1. Albrecht, R., Maitzen, H. M., and Rakos, K. D.: 1969, Astron. Astrophys. 3, 236.
2. Arvesen, J. C., Griffin, R. N., and Pearson, B. D.: 1969, Appl. Opt. 8, 2215.
3. Barber, C. R.: 1969, Nature 222, 929.
4. Comit� International des Poids et Mesures: 1969, Metrologica 5, 35.
5. Condon, E. U.: 1958, in Handbook of Physics (ed. by E. U. Condon and H. Odishaw), McGraw-Hill Book Comp. New York, Toronto, London, p. 5?13.