A test used in microscopy to determine the index of refraction of a mineral
grain relative to that of an immersion liquid. When transmitted light is
blocked, the grain acts as a lens and the ocular inverts the image of the
obstacle, causing a shadow to appear on the same side as the obstacle when the
grain has the higher refractive index, but on the opposite side when the grain
has the lower index. Syn: oblique illumination method
Source:
Dictionary of Mining, Mineral, and Related Terms
