The Differential Saturation Mechanism: an Application of the Zeeman Effect to the Diagnostic of Magnetic Fields

Abstract
A review of broad-band polarimetric observations is presented, from the first systematic observations of Jean-Louis Leroy in sunpots and active regions to the more recent applications concerning the polarization diagrams of chemically peculiar stars. The historical evolution of the interpretation is also discussed, with particular emphasis on the differential saturation mechanism and on its physical meaning.