1. For a general-reference to optical equations see M. Born and E. Wolf, ‘Principles of Optics’, 2nd Ed. , Pergamon, London (1964). The sign conventions used here, however, are those suggested by R. H. Muller, Surf. Sci. , 16, 14 (1969).
2. R. C. Jones, J. Opt. Soc. Am., 31, 23 (1941).
3. M. J. Dignam and M. D. Baker, Appl. Spectrosc., 33, 186 (1981).
4. M. J. Dignam, M. Moskovits and R. W. Stobie, Trans. Faraday Soc., 67, 3306 (1971).
5. O. S. Heavens, ‘Optical Properties of Thin Solid Films’, Butter-worths, London (1955) p. 55.