Visby lenses

The Visby lenses are a collection of lens-shaped manufactured objects made of rock crystal (quartz) found in a viking grave in Gotland dating from approximately the 10th century. Some of them are mounted in silver and may have been carried as a pendant, but others appear not to have been used as jewellery. The lenses have good optical properties, comparable to lenses manufactured in the 20th century. They are unusual in being aspheric, having an ellipsoidal profile.

The optimization was achieved by craftsmen long before mathematicians could describe the shape and properties of aberration-corrected lenses. It seems that this knowledge was lost for at least 500 years, until Descartes calculated the idea [sic] shape of a focusing lens but, lacking the necessary equipment, he could not produce it. Aspheric lenses for spectacles were not made until the 1950's. [ [http://www.frojel.com/Documents/Document03.html Medieval lenses exhibit modern performances. By Oliver Graydon.Featured in Opto & Laser Europe, Issue 56. November 1998.] ]

They may have been used for magnification by craftsmen for fine work, as reading stones, or to start fires. [ [http://www.vikinganswerlady.com/fire.shtml#BurningGlasses Viking Age Fire-Steels and Strike-A-Lights] ] There has been some speculation that they may possibly have been used as part of a telescope. [ [http://news.bbc.co.uk/1/hi/sci/tech/702478.stm 5 April, 2000, BBC News: Did the Vikings make a telescope?] ] [ [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=10498003&dopt=Abstract The visby lenses. Schmidt O, Wilms KH, Lingelbach B. Aalen University of Applied Science, Germany.] ]

Some of the properties of one of the lenses are as follows: [Webarchive backup: [http://web.archive.org/web/20060220034712/www.medocular.se/ogonfakta/historia/historia-visbylinserna/index.html Visbylinserna—historiska förstoringsglas? (Swedish)] ] [ [http://www.capioeye.co.uk/eyeinfo/history/visby_lenses/index.html The Visby lenses—Historical Magnifying Glasses?] ]

:Diameter: 50 mm:Focal Length: 22–35 mm:Angular resolution: 25–30 μm

The Vikings at Visby on Gotland are known to have participated in trading networks reaching as far as Constantinople, so it is possible the lenses originated in the Middle East with only the silver mounts being of local manufacture. However, subsequent excavations at Fröjel on Gotland have shown evidence of manufacture of beads and lenses from rock crystal as unworked items of crystal coexist with partially completed objects

...we have found many traces of rock crystal in the area, from raw material to finished beads and lenses.... [ [http://frojel.hgo.se/report8/Re8.html Carlsson, Fröjel Excavation Report 8, 23rd of August 1999] ]

The rock crystal itself would have been imported, as it is not native to Gotland.

Where they got the raw material is still a matter of discussion, but probably got if [sic] from the area around the Black Sea. [ [http://frojel.hgo.se/Report7/Report7.html Carlsson, Fröjel Excavation Report 7, 18th of August 1999] ]

Pieces of rock crystal, both in the form of raw material, half finished beads, lenses and well made faceted beads have been found at Fröjel during the excavation. This summer, we found something like 4 or 5 crystal lenses and several beads, and it all points to the conclusion that rock crystal was imported to Fröjel and used for making beads as well as lenses. [ [http://frojel.hgo.se/report9/Re9BAK.html Carlsson, Fröjel Excavation Report 9, 1st of September 1999] ]

The excavation reports have good accompanying pictures.

Some of the lenses can be viewed at Gotlands fornsal, a historical museum in Visby.

References

External links

* [http://www.gotmus.i.se/1engelska/skatter/engelska/jewellery_as_form_of_personal_expression.htm Jewellery as Form of Personal Expression (includes picture of a silver mounted lens)]
* [http://www.optiker.at/archiv/museum/visby/visby.htm Die Visby-Linsen (German, includes pictures of all the lenses found at Visby)]
* [http://www.leinroden.de/makeframe.html?22visb.htm Institut für Augenoptik Aalen, Projekte & Aktivitäten, Visby Linsen (German)]
* [http://www.leinroden.de/22visby.pdf Der Zeit voraus: Asphärische Linsen aus dem 11. Jahrhundert; Bernd Lingelbach, Olaf Schmidt; Das Fröjel Discovery Programme (German, extensive paper with many illustrations)]
* [http://frojel.hgo.se/Frojel99.pdf Link to summary of all of 1999 excavation reports at Visby]