In a previous paper, denoted I [C. Duprez and J. Tillieu, Int. J. Quantum Chem. 17, 155 (1980)], a global tensor, invariant in an origin change, has been introduced for all optical phenomena. The results obtained have been applied to two particular cases: refraction and the Faraday effect, in Paper II [C. Duprez, Int. J. Quantum Chem. 17, 169 (1980)]. In this paper, we study the various methods suitable for the calculation of these tensors. Then, we compare the results obtained in II with those previously obtained by other authors who, when studying the two particular cases mentioned above, omitted some terms necessary to obtain the invariance.