. The electron microscope, its development, present performance and future possibilities. Electron microscopes. Geometrical Electron Optics 11 transversal spherical aberration. In fact, the bundle has its smallest cross section not at p but at m, which is called the disc of minimum confusion. Its radius is one quarter of the spherical aberration. When an electron lens is focused, it is always m, not p, which is made to coincide with the screen or plate. Scherzer ^ has proved the important theorem that in electron lenses, whether electrostatic, magnetic, or combined, the spherical. Fig. 4. Sphe
. The electron microscope, its development, present performance and future possibilities. Electron microscopes. Geometrical Electron Optics 11 transversal spherical aberration. In fact, the bundle has its smallest cross section not at p but at m, which is called the disc of minimum confusion. Its radius is one quarter of the spherical aberration. When an electron lens is focused, it is always m, not p, which is made to coincide with the screen or plate. Scherzer ^ has proved the important theorem that in electron lenses, whether electrostatic, magnetic, or combined, the spherical. Fig. 4. Spherical aberration aberration can never be eliminated as long as there are no space charges or currents in the space traversed by electrons. All lenses used to date in electron optics fall into this class. The spherical aberration has always the sign as shown in figure 4, , the strength of the lens increases with the angle a. In optics this is called an undercorrected lens. Corrected electron lenses are impossible. Scherzer has proved also that the same fundamental difficulty exists in the case of chromatic aberration which is illustrated in figure 5. The faster electron (dotted line) will intersect the axis always beyond the slower electron. This defect cannot be cor- rected, because dispersing electron lenses cannot be realized with the means employed in present-day electron optics. We have proved this in the case of magnetic lenses. In electrostatic lenses, it is possible to realize a slight dispersing eiifect in a part of the field, but this is always more than compensated by the condensing efifect. Quantitative examples will be given later. The spherical and the chromatic aberrations are the only lens errors which exist at axial and at extra-axial points of the image field. The other defects—coma, astigmatism, image curva- ture, and distortion—are zero at the axis, and manifest them-. Please note that these images are extracted from scanned page images that may have
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