Editing Petrography (section)

==History==
Petrography as a science began in 1828 when Scottish physicist [[William Nicol (geologist)|William Nicol]] invented the technique for producing [[polarized light]] by cutting a crystal of [[Iceland spar]], a variety of [[calcite]], into a special prism which became known as the [[Nicol prism]]. The addition of two such prisms to the ordinary microscope converted the instrument into a polarizing, or [[petrographic microscope]]. Using transmitted light and Nicol prisms, it was possible to determine the internal crystallographic character of very tiny mineral grains, greatly advancing the knowledge of a rock's constituents.

During the 1840s, a development by [[Henry Clifton Sorby|Henry C. Sorby]] and others firmly laid the foundation of petrography. This was a technique to study very thin slices of rock. A slice of rock was affixed to a microscope slide and then ground so thin that light could be transmitted through mineral grains that otherwise appeared opaque. The position of adjoining grains was not disturbed, thus permitting analysis of [[rock texture]]. [[Thin section]] petrography became the standard method of rock study. Since textural details contribute greatly to knowledge of the sequence of crystallization of the various mineral constituents in a rock, petrography progressed into petrogenesis and ultimately into petrology.

Petrography principally advanced in Germany in the latter 19th century.