Some people have trouble focusing on close-up things. Others don’t see distant things very well. Many people develop one of these problems as they age. That’s why glasses were one of the greatest inventions of the fifteenth century.

Glasses could be made one way to help long-sighted people see close-up things better. They could be made another way to help short-sighted people see far-away things more clearly. However, some people have both a problem seeing things up close and a problem seeing things far away. For centuries, these folks had to endure the irritation of switching between two different pairs of glasses, depending on what they were looking at. Then Benjamin Franklin came along and made their lives a lot easier.

Franklin suffered from bad eyesight all his life, and it got worse as he aged. Benjamin Franklin struggled to see things at a distance and had an even harder time seeing things up close. He used glasses but found having to switch between different glasses to be intolerable. As he was a prolific inventor, he decided to do something about it. So on May 23, 1785, he designed glasses with two lenses in each frame hole, one on top of the other. The upper one allowed the wearer to see well at a distance. The lower one allowed the wearer to see things that were up close. You just had to look a little up or down to change the distance you could see at.

There were some problems with the original design. There was a line across the wearer’s field of vision, of course, where the two lenses met. Early models tended to have one or both lenses fall out, as it wasn’t easy to hold two separate lenses in one frame. Still, Franklin and many other users found it greatly preferable to the alternative. And, of course, evolving technology has improved the design.

Today’s progressive bifocals are made with a single lens that changes its thickness gradually, so wearers can go from near to far vision seamlessly, with no line across their glasses. There are bifocal sunglasses and diving goggles. There are even bifocal contact lenses that can be worn for a month at a time.