  We present new results on gravitational lensing over a cosmological
Robertson-Walker backgrounds which extend and generalize previous works. Our
expressions show the presence of new terms and factors which have been
neglected in the literature on the subject. The new equations derived here for
the optcal scalars allow to deal with more general matter content including
sources with non Newtonian components of the energy-momentum tensor and
arbitrary motion.
  Our treatment is within the framework of weak gravitational lenses in which
first order effects of the curvature are considered. We have been able to make
all calculations without referring to the concept of deviation angle. This in
turn, makes the presentation shorter but also allows for the consideration of
global effects on the Robertson-Walker background that have been neglected in
the literature.
  We also discuss two intensity magnification, that we define in this article;
one coming from a natural geometrical contruction in terms of the affine
distance, that here we call $\tilde{\mu}$, and the other adapted to
cosmological discussions in terms of the redshift, that we call $\mu'$. We show
that the natural intensity magnification $\tilde{\mu}$ coincides with the
standard solid angle magnification ($\mu$).
