Goniometer , (Gr. an angle, and a measure), an instrument for measuring the angles of crystals. Two kinds of goniometers are in use, one designed to measure the angles by direct application of the instrument to the faces of the crystal, and the other by the arc through which the crystal must be turned for two adjoining faces to reflect in succession the same object to the eye. The first and simplest form is the common or Hauy's goniometer. It is a graduated semicircular arc with a fixed and a movable radius, between which the crystal is placed, each radius being made to coincide with the plane of one of its faces. The angle of their opening may then be read off on the arc. This instrument cannot be depended upon for nicety of measurement. The reflecting goniometer was invented by Dr. Wollaston, and several modified forms of it have been introduced by others. It requires for its use crystals with clear faces, that can distinctly reflect the image of a dark line across a clear light, as the bar of a window sash. The instrument is made with great precision, and its graduated arc is furnished with a vernier, by which the degrees are divided into minutes.
The French goniometer of Adelman combines the principles of both the common and reflecting instruments, and is much less expensive than Wollaston's.