Reflecting crystal goniometer c. late 19th century J. Duboscq, Paris Brass, glass, steel Used to measure the angle between the faces of a mineralogical crystal 7-1/2” high This instrument was constructed in the style of a Wollaston reflecting goniometer. It is approximately 7-1/2” high. The weighted base is 4-1/4” in diameter and leveled by three brass leveling screws. The pillar-mounted vertical circle is silvered and engraved by degrees to 1/2 degree. The silver has been somewhat roughly polished in the instrument’s past history. The circle is read by means of a vernier which splits each degree by 60 although readings under 5 minutes of arc would be difficult. There is an adjustable black mirror measuring 1-1/4” x 1-5/8” attached to the base. The horizontal axle is centered in the circle and it rotates the circle by means of the flanged disk. There is a clamp and tangent slow motion screw for fine adjustments attached to the flanged disk. At the outer end of the axle there is a thumb screw that rotates the crystal holder independent of the flanged disk and circle. This style of goniometer was used to measure the angle between each face of various mineralogical crystals. It was determined with goniometers that the angle of crystal faces are consistent with the crystal’s building blocks. The makeup of a crystal can be determined by its face angles with this goniometer. 

This was done by mounting the crystal to the fixture attached to the center of the vertical circle and shining a beam of light on it which reflects to a reference point some distance away - perhaps on a wall or the floor. When the reference point is established, the vertical circle setting is noted. The crystal is then rotated to the next face until the beam of light is again hitting the same reference point and the vertical circle setting is again noted. The face angles can be calculated by the difference of the two noted readings. The black mirror is used for convenience as it allows the light beam to be adjusted to a suitable reference point. This instrument is complete, fully functional and is an excellent example of a Wollaston reflecting goniometer. Considering its age, it is in fine condition. “J DuBoscq Paris” is engraved on the brass face of the vertical circle. His firm was considered one of the finest instrument makers of his day. Circa late 19th Century. Antique Scientific Instrument