Relief is essentially the extent to which a mineral stands out with respect to its surroundings (i.e., adjacent minerals, enclosing glass or immersion oil).  This is a function of the relative refractive indices (n) of the two media.  (In some texts, relief has an absolute connotation, as it a comparison of refractive index of the mineral to that of the mounting medium, such as Canada Balsam, with n = 1.54.  Here, it is used as a comparison of a mineral and its surroundings, whatever they are.)  If a mineral has a lower refractive index than its surroundings, it is said to have negative relief, and if the mineral’s refractive index is higher that the enclosing medium, the mineral has positive relief.  If the difference in the two refractive indices is small, it does not show up well in the enclosing material, and the mineral is said to have low relief (positive or negative).  With progressively larger differences, the relief is termed moderate, high, very high or extreme.   These are qualitative assessments.  In A, grains of pyrope-rich garnet (n ~ 1.75) with conchoidal fracture stand out in sharp contrast to enclosing epoxy (n ~ 1.54) and exhibit high relief.  In B, grains of halite (n = 1.544), with traces of cubic cleavage, have low relief relative to the enclosing immersion oil (n = 1.560).  Using the Becke line method, one can determine whether the relief is positive or negative, that is, which phase has the higher refractive index (or indices).  This would be useful information in a situation in which the identity of one or both of the minerals was uncertain.  In C, the stage has been lowered, and there is an increased brightness around the outside of the edges, which is referred to as the Becke line.  Because the grains act as crude lenses, and refract the light towards the medium with the higher refractive index, the light has been bent outwards by the grains, and focussing at a plane above the mineral makes the exterior look brighter.  In D, the stage has been raised above focus (the distance between the grains and the objective has been decreased from “in focus”), and the bright regions are inside of the grain boundaries.  The Becke line behaviour indicates that the halite has a lower refractive index than its surroundings.