(iii) Screen (for viewing) and (iv) Vacuum pump The electron generator is the source of illumination.
Also called electron gun, it is a tungsten filament, which when heated by electric current emits a stream of electrons. The electrons accelerated by the high voltage energy are forced through a collimating aperture which renders the rays in parallel lines and fashions them into a beam. From there the beam passes through the condenser. The condenser ‘lens’ is a magnetic coil which corrects the aberrations (bending) in the beam and directs them towards the object the strength of the magnetic lens depends on the amount of current that is allowed to flow through it. Greater the flow of current, greater will be the strength of the magnetic field and greater will be the bending effect on the rays of electron. In other words, by varying the magnetic field strength, the beam can be properly focused on to the object. The specimen to be observed must be exceedingly thin (at least 200 times thinner than the ones used for routine optical observation) and must be supported on an equally exceedingly thin but strong film or metallic grid. As the electron microscope always functions only in a vacuum, the specimen must be absolutely dry.
Specimen supporting grid is usually a collusion film or Fonda or polymerized plastic plus a screen grid of copper having 200 meshes to an inch. The specimen is kept below the condenser and the electron beam passes through it and is scattered depending on the varying refractive index of the specimen. From there, the electron beam (coming out of the specimen) passes through the second set of magnetic coils (objective lens) which focus the electrons and form an intermediate image.
A third set of magnetic lenses (equivalent to the oculars) – the projection lenses, produce the final images the final image however cannot be directly viewed as the rays of electrons are harmful to the eye. They are either projected on to a fluorescent screen for viewing or allowed to expose a photographic plate kept for that purpose. The entire electron microscope must be in a vacuum or otherwise the electrons get scattered due to collisions with air molecules and fail to get focused. Hence the equipment unit is housed in a single unit which is continuously evacuated with a high speed vacuum pump.