Yes they do.
four
Yes.
Bifocal lenses as a concept were not patented; there are dozens of patents related to bifocals, bifocal contacts, and processes to make bifocals.
On eyeglasses? Two lenses = bifocals Three lenses = Trofocals
Bifocals are a type of prescription eyeglasses designed for people who need both near and farsighted vision correction. Different from single vision lenses, bifocals have two corrective lenses on each side of a pair of glasses, for a total of four lenses per pair.
The bifocals he bought were used. The lower lenses on my bifocals allow me to read more easily.
Bifocals
The bifocals were invented in 1784, by Benjamin Franklin, because he had seeing problems, and put two lenses of glass together to magnify his vision.
Progressive lenses provide presbyopia correction without the tell-tale line of...the bottom of their bifocals, to determine which part of the lens worksbetter.
Bifocals are a type of classes that have the lenses divided into an upper and lower prescription so that the wearer can see in the distance and read close too. As such they benefit only the wearer not the community.
Lucian Willis Bugbee has written: 'Bifocals' -- subject(s): Lenses
John Isaac Hawkins, the inventor of trifocal lenses, coined the term bifocals in 1824 and credited Dr. Franklin.