. The electron microscope, its development, present performance and future possibilities . FiG. 5. Chromatic aberration scopes in which the field is seldom more than a few degrees of arc. The coma is illustrated in figure 6. A lens, corrected for spherical aberration but not corrected for coma, images extra- axial points in the shape of an arrowhead pointing toward the axis. The point of the arrow corresponds to the ray which passes through the center of the lens; all other rays strike the screen at a greater distance from the axis. Rays passing through coaxial circles of the lens strike the s
. The electron microscope, its development, present performance and future possibilities . FiG. 5. Chromatic aberration scopes in which the field is seldom more than a few degrees of arc. The coma is illustrated in figure 6. A lens, corrected for spherical aberration but not corrected for coma, images extra- axial points in the shape of an arrowhead pointing toward the axis. The point of the arrow corresponds to the ray which passes through the center of the lens; all other rays strike the screen at a greater distance from the axis. Rays passing through coaxial circles of the lens strike the screen again in circles; these are, however, not concentric, but fit between two straight lines which pass through the point A' and form an angle of 60°. This is the characteristic coma figure which increases linearly with the distance from the axis. It is possible to correct electron lenses for coma alone, but this is of little interest as long as the spherical aberration cannot
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