Visby lenses
Encyclopedia
The Visby lenses are a collection of lens
Lens (geometry)
In geometry, a lens is a biconvex shape comprising two circular arcs, joined at their endpoints. If the arcs have equal radii, it is called a symmetric lens.A concave-convex shape is called a lune...

-shaped manufactured objects made of rock crystal (quartz) found in several Viking graves on the island of Gotland
Gotland
Gotland is a county, province, municipality and diocese of Sweden; it is Sweden's largest island and the largest island in the Baltic Sea. At 3,140 square kilometers in area, the region makes up less than one percent of Sweden's total land area...

, Sweden, dating from the 11th or 12th century. Some were in silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

 mounts with filigree
Filigree
Filigree is a delicate kind of jewellery metalwork made with twisted threads usually of gold and silver or stitching of the same curving motifs. It often suggests lace, and in recent centuries remains popular in Indian and other Asian metalwork, and French from 1660 to the late 19th century...

, 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
-See also:* Battle of Visby* Gotland University College* List of governors of Gotland County-External links:* - Visby*...

, while some are in the Swedish National Museum in Stockholm
Stockholm
Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...

, and others have been lost.

It was reported by Otto Ahlström in 1950 that most have aspheric
Aspheric lens
An aspheric lens or asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens....

 surfaces, demonstrating that knowledge of optical design had been much further developed in the Middle Ages than previously assumed. Their aberration curve is very similar to modern aspheric lenses as used in spectacles.

Description

The lenses are bi-aspheric and have excellent imaging properties. Their surface appears to be an oblate ellipse
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...

, while the surface nearest the eye approaches a parabola
Parabola
In mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...

. They are so well produced that even computer optimisation has not been able to improve their performance.

The best example of the lenses measures 50 mm (2 in) in diameter and 30 mm (1.2 in) thick at its centre, with an angular resolution
Angular resolution
Angular resolution, or spatial 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...

 of 25–30 μm.

The Visby lenses provide evidence that sophisticated lens-making techniques were being used by craftsmen 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 coworkers, it is clear that the craftsmen 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 craftsmen, and perhaps a single person.

It has been suggested that the lenses were not produced by the Vikings, as there are hints that they were in fact produced in Byzantium
Byzantium
Byzantium was an ancient Greek city, founded by Greek colonists from Megara in 667 BC and named after their king Byzas . The name Byzantium is a Latinization of the original name Byzantion...

 or Eastern Europe. The Vikings of Gotland were known to have participated in trade networks that reached as far as Constantinople
Constantinople
Constantinople was the capital of the Roman, Eastern Roman, Byzantine, Latin, and Ottoman Empires. Throughout most of the Middle Ages, Constantinople was Europe's largest and wealthiest city.-Names:...

. However, subsequent 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.

Proposed uses

Various uses have been proposed for the lenses:
  • They may have been used for by craftsmen for magnification in fine work, as reading stone
    Reading stone
    A reading stone was an approximately hemispherical lens that was placed on top of text to magnify the letters so that people with presbyopia could read it more easily. Reading stones were among the earliest common uses of lenses....

    s, or to start fires.
  • Olaf Schmidt has speculated that they may have been used as part of a telescope
    Telescope
    A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

    .

External links

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