The electron microscope, its development, The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 10 The Electron Microscope Field linzs Ar = const. / Lines A ^ consC (equivalent poteritial linesj Fig. 3. Magnetic lens This at once explains the lens effect of axially arranged coils. By its definition (3) the vectorpotential A is zero at the axis and increases approximately linearly with the radius r. This means that the equivalent potential decreases outside the axis and that the magnetic field has a repellent effect, , i
The electron microscope, its development, The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 10 The Electron Microscope Field linzs Ar = const. / Lines A ^ consC (equivalent poteritial linesj Fig. 3. Magnetic lens This at once explains the lens effect of axially arranged coils. By its definition (3) the vectorpotential A is zero at the axis and increases approximately linearly with the radius r. This means that the equivalent potential decreases outside the axis and that the magnetic field has a repellent effect, , it drives the elec- trons back, toward the axis. Therefore, a magnetic lens is always a condensing lens. An example is shown in the lower half of figure 3. Increasing equivalent potential is indicated by increas- ing density of shading. Having shown that axially symmetrical fields act as lenses, it remains to investigate whether they are good lenses. Of the numerous defects which lenses can have, only three are of in- terest in microscopy: the spherical aberration, the chromatic aberration, and the coma. Spherical aberration, the most important of the lens defects in electron microscopy, is illustrated in figure 4. An axial point is imaged as a point strictly speaking only while the imaging rays are infinitesimally close to the axis. These are called paraxial rays, and their intersection with the axis is the paraxial or Gaussian image. At larger angles a, the intersection moves away from the Gaussian point by a distance which is called the longitudinal spherical aberration, and which in first approxima- tion is proportional to a^. If a screen p is placed at the Gaussian point, the bundle will intersect it instead of in a point in a disk of a diameter proportional to a^. This diameter is called the
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