On Milne-Barbier-Uns\"old relationships

Abstract

This short review aims to clarify upon the origins of the so-called Eddington-Barbier relationships, which relate the emergent specific intensity and the flux to the photospheric source function at specific optical depths. Here we discuss about the assumptions behind the original derivation of Barbier (1943). We also point to the fact that Milne had already formulated these two relations in 1921.