| The study of polarized light is divided into two eras.
In the last third of the 17th century double refraction by calcite was
discovered, and the longitudinal theory of light applied to the phenomenon.
In 1808 the modern era began, with the discovery of polarization
by reflection. After a false start the transverse theory of light was applied
to the observations. The subject of polarized light, including mechanical
demonstrations of rather esoteric topics such as circular polarization,
made its way into the curriculum of the Natural Philosophy course, and
a number of instruments were designed to demonstrate and utilize its properties.
At the right is a plate showing the consequence of viewing
certain various crystals between crossed polarizers. This appeared in most
editions of Ganot's Physics up to the early 20th century.