1. A method of precision calibration of magnification of a scanning microscope with the use of a test diffraction grating, comprising the steps of positioning and orientation of a test object on stage of microscope so that strips of a test diffraction grating are perpendicular to a direction along which a calibration is performed; scanning of a selected portion of the test object along axes X and Y; measuring values of a signal S versus on coordinates x and y in a plane of scanning and storing of said values S (x, y) in a digital form as a two-dimensional digital array; transforming the two-dimensional array of signals S(x, y) into a two-dimensional array S (u, v) by turning of the axes so that a direction of a new axis u is perpendicular to the strips of the grate and a direction of a new axis v coincides with the strips of the grate; line-by-line mathematical processing of the array S(u,v) for each line S(u) by calculating of a Fourier spectrum of the line SP(&ohgr;) in correspondence with the formula 8 SP &af; ( ω ) = 1 2 - &it; &Integral; - ∞ ∞ &it; S &af; ( u ) &CenterDot; exp &it; &it; ( iu &it; &it; ω ) &it; &it; &dd; u, wherein &ohgr; is a coordinate in a reciprocal space which represents a space frequency; SP (&ohgr;) is a complex spectrum density which corresponds to the space frequency &ohgr;; S(u) is a function which describes a one-dimensional profile of a signal; and i&equals;{square root}−1 is an imaginary unit; converting of the one dimensional, complex function SP (&ohgr;) into a one-dimension spectrum of real values of a module &verbar;SP(&ohgr;)&verbar; by multiplying of each value SP (&ohgr;) by a complex-conjugate value; finding from the spectrum of the real values T h a greatest spectral maximum and determining its characteristic frequency &ohgr; h as an abscissa of a point with the maximum value &verbar;SP&verbar;; calculating an average value of a pitch T h of the diffraction grating in accordance with the formula: 9 T h = 1 W h performing the mathematical processing, the conversion, the determination and the calculation for subsequent lines S(u) with a new value of a coordinate v; statistically processing the thusly obtained set of values T h for all lines and determining an average value T and a standard deviation &Dgr;T over a whole frame; and determining a magnification M u in accordance with the selected direction u in accordance with the formula: 10 M u = T &CenterDot; L T 0 &CenterDot; N, where L is a width of a medium of image in direction of the calibration, T 0 is an independently obtained value of the pitch of the test object, and N is a number of pixels in the line along the direction u.