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One of the Visby lenses in a silver setting The Visby lenses are a collection of
lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
-shaped manufactured objects made of
rock crystal Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical for ...
(quartz) found in several Viking graves on the island of Gotland, Sweden, and dating from the 11th or 12th century. Some were in
silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical co ...
mounts with
filigree Filigree (also less commonly spelled ''filagree'', and formerly written ''filigrann'' or ''filigrene'') is a form of intricate metalwork used in jewellery and other small forms of metalwork. In jewellery, it is usually of gold and silver, ma ...
, the mounting covering the back of the lens, and were probably used as jewellery; it has been suggested that the lenses themselves are much older than their mounts. Some of the lenses can be seen at the Fornsal historical museum in
Visby Visby () is an urban area in Sweden and the seat of Gotland Municipality in Gotland County on the island of Gotland with 24,330 inhabitants . Visby is also the episcopal see for the Diocese of Visby. The Hanseatic city of Visby is arguably th ...
, while some are in the Swedish National Museum in
Stockholm Stockholm () is the capital and largest city of Sweden as well as the largest urban area in Scandinavia. Approximately 980,000 people live in the municipality, with 1.6 million in the urban area, and 2.4 million in the metropolita ...
, and others have been lost.


Discovery

Excavations at Fröjel on Gotland in 1999 discovered evidence of local manufacture of beads and lenses from rock crystal, with unworked pieces of crystal coexisting with partially finished beads and lenses.


Characteristics

The lenses are bi-aspheric and two of them have very good imaging properties. Their surface appears to be an oblate
ellipse In mathematics, an ellipse is a plane curve surrounding two focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a circle, which is the special type of ellipse i ...
, while the surface nearest the eye approaches a
parabola In mathematics, a parabola is a plane curve which is mirror-symmetrical and is approximately U-shaped. It fits several superficially different mathematical descriptions, which can all be proved to define exactly the same curves. One descri ...
. The best example of the lenses measures in diameter and has a thickness of at its centre, with an
angular resolution Angular resolution describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object, thereby making it a major determinant of image resolution ...
of 25–30 μm. It was reported by Otto Ahlström in 1950 that most have
aspheric An aspheric lens or asphere (often labeled ''ASPH'' on eye pieces) is a lens (optics), lens whose surface profiles are not portions of a sphere or Cylinder (geometry), cylinder. In photography, a camera lens, lens assembly that includes an aspheri ...
surfaces. The best of the lenses have low
spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike ...
, indicating that their surface profile was optimized to improve image quality. Most of the lenses, however, do not show any sign of optimization and produce worse images than a simple spherical lens. Prior to the Fröjel finds it had been suggested that the lenses were not produced by the Vikings, as there are hints that they were in fact produced in
Byzantium Byzantium () or Byzantion ( grc, Βυζάντιον) was an ancient Greek city in classical antiquity that became known as Constantinople in late antiquity and Istanbul today. The Greek name ''Byzantion'' and its Latinization ''Byzantium'' con ...
or Eastern Europe. The Vikings of Gotland were known to have participated in trade networks that reached as far as Constantinople.


Proposed uses

The Visby lenses provide evidence that sophisticated lens-making techniques were being used by artisans over 1,000 years ago, at a time when researchers had only just begun to explore the laws of refraction. According to Schmidt and his co-workers, it is clear that the artisans worked by trial and error, since the mathematics to calculate the best form for a lens were not discovered until several hundred years later. It has been suggested that the knowledge required to make such lenses was restricted to only a few people, and perhaps only one. Various uses have been proposed for the lenses. They may have been used by artisans for magnification in fine work, as
reading stone A reading stone is an approximately hemispherical lens that can be placed on top of text to magnify the letters so that people with presbyopia can read it more easily. Reading stones were among the earliest common uses of lenses. The invention ...
s, or to start fires. Olaf Schmidt has speculated that they may have been used as part of a
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
.


See also

*
Nimrud lens The Nimrud lens, also called Layard lens, is an 8th-century BC piece of rock crystal which was unearthed in 1850 by Austen Henry Layard at the Assyrian palace of Nimrud in modern-day Iraq. It may have been used as a magnifying glass or as a burni ...


References


External links


Jewellery as Form of Personal Expression (includes picture of a silver mounted lens)


* ttp://www.leinroden.de/makeframe.html?22visb.htm Institut für Augenoptik Aalen, Projekte & Aktivitäten, Visby Linsen (German)
Der Zeit voraus: Asphärische Linsen aus dem 11. Jahrhundert; Bernd Lingelbach, Olaf Schmidt; Das Fröjel Discovery Programme (German, extensive paper with many illustrations)

Link to summary of all of 1999 excavation reports at Visby
{{Viking Germanic archaeological artifacts Viking art Lenses