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.
The instrument proposed in 1822 by Andreas Ritter von Baumgartner (1793-1865) was a hybrid between a contact and a reflecting goniometer.
Surprisingly, this very early reflecting goniometer survived; it has a semicircular graduated circle [ILLUSTRATION FOR FIGURE 27 OMITTED], although it is not so shown in Biot's drawing (1821).
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.
Yet in the literature a differentiation of types of the reflecting goniometer according to the position of the graduated circle exists, the types being named after the respective designers:
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 (
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.
He claimed to have invented the principle of the reflecting goniometer as early as 1807, two years before Wollaston, and this is confirmed by Alexander Tilloch in his Philosophical Magazine (1807), in a sentence under "List of patents and inventions": "M.