“…The new work presented here is a discussion of those effects which are specific to non-centrosymmetric crystals. In this case, the Fourier coefficients Vg and Vg of the elastic and absorptive potentials repectively are both complex numbers and there is no reason in principle why the phases, ~pg and ~pg, of these coefficients should be the same (Gevers, Blank & Amelinckx, 1966;Snykers, Serneels, Delavignette, Gevers, Van Landuyt & Amelinckx, 1977;Bird, James & King, 1989;. However, in the vast majority of diffraction calculations it is assumed that the absorptive potential is directly proportional to the elastic potential, in which case the phase difference, A~pg = ~pg-~pg, vanishes.…”