1. In this review we shall not try to present an exhaustive bibliography; the references we cite however do contain most of the relevant literature on the subject. General reviews on fundamentals can be found in: R. Kubo, Rep. Progr. Physics 29, 255 (1966);
2. D. Forster, “Hydrodynamics, Fluctuations, Broken Symmetry and Correlation Functions”, Benjamin, 1975;
3. B. J. Berne and R. Pecora “Dynamic Light Scattering”, Wyley, N.Y., 1976;
4. J. P. Hansen and I. R. McDonald, “Theory of Simple Liquids”, Academic Press, 1976;
5. A. Barker, R. Loudon, Rev. Mod. Phys. 44, 18 (1972);