1. Primarily developments since the following reviews: (a) R.E. Slusher in Progress in Optics XII,E. Wolf, ed. (North-Holland, Amsterdam, 1974), 55-100
2. (b) L. Allen and J.H. Eberly, Optical Resonance and Two-Level Atoms (John Wiley, New York, 1975)
3. (c) I.A. Poluektov, Yu. M. Popov and V.S. Roitberg, Sov. J. Quant. Electron. 4,423 and 719 (1974).
4. S.L. McCall and E.L. Hahn, Phys. Rev. Lett. 18, 908 (1967) and Phys. Rev. 183, 457 (1969).
5. B. Bölger, L. Baede and H.M. Gibbs, Opt. Commun. 18, 67 (1976).