I cannot distinguish a letter even of large print; but am happy in the invention of double spectacles, which serving for distant objects as well as near ones, make my eyes as useful to me as ever they were: If all the other defects and infirmities were as easily and cheaply remedied, it would be worth while for friends to live a good deal longer–Benjamin Franklin
Two-hundred-forty years ago, on May 23, 1785, Benjamin Franklin offered a detailed description of his invention, “double spectacles,” to London optician Peter Dollond, who’d previously criticized Franklin’s idea as only good for “particular eyes.” Franklin’s response:
By M. Dollond’s saying that my double spectacles can only serve particular eyes, I doubt he has not been rightly informed of their construction. I imagine it will be found pretty generally true that the same convexity of glass through which a man sees clearly at a distance proper for reading is not the best for greater distances. I therefore had formerly two pairs of spectacles which I shifted occasionally, as in traveling I sometimes read, and often wanted to regard the prospects. Finding the change troublesome and not always sufficiently ready, I had the glasses cut and half of each kind associated in the same circle, thus–
(Franklin’s original illustration was hand-drawn, and hand-written. This is the most common representation in use today, only because it’s more legible.)
Ben Franklin is generally credited with the invention of the bifocal lens, although there is some speculation that others may have also experimented with the concept; in any event, Franklin certainly seems to have been the first to explain the process of making them so clearly, and to provide an illustration by which others could make them as well.
Bifocal glasses (a term coined in 1824 by John Isaac Hawkins), were made with two separate pieces of glass contained in each frame until early in the twentieth century, when a method was developed for fusing the sections together into one, thus producing a stronger lens, and a less-disruptive transition between the two prescriptions. Future advances in technology would lead to the lightweight polycarbonate lenses, the “invisible” bifocal, and the “progressive” (small “p,” non-ideological) modern lenses of today.
Benjamin Franklin, my tired old eyes salute you. Thank you, good sir.
