GB705867A - - Google Patents

Abstract

705,867. Electron microscopes. SIEMENS & HALSKE AKT.-GES. Aug, 31, 1951 [Sept. 2, 1950], No. 20617/51. Class 39(1) [Also in Group XXXV] An electron microscope apparatus comprises four image forming magnetic lenses, arranged in order along the electron optical axis, wherein the two centre lenses have associated therewith a common regulating element carrying the two lens forming parts, this element being displaceable backwards and forwards axially for regulating the magnification. In one form, Fig. 1, objective and projection lenses 1, 4 are formed by pole pieces 5, 6; 7, 8 energized by cylindrical permanent magnets 12, 13 and the two intermediate lenses 2, 3 are formed at constrictions of magnetic member 20. The latter member is axially adjustable in bores in pole pieces 6, 7 by means of a driving member such as rack and pinion gearing 26, 25 housed in magnetic disc 16. Flux is fed to the central portion of member 20 and so to lenses 2, 3 via magnetic covers 9, 10 with which pole pieces 5, 8 may or may not be integral, magnetic shell 11 and a rotary regulator ' consisting of annular toothed members the inner one secured to disc 16 being fixed and the outer one being in contact with shell 11 but rotatable therein. Axial adjustment of 20 varies the final image magnification and adjustment of the regulator varies the focusing. Non-magnetic rings 14, 15 space the regulator and magnetic disc 16 from pole pieces 6, 7. In a modification the two centre lenses are constructed with separate pole pieces spaced by non-magnetic rings. The means for adjusting the focusing and magnification may each have a scale for indicating the magnification and by suitable coupling of the two adjusing means the magnification may be varied while keeping the image in focus. Alternatively the two adjusting means may be independently set to the same calibrated magnification. An auxiliary fine focusing means may be provided for exact focusing adjustment. In another modification, Fig. 3 (not shown), non-magnetic rings 14, 15 are replaced by permanent magnets, making four permanent magnets in all, and the rotary regulator is replaced by a member parallel to the axis which is movable to and fro transversely to the axis. In another form, Fig. 4, the magnet system comprises four permanent magnets 50 which may be variably short circuited by magnetic members 54, 55 pivoting on pins 52, 53 so as to allow for working with different accelerating potentials. This system may be detached from yoke 51 and withdrawn or introduced through one end of magnetic casing 41 by removing one of the end covers 42. 43 is the lower pole piece of the objective, 44, 45 the pole pieces of the projection lens, and 47 the axially movable magnetic body. Regulator 48 pivots about axis 49 to adjust the focusing. Prior art devices comprising three or four lenses in electron microscopes are referred to. In one such device three electromagnetic lenses are employed, the first or objective lens being adjustable for focusing, the centre lens being adjustable for varying the magnification and the last or projection lens being fixed.