“…Many of the earlier O 1989 International Union of Crystallography papers discussed two-beam theory (e.g. Kainuma, 1955;Fukuhara, 1963;Fujimoto & Kainuma, 1963;Ishida, 1970Ishida, , 1971Okamoto, Ichinokawa & Ohtsuki, 1971;Chukhovskii, Alexanjan & Pinsker, 1973), this giving way to more general single scattering theories (Gj0nnes, 1966;Gj0nnes & H0ier, 1971;Rez, Humphreys & Whelan, 1977;Rossouw, 1985;Rossouw & Bursill, 1985) and analysis of the relationship between single and multiple diffuse scattering (H0ier, 1973;Yamamoto, 1980). In two recent papers Taft0 (1983Taft0 ( , 1987 has revived interest in two-beam theory by showing that structural and even phase information is retained in two-beam inelastic scattering.…”