  This manuscript deals with the question "How does light couple to an
antiferromagnetic order parameter"? For that we develop a microscopic model
that explains the non-reciprocal optical effects in centrosymmetric Cr_2 O_3.
It is shown that light can couple {\em directly} to the antiferromagnetic order
parameter. This coupling is mediated by the spin-orbit interaction and involves
an interplay between the breaking of inversion symmetry due to the
antiferromagnetic order parameter and the trigonal field contribution to the
ligand field at the Cr^{3+} ion.
