Here the first magnetic lens is shown cut in half.
An electro-magnet is wound in a coil (indicated in pink). This is embedded inside of an iron ring. Because iron tends to draw in magnetic fields, the shape of the iron ring can be designed to optimize the form of the magnetic field within the core of the lens.
The goal is to design a field that will collect almost all of the electrons and bend them towards a single point below the lens.
