Early Tourmanile Tongs for the determination of crystal geometry in optical mineralogy, ca. 1860

Early Tourmanile Tongs for the determination of crystal geometry in optical mineralogy, ca. 1860

Stock Number: FG_22031

£550

A pair of Tourmaline Tongs (c.1860). made of two tourmaline crystals mounted in cells on a brass fork-like holder with a milled knob screw for setting the internal distance, used for testing the optical sign of mineral crystals, with four cork-mounted mineral crystals cut to provide their interference figure.

Circa

1860

Countries of manufacture

UK and Ireland, Germany, France

Categories: Scientific, Microscopy, Mineralogy & Gemmology, Natural history, Physics & Chemistry, Other Technology Antiques

Description

Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminum, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone, and the gemstone can be found in various colors. Tourmaline crystals are highly birefringent, a crystallographic feature resulting in the split of a ray of light crossing the crystal outside its optical axis into an ordinary and an extraordinary ray, being polarized at right angles to each other. However, the E-ray is rapidly absorbed, so if the crystal is a few millimeters thick, the transmitted light is linearly polarized. Tourmaline tongs is a primitive polariscope device having a transparent tourmaline crystal thin section in each half, mounted in cork in independently rotatable oxidized brass disks at the ends of a pair of spring tongs mounted with the transmission axes at right angles to each other. Specimens, such as the mineral crystal specimens mounted in square paper-covered cork plates (provably by Steeg & Reuter) seen here, are slipped between the two tongs and observed by holding the system up to the eye. These specimens contain each a crystal slice set in a specific vibration axis of its optical indicatrix to yield an interference figure (seen in the photos taken with a cellphone camera through the instrument seen here). The device was invented by Jean-Baptiste Biot (1774-1862), as an inexpensive, effective, and easy-to-use polariscope. In 1816, the Scottish physicist, David Brewster (1781-1868), had devised a simple polariscope in which the object to be examined was held between two plates of a singly refracting crystal (he used agate) placed transversally to one another. This early experimental work in physical optics, mostly concerned with studying the polarization of light and the birefringence of crystals, was the first step in creating the field of optical mineralogy.

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IL Gilgamesh

Gilgamesh was the mythological hero of the cultures of ancient West Asia, who set out on a journey in which he sought youth and eternal life.
Fleaglass Gilgamesh is located in Israel. As an archaeologist researching the material culture of the distant past and using the microscope as a major research tool, for more than two decades I have collected microscopes from the first 300 years of existence of this amazing tool and researched the cultural context of their use. Passion has become an obsession and I cultivate the full and almost unique West Asian collection of historical microscopes. Respectively, I put up for sale surplus or interesting items from the collection. I would be happy to advise any interested collector free of charge. Needless to say, I would love to send photos, information and bibliographic references, and discuss the sale details of the items offered here for sale.

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