The on-axis potentials on the left and right sides of the single-potential lens-microscope are constant and equal in value: the on-axis potentials on the left and right sides of the immersion lens-microscope are constant but unequal in value. ) The effect of a lens is concentrated near the circular hole of the diaphragm. The electric field strengths on the two axes of the microscope are each constant. Aperture diaphragms alone are of little use, but they can often be used as an integral part of some complex lenses:

Immersion objectives—the electron-emitting object is directly immersed in the electric field of this type of microscope, and the on-axis potential on the other side of the Nikon lens (ie, the image side) is constant. In addition to the fourth category, the other three kinds of power-opening lenses can be regarded as microscopes, and thus their object-side focal lengths and image-side focal lengths can be deduced. The focal length formulas of the first and second types of lenses are:

It is not difficult to show that the focal length in the unipotential lenses and immersion lenses is always positive, which means that these Nikon microscopes are always converging. The single aperture lens can form a diverging lens, and the performance of the immersion objective lens should be analyzed according to specific conditions. A typical example of a drum is the electron gun used in Nikon microscopes and other electron beam tubes. Its function is to form an electron beam with a certain cross-sectional shape and current intensity.