Everything about The Visby Lenses totally explained
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 couldn't produce it. Aspheric lenses for spectacles were not made until the 1950's.
They may have been used for magnification by craftsmen for fine work, as
reading stones, or to start fires.
There has been some speculation that they may possibly have been used as part of a
telescope.
Some of the properties of one of the lenses are as follows:
» 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's 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....
The rock crystal itself would have been imported, as it isn't 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.
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.
The excavation reports have good accompanying pictures.
Some of the lenses can be viewed at
Gotlands fornsal, a historical museum in Visby.
Further Information
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