1. Standard texts are:M. Born andE. Wolf:Principles of Optics (Pergamon, New York, N.Y., 1959), Sect. 4.2.2;O. Stavroudis:The Optics of Rays, Wavefronts, and Caustics (Academic, New York, N.Y., 1959);R. K. Luneburg:Mathematical Theory of Optics (University of California, Berkeley, Cal., 1964), Sect. 28.
2. D. T. Moore:Appl. Opt.,19, 1035 (1980).
3. A. Frank, F. Leyvraz andK. B. Wolf:J. Math. Phys.,31, 2757 (1990);Yu. N Demkov andV. N. Ostrovskii:Ž. Ėksp. Teor. Fiz.,60, 2011 (1971) (Sov. Phys. JETP,33, 1083 (1971)).
4. A. C. Chen:Am. J. Phys.,46, 214 (1978);H. A. Buchdahl:Am. J. Phys.,46, 840 (1978).
5. B. Ratra:Phys. Rev. D,31, 1931 (1985).