  The fundamental quadrature governing light rays in a spherically symmetrical
medium is first recalled. A rigorous discussion of some qualitative properties
of its solutions follows, using the Young-Kattawar diagram which leads to a
geometric formulation of the ray curvature. The case of an optical duct is
deepened, analyzing transfer curves for different positions of the observer
with respect to the duct. New analytical expressions for their wavelength
dependence are derived, and their numerical consequences are coherent with
computer simulations.
