Quadrupole Lens
The quadrupole lens consists of two single quadrupoles turned 90° with respect to each other. Let z be the optical axis then one can deduce separately for the x and the y axis that the refractive power is again the square of the refractive power of a single lens.
A magnetic quadrupole works very similar to an electric quadrupole. But the Lorentz force increases with the velocity of the charged particle. In spirit of a wien filter a combined magnetic, electric quadrupole is achromatic around a given velocity. Bohr and Pauli claim that this lens leads to aberration when applied to ions with spin (in the sense of chromatic aberration), but not when applied to electrons, which also have a spin. See Stern-Gerlach experiment.
Read more about this topic: Electrostatic Lens