The Becke Line Test is a technique in optical mineralogy that helps determine the relative refractive index of two materials. It is done by lowering the stage (increasing the focal distance) of the petrographic microscope and observing which direction the light appears to 'move' toward. This movement will always go into the material of higher refractive index. Typically, this is done by comparing 1) two (or more) minerals, 2) a mineral versus thin section epoxy (such as Canada Balsam which has a moderate refractive index of 1.54), and/or 3) a mineral versus an oil of known refractive index (as in Oil immersion studies).
The method was developed by Friedrich Johann Karl Becke (1855-1931) .[1]