The Visby lenses are a collection of lens-shaped manufactured objects made of rock crystal (quartz) found in several Viking graves on the island of Gotland, Sweden, dating from the 11th or 12th century. Some were in silver mounts with filigree, 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.[1] Some of the lenses can be seen at the Fornsal historical museum in Visby, while some are in the Swedish National Museum in Stockholm, and others have been lost.
It was reported by Otto Ahlström in 1950 that most have aspheric surfaces,[2] 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.[1]
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The lenses are bi-aspheric and have excellent imaging properties. Their surface appears to be an oblate ellipse, while the surface nearest the eye approaches a parabola. They are so well produced that even computer optimisation has not been able to improve their performance.[3]
The best example of the lenses measures 50 mm (2.0 in) in diameter and 30 mm (1.2 in) thick at its centre, with an angular resolution 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 or Eastern Europe.[1] The Vikings of Gotland were known to have participated in trade networks that reached as far as Constantinople. 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.[4][5]
Various uses have been proposed for the lenses: