Visby lenses
As Visby lenses ( Swedish : Visbylinserna ) are more than ten major and numerous smaller, mostly bi-convex lenses of rock crystal called, now largely in the Museum of Gotland Fornsal in Visby are. The original find is part of a treasure that Vikings deposited around 1050 ; In the course of an excavation program in 2002 , several similar lenses were found in Fröjel , a port town of the Viking Age on Gotland . Rodenstock also made several replicas in 1989, one of which is now known as the reading stone in the optics exhibition of the Deutsches Museum in Munich . a
execution
Some of the lenses, especially smaller ones, have silver frames and were apparently worn on necklaces. With some mounted lenses, the underside is covered with thin silver foil, "so that the lenses act like mirrors." The largest chain is a necklace of seven larger, mirrored lenses. In each of these lenses, the person opposite the wearer sees his own, reduced reflection.
The largest lens has a diameter of 50 mm and is 28.5 mm thick. From the excavations in Fröjel comes an almost exactly spherical cut rock crystal, which with a diameter of 45 mm is also one of the largest lenses found.
Optical properties
Almost all Visby lenses are aspherical - their most striking feature. Otto Ahlström already described the unusual shape of the lenses in 1950. A lathe was probably used to produce the rotationally symmetrical shapes .
Karl-Heinz Wilms, an employee of the company Rodenstock, surveyed one of the Visby lenses based on an enlarged photograph, b analyzed the optical properties and settled at Rodenstock make multiple replicas. Wilms stated that “one surface was designed to be prolong c ellipsoid - perhaps close to the ideal shape, but the other surface was designed to be oblong d ellipsoid”. The examined lens has a magnification of about two times with extremely low spherical aberration . In 1998, Bernd Lingelbach and Olaf Schmidt from the Aalen Institute for Ophthalmic Optics measured several Visby lenses contactlessly using the light section method and determined “almost ideal optical properties ” for at least some . These lenses, manufactured around 1000 years ago, have excellent imaging properties even by today's standards - they are far superior to later, hemispherical plano-convex reading stones from the Middle Ages . Comparable properties were only achieved in optical lenses again in the middle of the twentieth century.
Optical lenses were first mentioned in writing in the treasure trove of optics of the Arab scholar Ibn Al-Haitham (996-1038). The book was translated into Latin around 1240 . European monks took up the idea and made spherical plano-convex lenses for visual aids. However, finds of rock crystal lenses from ancient times are nothing unusual. One of the oldest known lenses, the lens of Nimrud , which Austen Henry Layard during his excavations in the royal palace of Nimrud in Mosul in the 19th century took place, is estimated at an age of about 3,000 years. The processing of rock crystal was already widespread in the 11th century, but the mathematical basis for aspherical surfaces has only existed since 1637 by René Descartes - the manual practice was thus 500 years ahead of theory: “Apparently some or perhaps only one lens manufacturer Years of trial and error have improved the imaging properties of the lenses and finally found the ideal shape. "
origin
The origin of the lenses has not been clearly established despite a more detailed analysis. The raw material, rock crystal, does not occur on Gotland . It was believed that the Swedish Vikings might have brought the lentils with them from their trade trains, which were concentrated in southeastern Europe and Asia Minor . It is possible that they were brought to Gotland by members of the Varangian Guard from Byzantium . Mounted and unmounted rock crystals appeared suddenly on Gotland towards the end of the 11th century and disappeared just as suddenly, suggesting that all of the pieces reached Gotland for the same reason. This contrasts with the fact that no rock crystal lenses with similar properties have been found outside of Gotland. Mårten Stenberger also thinks that at least the goldsmith's work is in Gotlandic.
During excavations in Fröjel in summer 2002 among other lenses for the first time the tool was found for processing rock crystal, along with raw rock crystal pieces and semi-finished lenses e and pearls . This allows the possibility that the Visby lenses may have originated on Gotland after all.
use
Nothing has been reported about the use of Visby lenses, so there is only speculation. The lenses may have been used by craftsmen to magnify filigree work, as a reading stone or as a burning glass .
Pieces of jewelry such as the large chain also served a representative and possibly also magical purpose.
See also
literature
- Olaf Schmidt, Karl-Heinz Wilms, Bernd Lingelbach: The Visby Lenses . In: Optometry and Vision Science . tape 76 , no. 9 , September 1999, p. 624-639 ( online ).
- Bernd Lingelbach, Olaf Schmidt: Ahead of Time: Aspherical Lenses from the 11th Century. ( Online ( Memento of February 5, 2012 in the Internet Archive ); PDF; 7.3 MB).
- Olaf Schmidt, Karl-Heinz Wilms, Bernd Lingelbach: The Visby lenses ( online ).
- Richard Greeff : The invention of the eye glasses. Representations of cultural history based on documented sources (= Optical Library. Vol. 1, ZDB -ID 988100-1 ). Verlag A. Ehrlich, Berlin 1921.
Web links
Collection of Visby lenses
Link to the picture
- Visby lenses on the website of the Institut für Augenoptik Aalen (IfAA)
Remarks
- aInv no. 89/753, illustration
- cd. H. stretched
- dd. H. compressed
- e Illustration of finds from Fröjel
Individual evidence
- ↑ a b Institute for Ophthalmic Aalen
- ↑ a b c d Bernd Lingelbach, Olaf Schmidt : Ahead of its time: Aspherical lenses from the 11th century.
- ↑ Otto Ahlström: Swedish Vikings used optical lenses. The Optician 1950, pp. 459-469.
- ↑ a b Viking Age Fire-Steels and Strike-A-Lights at Viking Answer Lady (English).
- ↑ Abu-'Ali Al-Hasan Ibn Al-Haytham: Kitab-al-Manazir. (German: "Treasure of optics").
- ^ Ian P. Howard: Basic Mechanisms. Seeing in Depth. Volume 1. Porteous, Toronto 2002, ISBN 0-9730873-0-7 , p. 16 ff.
- ^ Excavation reports from August 23 ( memento from February 20, 2012 in the Internet Archive ) and September 1, 1999