But the invention of the reflecting goniometer by W.
Dufrenoy mentions that the straightedge contact must be removed in order to use the instrument as a reflecting goniometer. This principle will be explained later.
The instrument proposed in 1822 by Andreas Ritter von Baumgartner (1793-1865) was a hybrid between a contact and a reflecting goniometer. Baumgartner was director of the royal porcelain factory, and from 1851 president of the Viennese Academy of Sciences.
As early as 1811 the French instrument maker Robert Agalae Cauchoix (1776-1845) constructed a similar reflecting goniometer. According to Biot (1811), this is "anterieur au goniometre de M.
The measurement of angles with a reflecting goniometer is based on the physical laws of the reflection of light from a plane mirror with equal angles of incidence and reflection: the incident light is reflected from smooth crystal faces.
What an important influence may be exercised over the progress of a single branch of science by the invention of a ready and convenient mode of executing a definite measurement, and the construction and common introduction of an instrument adapted for it, cannot be better exemplified than by the instance of the reflecting goniometer! This simple, cheap and portable little instrument has changed the face of mineralogy and given it all the characters of one of the exact sciences.
It appears, however, that no early form of the reflecting goniometer was available, since in 1809 Wollaston commented on his new invention: "A means of remedying this defect has lately (!) occurred to me." It is astonishing that Wollaston, with his reflective goniometer, ascertained an angle for the calcite rhombohedron which was less exact than his measurement of seven years earlier: "very nearly, if not accurately 105 [degrees]." Wollaston attacked Hauy's doctrine directly without referring to him by name:
This indisputable difference of over half a degree from Hauy's theoretical value was one of the first cracks in the edifice of the ideas of Hauy, who, by the way, declined to use the reflecting goniometer.
The reflecting goniometer in its original form appears to have been manufactured predominantly by English makers.
Dufrenoy (1856) refers to a horizontal-circle reflecting goniometer without accessories named after the mineralogist Friedrich Mohs (17731839), but Dufrenoy offers no proof or citation The author does not know of any example of such a Mohs goniometer.
The truth appears to be that in 1809 the reflecting goniometer was built independently by Wollaston and Brewster.
As already discussed in the section reviewing the elementary principles of measuring with a reflecting goniometer, accurate measurements depend on a constant, unchanged position of the observer's eye, as well as on the parallelism of the graduated circle plane and that of the imaginary plane formed by the signal, the reflecting crystal face and the observer's eye.