For many years the study of crystal physics was mainly of academic interest, but now it impinges on many branches of pure and applied science. In crystal-struoture determination
the study of the ph)Tsical properties is often of great value, especially in testing for the absence of a centre of symmetry. Valuable information concerning the crystal structure is to be deduced from the optical and magnetic properties of crystals built up of long or flat and therefore strongly anisotropic molecules. More directly still, the thermal, dielectric, piezoelectric, an{l ela.stic properties of the crystals used in the fabrication of crystal oscillato:L"s and resonators must be accurately known to lnanufacturers, and eqllally important is the magnetic anisotropy of the material used in permanent magnets.