The general principles of scanning electron microscopy

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Abstract
In the transmission electron microscope, as shown in figure 1 (Nixon 1962), the electron gun at the top illuminates the specimen with the beam angle controlled by the condenser lens. The lenses below the specimen are used to magnify the image of the specimen which is viewed on the final screen at many thousand times magnification. If a second electron gun is placed below the fluorescent screen at the bottom of the column and the electron beam is projected upwards through the same lenses this second electron source will be reduced in size by the same amount that the specimen image is magnified. This effect can be observed with both electron guns on at the same time and demonstrates the reversibility of rays through electron lens systems. In this way the resolved point in the specimen image on the fluorescent screen or on the photographic plate is equal to the focused electron probe in the plane of the specimen.

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