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=±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.Comment: 14 pages, LATEX file, ISAS/EP/94/30 (Figures added, originally missed
for E-mail transmission problem.