X-Rays and Crystal Symmetry

Abstract

IT has long been recognised that angular measurements do not always carry one beyond a determination of the system, and that other methods of investigation are needed if the crystal is to be assigned to its class of symmetry. But different methods do not always give the same result, so that some principle of discrimination has to be adopted. In the past the principle universally applied has been that of greatest common measure, the crystal being correspondingly relegated to the highest class, the symmetry of which is common to the various symmetries observed (in most cases this leads to the lower of two observed symmetries, since the symmetry of one is generally wholly contained in that of the other). It must be noted that all class assignments are provisional and liable to modification (necessarily in the direction of lower symmetry) as new evidence is forthcoming.