Stationary-phase prescription and specular reflection in high-energy potential scattering: Fermat principle

It is shown that in the Nth-order Born approximation of high-energy potential scattering the stationary-phase prescription in the angular domain chooses those paths obeying the specular reflection at each step of a multiple scattering with an over-all scattering angle θ. This is the generalized Reading-Bassichis theorem, or a restatement in Schr\"odinger theory of the familiar Fermat principle in geometrical optics.