The Goniometer and Measurement of Crystals.
The
fact just stated of the permanence of the angular dimensions of
crystals, shows the importance of some accurate method of measuring
their angles ; that is, the inclination of two faces to each other. Two
instruments have been specialty used for this purpose,—the common or
contact goniometer, invented by Ca-ringeau, and the reflecting
goniometer of Wollaston. The former is simply two brass rulers turning
on a common centre, between which the crystal is so placed that its
faces coincide with the edges of the rulers, and the angle is then
measured on a graduated arc. This instrument is sufficiently accurate
for many purposes and for large crystals; but for precise
determination is far inferior to the reflecting goniometer. This
requires smooth and even faces, but these may be very small, even
the hundredth of an inch, in skilful hands; and as small crystals are generally most perfect, far greater accuracy can
