These are mysterious artifacts that have fascinated me for many years. One of those extant pieces of our cultural development, that gives pause for thought. For those unfamiliar with them, one brief overview and a more professional assessment, as well.
Here is the wikipedia page for brevity: https://en.wikipedia.org/wiki/Visby_lenses
A more exhaustive description and analysis is under article tools, as a full pdf: http://journals.lww.com/optvissci/Abstr ... s_.19.aspx
Visby lenses.
Re: Visby lenses.
Thank you, apochronaut: very interesting indeed. I've never heard of visby lenses before.