  This paper discusses a theory of natural circular dichroism in the x-ray
region. Integrated spectra are interpreted in terms of microscopic effective
operators, which are derived in the framework of a localised (atomic) model. It
is shown that the generators of a de Sitter group, such as that introduced by
Goshen and Lipkin for nuclear structure, are suitable for describing electronic
properties of non-centrosymmetric crystals.
