1. Bergmann P. G. 1942 Introduction to the theory of relativity. New York: Prentice Hall Inc.
2. R.C;Fermi E.;Accad. Lincei,1922
3. Landau L. & Lifshitz E. 1951 The classical theory offields. Cambridge Mass.:Addison Wesley Press Inc.
4. Moller C. 1952 The theory of relativity. Oxford: Clarendon Press.
5. Helv. phys;Pirani F. A. E.;Acta Suppl.,1956