Unusual mineralogy tourmaline tongs
Stock Number: FG_21027
Tourmaline tongs are a primitive polariscope device having a transparent tourmaline crystal thin section or another polarizing medium in each side, mounted in independently rotatable disks at the ends of a pair of spring tongs. Specimens, such as the mineral crystals are slipped between the two tongs and observed by holding the system up to the eye.
Country of manufacture
UK and Ireland
This is a unique pair of polarizing tongs some 6″ long, unusual in that one polarizer is a small reflective glass plate that can be set in Brewster’s angle, named after the Scottish physicist, David Brewster (1781-1868), who discovered the law which gives the tangent of this angle as the ratio of the indices of refraction of the two media (the medium on whose surface the light is incident, and the medium through which the light travels to reach that surface). The other medium is a mineral crystal, most likely tourmaline.
Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminum, iron, magnesium, sodium, lithium, or potassium. 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 are a primitive polariscope device having a transparent tourmaline crystal thin section in each half, mounted in independently rotatable disks at the ends of a pair of spring tongs with the transmission axes at right angles to each other. Mineral crystal specimens mounted in plates 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. The device was invented by Jean-Baptiste Biot (1774-1862), as an inexpensive, effective, and easy-to-use polariscope. In 1816, Brewster 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 the study of the polarization of light and the birefringence of crystals, were the first steps in creating the field of optical mineralogy.
Ask the Dealer
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.