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XVI. Description of a reflective Goniometer. By William Hyde 
Wollaston, M. D. Sec. R. S . 
Read June 8, 1809. 
From the advances that have been made of late years in 
crystallography, a very large proportion of mineral substances 
may now be recognised, if we can ascertain the angular di- 
mensions of their external forms, or the relative position of 
those surfaces that are exposed by fracture. But though the 
modifications of tetrahedrons, of cubes, and of those other re- 
gular solids, to which the adventitious aid of geometry could 
be correctly applied, have been determined with the utmost 
precision, yet it has been often a subject of regret, that our 
intruments for measuring the angles of crystals are not pos- 
sessed of equal accuracy, and that in applying the goniometer 
to small crystals, where the radius in contact with the surface 
is necessarily very short, the measures, even when taken with 
a steady hand, will often deviate too much jfron* the truth to 
aid us in determining the species to which a substance be- 
longs. 
A means of remedying this defect has lately occurred to 
me, by which in most cases the inclination of surfaces may 
be measured as exactly as is wanted for common purposes, 
and when the surfaces are sufficiently smooth to reflect a dis- 
tinct image of distant objects, the position of faces only -- of 
mdcccix. L 1 
