"Visby" lenses were initially thought to be ornaments

"It seems that the elliptical lens design was invented much earlier that we thought and then the knowledge was lost," says Dr Olaf Schmidt, of Aalen University in Germany.

The late Dr Karl-Heinz Wilms first heard of the so-called "Visby" lens in 1990 when he was searching for exhibits for a Munich museum. It was named after the major town on Gotland. Dr Wilms found a picture of the lens in a book and planned to examine the original.

But it was not until 1997 that a team of three scientists went to Gotland to take a close look at what were actually 10 lenses locked away in the storeroom of a local museum.

Perfect shape

One of the team, Dr Olaf Schmidt, told BBC News Online: "I was excited, of course. The polish of some of the lenses was almost perfect. The second thing that caught our eye was that their imaging was very good."

This lens was an almost perfect ellipsoid

When the lenses were put through their paces, the team was amazed. The lenses passed a series of tests almost as well as modern optics.

Made from rock-crystal, the lenses have an accurate shape that betrays the work of a master craftsman. The best example of the lenses measures 50 mm (2 inches) in diameter and 30 mm (1 inch) thick at its centre.

"The surface of some of the lenses have an almost perfect elliptical shape," Dr Schmidt said. "They were obviously made on a turning lathe."

The lenses have a flattened central area that makes them excellent magnifiers.

"They could have been used as magnifiers, allowing fine carving to be carried out, or they could have been used to start fires or to burn wounds and cuts so that they did not get infected."

What intrigues the researchers is that the lenses are of such high quality that they could have been used to make a telescope some 500 years before the first known crude telescopes were constructed in Europe in the last few years of the 16th century.

Lost knowledge

The Gotland crystals provide the first evidence that sophisticated lens-making techniques were being used by craftsmen over a 1,000 years ago.

At that time, scientists had only just started to explore the laws of light refraction.

According to the researchers, it is clear that the craftsmen who figured the lenses knew more about applied optics than did the scientists of the time. They must have worked by trial and error because the mathematics to calculate the best shape for a lens did not become available for several hundred years.

The researchers speculate that the knowledge to make such an accurate lens was known to only a few craftsmen, perhaps only one person.

But it seems clear that the Vikings did not make the lenses themselves. "There are hints that the lenses may have been manufactured in [the ancient empire of] Byzantium or in the region of Eastern Europe," Dr Schmidt said.

Some of the lenses can be seen at Gotland's Fornsal, the historical museum in Visby. Some are in the Swedish National Museum in Stockholm. Others have been lost.