  We put forward the physical meaning of the conservation equation for the
helicity on scattering of an electromagnetic field with a generally
magnetodielectric bi-isotropic dipolar object. This is the optical theorem for
the helicity that, as we find, plays a role for this quantity analogous to that
of the optical thorem for energy. We discuss its consequences for helicity
transfer between molecules and for new detection procedures of circular
dichroism based on ellipsometric measurements.
