The bifocal was invented by Benjamin Franklin in the eighteenth century.
At that time, when someone wearing glasses became presbyopic, a separate pair of reading glasses was prescribed. When Ben Franklin found himself in that situation, he felt it was inefficient to have to change glasses when alternating his vision from distance to near and vice versa. Franklin realized that when viewing at distance through his distance glasses he was using the top portion of the lenses, and when reading at near with the reading glasses he was using the bottom portion of the lenses.
Therefore, he decided to cut each distance and reading lens in half and glue them to each other, with the distance lens at the top and the reading lens at the bottom. Thus, he had two pairs of glasses in one — and the bifocal was invented!