  A free field representation for the type $I$ vertex operators and the corner
transfer matrices of the eight-vertex model is proposed. The construction uses
the vertex-face correspondence, which makes it possible to express correlation
functions of the eight-vertex model in terms of correlation functions of the
SOS model with a nonlocal insertion. This new nonlocal insertion admits of a
free field representation in terms of Lukyanov's screening operator. The
spectrum of the corner transfer matrix and the Baxter--Kelland formula for the
average staggered polarization have been reproduced.
