This Day In History: May 23

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In a letter dated May 23, 1785, Benjamin Franklin reveals his design for what would later be called bifocal glasses. The Pennsylvania inventor, printer, author, diplomat and American Founding Father had grown tired of alternating between two different pairs of glasses to help his near or far vision. So he came up with an idea to, quite literally, split the difference. Franklin is widely credited as the inventor of bifocals.

In the letter to his friend George Whatley, a London merchant and pamphleteer, Franklin includes a sketch of his new invention, saying that he found the bifocals particularly useful while dining in France. With them, he wrote, he could see both the food he was eating and the facial expressions of people seated across the table, which helped him better interpret their words—crucial for a diplomat navigating a foreign country.

“I therefore had formerly two pair of spectacles, which I shifted occasionally, as in travelling I sometimes read, and often wanted to regard the prospects,” Franklin wrote. “Finding this change troublesome…I had the glasses cut, and half of each kind associated in the same circle. By this means, as I wear my spectacles constantly, I have only to move my eyes up or down, as I want to see distinctly far or near, the proper glasses being always ready. ”

The bifocal sketch came the year after Franklin made a special request to his optician: Slice in half the lenses of his reading glasses and long-distance glasses, then combine them together with the distance lenses on top and reading glasses on the bottom. Franklin called the glasses style “double spectacles,” later known as bifocals.

Like with his other inventions—including the lightning rod, swim fins and urinary catheter—Franklin had little interest in making money. He wanted his bifocal breakthrough to help other members of the community struggling with vision deterioration. Franklin never patented any of his inventions, intent on sharing them freely.