A test used to determine the approximate refractive index of a mineral by immersion in a liquid of known refractive index. A card is inserted below the rotating stage to cut out half the light. The condenser is then adjusted to focus or blur the card. If the mineral differs in refractive index from the liquid, one side of the grain will have a distinct shadow. If the shadow is on the same side as the card the mineral has a higher refractive index than the liquid; if it is on the opposite side of the grain the mineral has a lower refractive index than the liquid.