The expressions allowing one to present the logarithm of an index of refraction in two-dimensional non-uniform environment as a sum of harmonic and non-harmonic functions are given. By the example of a three-dimensional wedge, in which the calculation of the field of a directed source results in a system of two-dimensional Helmholtz' equations, it is shown that non-harmonic components of the refraction index logarithms in these equations remain negligible in a wide enough range of wedge opening corners, which permits one to obtain solutions in the form of geometric-optical series.