The focusing effect of a rotationally symmetrical magnetic field on the trajectory of electrons is the result of the complicated influence of a magnetic field on a moving electron. This effect is a force proportional to the induction "B" of the field and electron velocity "v". The vector product of the radial component of induction "Br" and axial component of velocity "va" is a force perpendicular to those vectors, causing the electron to move around the axis. Additional effect of this motion in the same magnetic field is another force "F" oriented radially to the axis, which is responsible for the focusing effect of the magnetic lens. The resulting trajectory of electrons in the magnetic lens is a curve similar to a helix. In this context it should be mentioned that variations of focal length (exciting current) cause a slight rotation of the beam cross-section.