It's not the lenses which are difficult, it's clear glass.

With a piece of pure glass, the rest is some emory and a great deal of patience.

Clear glass is also more about knowhow than raw technical difficulty, it isn't the Bessemer process, it's knowing how to flux with soda + add some lead to get the melting point down, then pouring it on lead to get nice flat sheets.

It’s not like you’d be distracted by Twitter, just knowing how it works you could probably work it out with a few skilled glass makers.