Abstract: The lens is formed by the association of two electromagnetic lenses sharing one and the same magnetic circuit crossed by one and the same central channel. The first lens is a shielded lens and has an annular gap at one of its ends. This annular gap cuts the central channel along a direction the plane of which is perpendicular to the optical axis. The second lens is a single-pole lens surrounding the annular gap of the first lens. It has an annular gap demarcated by two lips, the walls of which have shapes generated by revolution around the optical axis. Induction coils enable the magnetic flux of the gaps to be made to vary.

Abstract: The electronic optical apparatus provided is such that the parts situated in the immediate vicinity of the beam, anode, screen placed inside the condenser block, screen formed from a tube and limiter diaphragms inside the deflection block, screen placed between the final focussing lens and the sample to be observed and the secondary electron collector electrode have their faces exposed to the electron beam formed from pyrolytic graphite.