. The electron microscope, its development, present performance and future possibilities . Fig. 11. E. Ruska's magnetic electron microscope of 1934 between the windings would be a very slow process, the whole coil is encased in metal. The gap in the iron casing is covered with non-magnetic material, such as brass. The pole pieces are interchangeable. In addition to these features, which recur in practically all modern microscopes, Ruska's design provides water cooling, a complication which was later found unnecessary. Comparing the optical systems of an electron microscope and an optical micro
. The electron microscope, its development, present performance and future possibilities . Fig. 11. E. Ruska's magnetic electron microscope of 1934 between the windings would be a very slow process, the whole coil is encased in metal. The gap in the iron casing is covered with non-magnetic material, such as brass. The pole pieces are interchangeable. In addition to these features, which recur in practically all modern microscopes, Ruska's design provides water cooling, a complication which was later found unnecessary. Comparing the optical systems of an electron microscope and an optical microscope of the projection type, which are shown side by side in figure 10, it can be seen that they are qualitatively identical. There is, however, a striking quantitative difference in the illuminating beams. Optical microscopes w^hich aim at high resolution are fitted with wide-angle illuminator systems. The advantage of wide-angle illumination for a good resolution w-as a result of Abbe's theory of the microscope, and though later investigations proved that this conclusion was really a rather daring extrapolation from the original theory, the result remained true, though it had to be buttressed by another theory. The best result is obtained when about two thirds of the aperture is filled by the illuminating beam, but this condition is not critical. In electron microscopy, however, it appears from figure 10 as well as from practically all the literature of the subject, as if a radically dififerent method had been chosen: parallel illumina-
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