Visby lens

"The Vikings could have been using a telescope hundreds of years before Dutch spectacle makers supposedly invented the device in the late 16th century.... 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. ..." From Did the Vikings make a telescope? BBC Science
http://news.bbc.co.uk/2/hi/science/nature/702478.stm