  The factorization condition for the scattering amplitudes of an integrable
model with a line of defect gives rise to a set of Reflection-Transmission
equations. The solutions of these equations in the case of diagonal $S$-matrix
in the bulk are only those with $S =\pm 1$. The choice $S=-1$ corresponds to
the Ising model. We compute the transmission and reflection amplitudes relative
to the interaction of the Majorana fermion with the defect and we discuss their
relevant features.
