Visby lenses

The Visby lenses are a collection of lens-shaped manufactured objects made of rock crystal (quartz) found in several Viking graves on the island of Gotland, Sweden, and dating from the 11th or 12th century.

[5] The best of the lenses have low spherical aberration, indicating that their surface profile was optimized to improve image quality.

[1] Most of the lenses, however, do not show any sign of optimization and produce worse images than a simple spherical lens.

[citation needed] The Visby lenses provide evidence that sophisticated lens-making techniques were being used by artisans over 1,000 years ago, at a time when researchers had only just begun to explore the laws of refraction.

According to Schmidt and his co-workers, it is clear that the artisans worked by trial and error, since the mathematics to calculate the best form for a lens were not discovered until several hundred years later.