It turns out that the author meant the (more modern French) "besicles", meaning 'spectacles' (though people usually say lunettes): there's a glossary of spectacles terms here. Now: besicles was originally bericles, where the 'r' somehow transformed into 's' over the centuries (but you'd have to ask a linguist about this subtle shift). And bericles was in turn from beryl, a "smoke-coloured stone" (according to this fun page on spectacles history) used to grind the lenses from - although pure beryl is colourless, & it is the impurities in it which give it colour.
There's a bigger article on spectacles in the American Journal of Gastroenterology (yes, really!), though you'd need to pay to read it (boo, hiss).