Where did Benjamin Franklin invent the biofocals?

Answer 1

Franklin was in Paris from 1777 to 1785. He first mentioned his bifocal lenses in 1784, and refers to them as "my invention" without giving any more detail.

In 1779, he ordered some eye glasses, that were more time-consuming and expensive than normal, and it is POSSIBLE that the delay and cost were due to these being bifocal.

A 1789 letter from a friend of Franklin states that Ben had been wearing double lenses for decades.

In other words, the evidence is ambiguous. He might have invented them in Paris, or maybe while still in Philadelphia. Or he might have discovered them while in London, and simply claimed them as his idea.

Answer 2

Benjamin Franklin invented bifocals in Philadelphia, Pennsylvania.