A transmission electron microscope resembles a standard light microscope, except that an electron beam rather than light rays are used for illumination. The system is inverted with respect to a standard microscope. An Electron Gun at the top of the apparatus generates a beam of electrons, which are focused through two electro-magnetic fields that correspond essentially to coarse and fine focusing knobs (in early models, the first focusing ring had literally to be moved by hand because of the weight of the magnets. The interior path [grey] is a high-vacuum system to avoid air molecules that would de-focus the beam.

    An extremely thin sample, typically embedded in plastic resin and stained with metallic stains that will pass the electron beam differentially according to the material. The sample is placed on a thin metallic grid, load into a sample holder, and inserted into the electron beam through the sample port in two stages so as to introduce minimal air into the vacuum.

    The image is projected onto a fluorescent imaging screen. The image can be viewed through a binary optical microscope, which has about the same magnification as a dissecting scope. Images are typically visualized with a CCD camera (as in a modern digital camera), or formally with a film camera. (The imaging screen is retracted during photography).

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