1. A method of precision calibration of magnification of a scanning microscopes with the use of a test diffraction grating, comprising the steps of positioning and orientation of a test object on a stage of microscope so that strips of a test diffraction grating are perpendicular to a direction along which a calibration is performed; scanning 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 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 periodically repeating strips of a test-object; transformation of the array S(u) into an array P(w) in accordance with the formula 7 P &af; ( w ) = &Sum; j = 1 N - w &it; &it; { [ S &af; ( j ) - S &af; ( j + w ) ] 2 } where N is a number of members in the array S(u), determination of coordinates 1, w 2, w 3, w 4... of successive minimums of the function P(w), wherein w 4 >w 3 >w 2 >w 1 >0, and a determination of an average pitch T′ of the test grating in pixels for selected line S(u) in accordance with the formula 8 T ′ = 1 n - 1 &it; &Sum; i = 1 n - 1 &it; &it; ( w i + 1 - w i ), av and calculating a magnification M u in selected direction u in accordance with the formula 9 M u = T av &CenterDot; L T o &CenterDot; N 0 is a value of pitch of the test object attested by an independent method, and N is a number of pixels in a line along the direction u.