1. A method of precision calibration of magnification of a scanning microscope with the use of test diffraction grating, comprising the steps of positioning and orientation of a test object on a microscope stage so that strips of a test diffraction grating are perpendicular to a direction on 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 coordinates X and Y in a plane of scanning and storing 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 grating and a direction of a new axis V coincides with the strips of the grating; line-by-line mathematical processing of the array S(u,v) including: separation from it of a one-dimensional array-line S(u) which contains a profile of an image of periodically repeating strips of a test-object; multiplication of the line S(u) by shifting of its copy relative to an original by an integer number of periods, adjustment of a value of the shift for example by the method of least squares in accordance with a criterion of the best coincidence of the overlapping portions of the original and the copy of the line S(u), calculation of average values S(u) in the zone of overlapping as a semisum of values of the original and copy S(u) in each point of the zone; transformation of the multiplied array S(u) into an array P(w) in accordance with the formula 6 P &af; ( w ) = &Sum; j = 1 N / 2 &it; &it; [ S &af; ( j ) * S &af; ( j + w ) ] 1, w 2, w 3, w 4... of successive maximums of the function P(w) wherein w 4 >w 3 >w 2 >w 1 >0; determination of an average period T′ of the test grating in pixels for the selected line S(u) in accordance with the formula 7 T ′ = 1 n - 1 &it; &Sum; i = 1 n - 1 &it; &it; ( w i + 1 - w i ) av for all lines and calculating a magnification M u along the selected direction u in accordance with the formula 8 M u = T av &CenterDot; L T o &CenterDot; N 0 is an independently attested value of a pitch of the test object, N is a number of pixels on a line along the direction u.