Used for measuring anges on faces of crystals and gems. Incomplete.
Made in Germany approximately 1890's. It is brass and still has its original lacquered and black enamelled finish. Using a vernier and slow motion tangent screw, angles can be measured to 1 arcmin on the graduated scale. The crystal is mounted on the head, which is adjustable in the x, y and z directions, as well as tiltable in two axes and rotatable around the third. The collimator is fixed to the tripod, while the telescope (missing) is movable. Adjustments critical to the actual measurements are made using a number of thumb screws below the tripod. To get accurate measurements, small crystals with smooth faces must be used. The goniometer works on the principle of turning the crystal about an axis parallel to the edge between two faces. The image reflected from a second face may be brought into the same position as that formerly occupied by the image reflected from the first face; the angle through which the crystal has been rotated, as determined by a graduated circle to which the crystal is attached, is the angle between the normals to the two faces.
MW
