A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are used mainly for surveying applications, and have been adapted for specialized purposes in fields like metrology and rocket launch technology.:
This apparatus is listed in the 1885 catalogue of Jules Duboscq of Paris as the “Grand Circle of MM. Jamin et Sénnarmont.” It was designed for the study of the laws of polarized light reflected from crystalline substances, liquids and metals. It could also be used to measure indices of refraction. With its accessories, it cost 1,000 francs (about $200). The circle is an early example of an ellipsometer, used to study the reflection of light at or near Brewster's angle.