Pictured is one of the Visby lenses, a piece of jewellery used to magnify or distort images. It could be used as a telescope, reading stone, to start fires, or simply to look pretty. They were found at viking* graves in Gotland, Sweden.
It's thought that these were produced by artisans that lacked the complex mathematics to calculate the shapes of the lenses necessary for magnification, but worked by trial-and-error instead. It's unclear whether it was vikings* who produced these, or if they were acquired via trade networks stretching through Eastern Europe and Byzantium. However, excavation shows that the materials needed were being worked nearby to make jewellery.
* I object to the term "viking", because these were from the 11-12th centuries, really at the tail end of the Viking Age, which is generally accepted to have ended in 1066. Furthermore, not every Viking-Age-Scandinavian was a viking. Most were just chilling in their villages and cities, trading, crafting, etc.
It's thought that these were produced by artisans that lacked the complex mathematics to calculate the shapes of the lenses necessary for magnification, but worked by trial-and-error instead. It's unclear whether it was vikings* who produced these, or if they were acquired via trade networks stretching through Eastern Europe and Byzantium. However, excavation shows that the materials needed were being worked nearby to make jewellery.
* I object to the term "viking", because these were from the 11-12th centuries, really at the tail end of the Viking Age, which is generally accepted to have ended in 1066. Furthermore, not every Viking-Age-Scandinavian was a viking. Most were just chilling in their villages and cities, trading, crafting, etc.