The Becke Line Test is a technique in optical mineralogy

Optical mineralogy

Optical mineralogy is the study of minerals and rocks by measuring their optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope...

 that helps determine the relative refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 of two materials. It is done by lowering the stage (increasing the focal distance) of the petrographic microscope

Petrographic microscope

A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks...

 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

Thin section

In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...

 epoxy

Epoxy

Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

 (such as Canada Balsam

Canada balsam

Canada balsam, also called Canada turpentine or balsam of fir, is a turpentine which is made from the resin of the balsam fir tree of boreal North America...

 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

Oil immersion

In light microscopy, oil immersion is a technique used to increase the resolution of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens.Immersion oils are...

 studies).

The method was developed by Friedrich Johann Karl Becke

Friedrich Johann Karl Becke

Friedrich Johann Karl Becke was an Austrian mineralogist and petrograph.- Biography :After studying at the University of Vienna, where he specialized in the natural sciences, he became there a lecturer on geology. In 1882 he was appointed professor at the University of Czernowitz...

(1855-1931)
.