thumb|One of the Visby lenses in a silver setting
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, and 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.
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.
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
thumb|Aspheric lens as measured by Karl-Heinz Wilms
The lenses are bi-aspheric and two of them have very good imaging properties. Their surface appears to be an oblate ellipse, while the surface nearest the eye approaches a parabola.
The best example of the lenses measures in diameter and has a thickness of at its centre, with an angular resolution of 25–30 μm.
It was reported by Otto Ahlström in 1950 that most have aspheric surfaces. The best of the lenses have low spherical aberration, indicating that their surface profile was optimized to improve image quality. Olaf Schmidt has speculated that they may have been used as part of a telescope.
See also
- Nimrud lens
References
External links
- Jewellery as Form of Personal Expression (includes picture of a silver mounted lens)
- Die Visby-Linsen (German, includes pictures of all the lenses found at Visby)
- 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