Goniometer is an instrument that measures angles and allows for objects to be rotated to a particular angular position. In 1669, Niels Stensen identified a very important characteristic of crystals which stated that the interplanar angles of the quartz crystals are constant. This characteristic was later determined to hold for all crystals (not just quartz), according to Virtual Museum of History of Mineralogy (mineralogy.be), and formalized into Steno's Law by Jean Baptist Louis de Rome de l'Isle in the mid-1700's.
Crystal
Early History
In 1780, Arnould Carrangeot developed a primitive instrument to measure angles between the faces of the crystals during the process of making models of crystals for l'Isle's collection. This instrument was the first of its kind and came to be known as a contact or an application goniometer. It consisted of two sections connected at the joint, according to mineralogy.be. The two sections, or limbs, were applied to adjacent crystal faces and the angle between them was measured. The approximate accuracy of this primitive goniometer was around 15 minutes.
Optical Goniometer
In 1809, William Hyde Wollaston, an English chemist, invented the optical goniometer. This type of goniometer uses light reflection. The crystal is attached on a rotatable horizontal axis which is connected to a graduated (divided) circle, according to mineralogy.be. When the circle rotates, the second surface reflects the light ray in the same direction and the angle between the faces is determined by the difference between the reading of the graduated circle.
Features
The optical goniometer has an accuracy of about 5 degrees. Soon after, a mirror was added to this type of goniometer in order to improve the positioning of the light reflection. This feature improved accuracy from 5 minutes to 1 minute.
Vertical and Horizontal Goniometers
Vertical circle goniometers were introduced in 1843 by Eilhard Mitscherlich. This name comes from the fact that it is a high precision goniometer with a vertical circle and a telescope attached to its base, according to mineralogy.be. Horizontal circle goniometers were introduced earlier in 1839 by Jacques Babinet and they consisted of two attached telescopes. They had an accuracy of 30 seconds and were actually an improvement on the vertical goniometer.
Modern Goniometers
A modern goniometer was first invented by W.H. Miller in 1874 and it consisted of two circles, instead of one. Two circles allowed for further measurements without the remounting of the crystal, according to mineralogy.be. Further developments included the three-circle instrument in the late 1800's, but these were used rarely because they were too expensive and complicated to adjust. In fact, all goniometers fell out of fashion soon after 1900 as a result of X-ray diffraction. Most goniometers are now considered collector's items and are typically only found in museums and special collections.
Tags: according mineralogy, angle between, between faces, circle goniometers, circle goniometers were
