English, c.1810, engraved to the front 'Thomas Jones, 62 Charring Cross' the goniometer on a turned French polished wooden base with supporting 'A' frame terminating in a split bearing, with 2 knurled wheels at the rear, 1 to turn the spindal holding the armature the other to turn the spindal and the scale, with a silvered vernier scale at the side reading to arc-minutes, this is the first form of Wollaston goniometer with the the sprung stop at the 0 and 180 positions which was a feature dispensed with on later devices, in the original Mahogany case with catches, case height 15cm, with period glass dome cover

Jones, Thomas, according to Clifton Jones was working at 62 Charing cross from 1816, was apprentice to Jesse Ramsden

History and Operation of the Wollaston Crystallographic Goniometer

The Wollaston crystallographic goniometer was a significant development in the field of crystallography, enabling scientists to measure the angles between crystal faces with great precision. The invention of this device is attributed to William Hyde Wollaston, a British scientist and polymath, in the early 19th century. His work not only advanced the study of crystallography but also laid the foundation for future innovations in mineralogy and materials science.

Before the development of the goniometer, scientists faced challenges in accurately measuring the geometry of crystals. Crystals, with their symmetrical and structured shapes, had long intrigued scientists and mathematicians alike. However, understanding their internal symmetry required precise measurements, particularly of the angles between their faces. Traditional tools used for such measurements, were inadequate for obtaining the level of accuracy needed for detailed scientific study.

In 1809, Wollaston introduced his crystallographic goniometer. This instrument allowed for highly precise angular measurements of crystal faces, down to fractions of a degree. Considered the first reflecting goniometer, a type of instrument that relies on measuring the reflection of light to determine the angle between crystal faces, as opposed to contact goniometers, which require physical interaction with the crystal.

Wollaston’s invention significantly impacted mineralogy and crystallography because it allowed for the accurate measurement of the fundamental angles of crystals, leading to better classification and understanding of crystal structures. By measuring the angles between different faces, researchers could deduce the symmetry and internal structure of the crystal. This was particularly useful for the study of minerals, as crystals often form naturally in these materials.

Operation of the Wollaston Goniometer

The Wollaston goniometer is classified as a reflecting goniometer, where the measurement of angles is based on the reflection of light. The basic principle involves aligning the reflection of a light source on one face of the crystal, followed by measuring the angle at which the light reflects from another face.

The operation of the Wollaston goniometer can be broken down into a few key steps:

1. Mounting the Crystal: The crystal specimen is carefully mounted on a spindle that can rotate using a small piece of wax. The crystal is positioned so that one of its faces can reflect light. Precise mounting is critical, as the measurement accuracy depends on the crystal's stability and the reflected light's clarity.

2. Aligning the Light Source: A light source is directed at the mounted crystal. In Wollaston’s design, this was often natural light or a simple lamp. The goal is to ensure that the light strikes one of the crystal's faces at a specific angle and reflects back toward an observer.

3. Measuring the Reflected Angle: The observer views the reflection of the light from one face of the crystal. The goniometer is equipped with a graduated circle that can rotate along with the crystal. Once the reflection is observed, the goniometer is rotated until the light reflects from another face of the crystal. The angle of rotation is measured using the graduated circle. This angle corresponds to the interfacial angle between the two crystal faces.

4. Repeating for Different Faces: The process is repeated for different pairs of faces to fully characterize the crystal. By systematically measuring the angles between different faces, it is possible to derive the symmetry and geometry of the crystal. The precision of the Wollaston goniometer allowed for measurements to within 30 seconds of arc, which was a major improvement over previous instruments.

The reflecting goniometer, unlike contact goniometers, did not require any physical contact with the crystal, which could damage fragile specimens. Furthermore, by using the reflection of light, it was possible to obtain much more accurate measurements, as light reflection is a highly precise phenomenon that can be accurately measured.

Impact on Crystallography - The Wollaston goniometer became an essential tool in crystallography, enabling researchers to explore the geometrical properties of crystals with a great level of detail. Its use led to the discovery of several important principles in crystallography, including the identification of crystal symmetries and the formulation of laws such as the law of constant interfacial angles, which states that the angles between equivalent faces of crystals of the same substance are constant.

Wollaston’s contribution also paved the way for the development of more advanced goniometers and crystallographic tools. Later innovations included more precise optical systems and automated goniometers, which could be used to measure crystals with even greater accuracy and efficiency. In modern crystallography, the basic principles of angle measurement established by the Wollaston goniometer remain relevant, even as X-ray diffraction and other techniques have expanded the toolkit available to researchers.

References:

1. P. Gay, *Wollaston and His Reflecting Goniometer: A Historical Review*, Nature, 1966.
2. C. Giacovazzo, *Fundamentals of Crystallography*, Oxford University Press, 2011.
3. G. L. Clark, *Applied X-ray Crystallography*, McGraw-Hill, 1955.
4. B. E. Warren, *X-ray Diffraction*, Dover Publications, 1990.