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
at the top of the apparatus generates a beam
of electrons, which are focused through two electro-magnetic
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
in two stages so as to introduce minimal air into the
The image is projected onto a fluorescent imaging
. 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).