In polarized-light microscopy, a method or test for determining relative indices of refraction between two adjacent mineral grains or between a mineral grain and its host medium; e.g., Canada balsam, Lakeside cement, an epoxy resin, or an immersion oil of known index of refraction. On defocusing by increasing the working distance between 274 the microscope stage and the objective lens, the Becke line moves toward the higher index of refraction. Compare: van der Kolk method