The propagation of X-ray waves through an optical system consisting of 33 aluminum X-ray refractive lenses
is considered. For solving the problem, a finite-difference method is suggested and investigated. It is shown
that very small steps of the difference grid are necessary for reliable computation of propagation of X-ray waves
through the system of lenses. It is shown that the wave phase is a function very quickly increasing with distance
from an optical axis, after the wave has propagated through the system of lenses. If the phase is a quickly
increasing function, then the wave electric field is a quickly oscillating function. We suggest and recommend
using the equation for a phase function instead of the equation for an electric field. The equation for a phase
function is derived.