Visby lenses

Execution

Some of the lenses, especially smaller ones, have silver versions and were apparently worn on necklaces. In some mounted lenses the bottom is covered with thin silver film, " so that the lenses act like mirrors. " The largest chain is a necklace of seven main, mirrored lenses. The Opposite of the wearer of this necklace looks so seven times its own, reduced reflection.

The largest lens has a diameter of 50 mm and is 28.5 mm thick. From the excavations in Fröjel also comes almost exactly a spherically polished rock crystal, which counts with a diameter of 45 mm also among the largest found lenses.

Optical Properties

Almost all Visby lenses are aspheric executed - her most striking feature. Even Otto Ahlström 1950 described the unusual shape of the lenses. For the production of rotationally symmetric shapes probably used a wood lathe.

Karl -Heinz Wilms surveyed one of the Visby lenses based on a zoomed photo, b analyzed the optical properties and had to make multiple replicas at Rodenstock. Wilms noted that " an area prolongc ellipsoid - perhaps close to the ideal shape, but the other surface oblongd ellipsoid designed " was. The tested lens has an approximately two- fold magnification with extremely low spherical aberration. Bernd Lingelbach and Olaf Schmidt from the Institute of Eye Optics exact measurements Aalen 1998, several Visby lenses contact by means of light-section method and presented at least some " almost ideal optical Properties". This made ​​around 1000 years ago lenses have also by today's standards with excellent imaging properties - later, hemispherical plano-convex stones gathered from the Middle Ages, they are far superior. Comparable properties were achieved again until the mid-twentieth century in optical lenses.

Were first mentioned in writing optical lenses in the treasure of the optics of the Arab scholar Ibn Al- Haitham ( 996-1038 ). Around 1240 the book was translated into Latin. European monks took up the idea and made ​​spherical plano-convex lenses for vision aids. However, finds of rock crystal lenses from antiquity are nothing extraordinary. One of the earliest known lenses, the lens of Nimrud, which was Austen Henry Layard in his excavations in the royal palace of Nimrud in Mosul in the 19th century is estimated at an age of about 3000 years. The processing of rock crystal was already widespread in the 11th century, the mathematical foundations of aspherical surfaces, however, have only existed since 1637 by René Descartes - the artisanal practice was thus the theory by 500 years ahead: " Apparently, some or perhaps only a single lens manufacturer through years of trying out the imaging properties of lenses improves and eventually found the ideal form. "

Origin

With spending in Fröjel in summer 2002 as well as other lenses for the first time tool for processing rock crystal was found next unprocessed rock crystal pieces and semi-finished Linsene and pearls. This leads to the possibility that the Visby lenses may yet emerged on Gotland.

Use

About the use of the Visby lenses, nothing is handed down, therefore, there is only speculation. Maybe the lenses of craftsmen were for the enlargement of filigree work, used as a reading stone or as a magnifying glass.

Gems such as the large chain served beside probably also a representative and possibly magical purpose.