For astonishing accounts of Viking age lens discoveries on Gotland: see Schmidt O, Wilms K, Lingelbach B. The Visby Lenses. in Optometry and Vision Science Vol. 76, No 9, pp 624-630.
Description in Swedish Description in German
For possible accounts in English see: Ahlström, Otto "Swedish Vikings used Optical Lenses" in "The Optician" 1950; 459-469; and/or Enoch, Jay M.
"Early lens use: lenses found in context with their original objects" in "Optometry and Vision Science", 1996, Vol.73:707-715. However, it seems that these lenses were not used for searching the sky.
With thanks to John Larsson