I was thinking of glasses to put on one's face, though I guess you're right that lenses or prisms of certain types could very well have been used for a long time. 

I ask the question because in a painting from Jan van Eyck, from 1436, it shows a man with reading glasses.  Here's a detail of the painting:http://www.jan-van-eyck.org/The-Madonna-with-Canon-van-der-Paele-%28detail-3%29-1436.htmlI didn't realize that they were around in the early 15th century.