuterium compounds must be used in their place. With single-crystal methods the limitation arises from the severe extinction of the neutron beam. A single crystal suitable for structure analysis must in general have dimensions of about 1 mm.1.