In transmitted-light microscopy, a comparative test used to determine the approximate refractive index of a mineral. In plane-polarized light, when the substage iris diaphragm is partly closed to accentuate grain boundaries, a thin line of light, the ‘Becke white line’, appears. If the microscope tube is then racked up, or the microscope stage racked down, the light line will move into the medium with the higher refractive index. The medium could be either an adjacent mineral of known refractive index or, more commonly, the mounting medium, usually a cold-setting resin or Canada balsam (refractive index 1.54).