1. A. Kastler:Journ. Phys.,11, 255 (1950).
2. A. Gozzini:Compt. Rend.,255, 1905 (1962);A. Gozzini:Proc. III Conf. Quant. Elect., (1963), p. 275.
3. G. Fornaca, A. Gozzini andF. Strumia:Proc. XII Coll. AMPERE (1963), p. 554.
4. Glass obtained from commercial lamps has been used. A special type of glass has also been used, proided by Dr.J. Kroon of Philips (Eindhoven), to whom we are indebted.
5. See for example:G. Bruhat:Optique, VI éd., cap. XXXVI (Paris, 1965).