Goldschmidt two-circle reflecting goniometer
Date: circa 1890
Inventory Number: 2004-1-0317
Classification: Goniometer
Subject:
Maker: Peter Stoe
Maker: Stoe & Cie (1887-present)
User: Benjamin Martin Shaub (1893 - 1993)
Cultural Region:
Place of Origin:
City of Use:
Dimensions:
41.5 × 65 × 50 cm (16 5/16 × 25 9/16 × 19 11/16 in.)
DescriptionThe instrument consists of four essential parts: two rotating divided circles for the measurement of angles, one vertical and one horizontal; a collimator and a telescope, the later provided with changeable lenses, for the observation of the signal from the crystal faces; a delicate but easily and rapidly manipulated apparatus for adjusting the crystal fixed in the center of the vertical circle. The whole is carried by a stout circular table supported by a tripod. The circles are read by means of their verniers with the aid of eyepieces.
The conical axis of the horizontal circle, which is rotatable in the rigid central hollow cone of the tripod, carries in a central boring a narrow column, tapped below, and capable of being raised or lowered by means of a driving nut, and terminating in a little disc, which serves as a tabular stand for a glass cell or prism containing an immersion or highly refracting liquid, which may occasionally be required. This conical column also supports the vertical wheel balanced by a counterweight. The vertical circle can be moved along a horizontal rail.
The crystal holder is mounted on two cross slides positioned at right angles, one on top of the other, that can be moved by rotating the adjustment knobs. On top of these flat traverses boxes are located two segmented cradles superimposed at right angles which can be used to tilt the crystal mount in an arc from the vertical to the horizontal. Using these traverse and tilt movements the edge of the crystal may be precisely aligned parallel with the axis of the disk. This mechanism (also called the "goniometer head") is essential for high precision measurements.
The conical axis of the horizontal circle, which is rotatable in the rigid central hollow cone of the tripod, carries in a central boring a narrow column, tapped below, and capable of being raised or lowered by means of a driving nut, and terminating in a little disc, which serves as a tabular stand for a glass cell or prism containing an immersion or highly refracting liquid, which may occasionally be required. This conical column also supports the vertical wheel balanced by a counterweight. The vertical circle can be moved along a horizontal rail.
The crystal holder is mounted on two cross slides positioned at right angles, one on top of the other, that can be moved by rotating the adjustment knobs. On top of these flat traverses boxes are located two segmented cradles superimposed at right angles which can be used to tilt the crystal mount in an arc from the vertical to the horizontal. Using these traverse and tilt movements the edge of the crystal may be precisely aligned parallel with the axis of the disk. This mechanism (also called the "goniometer head") is essential for high precision measurements.
SignedEngraved in cursive on main circle: Stoe & Cie m.b. H./ Heidelberg, Germany
FunctionThe purpose of crystallographic goniometry is to measure the interfacial angles of crystals to determine their type. Single-circle measurement methods determine directly the angle between two crystal faces. In the two-circle method, however, two angles are measured representing the so-called "position angles" from which the relative angle between the faces can be then computed. The "position angles" can be visualized as the intersection points between an imaginary sphere surrounding the crystal and the perpendicular lines to the surfaces of the crystal that pass through the center of the sphere. Each of these points is fully determined by two coordinates, a vertical and an azimuthal angle. The interfacial angles can be then computed using spherical trigonometry. With a two-circle goniometer, under favorable conditions, all faces of a crystal can be measured with a single mounting.
The measurements are done as in the case of a single-circle reflecting goniometer. The crystal would be affixed (usually by wax) to the holder with the edge precisely aligned parallel with the axis of the disk. The axes of both the signal and the observation telescopes form a plane parallel to that of the disk. Since the lens system significantly reduces the reflection on the crystal, the signal telescope can be illuminated by a lamp placed behind it. The objective of the collimator is directed to the crystal edge. The crystal is rotated until the light reflected by the crystal face is seen in the telescope as a narrow strip of light. One would write down the angles at which this occurred, and then rotate the vertical disk until the next crystal face reflected the collimated light in exactly the same way.
The measurements are done as in the case of a single-circle reflecting goniometer. The crystal would be affixed (usually by wax) to the holder with the edge precisely aligned parallel with the axis of the disk. The axes of both the signal and the observation telescopes form a plane parallel to that of the disk. Since the lens system significantly reduces the reflection on the crystal, the signal telescope can be illuminated by a lamp placed behind it. The objective of the collimator is directed to the crystal edge. The crystal is rotated until the light reflected by the crystal face is seen in the telescope as a narrow strip of light. One would write down the angles at which this occurred, and then rotate the vertical disk until the next crystal face reflected the collimated light in exactly the same way.
ProvenanceCarl Francis, Mineralogy Museum, Harvard University, 2004. From Benjamin Martin Shaub, Professor of Mineralogy at Smith College. Purchased by Carl Francis in 1955.