I’ve been reading about the history of glasses today, and I came across the most interesting article on the use of bifocals in the animal world.
It’s kind of surprising to me that eyeglasses are nearly 800 years old in the West. Invented sometime in the 13th century, they’ve become a necessary part of life for many of us in the computer age. Interestingly enough, before their invention, the use of reading stones was fairly common in the Middle Ages (I guess before these came along you just squinted). Not much happened between the invention of glasses aside from significant improvements in optics and changes in style until Benjamin Franklin came along.
The problem with glasses for the longest time was that if you had one or more issues with your vision, say myopia (short-sightedness) or hypermetropia (far-sightedness) in addition to the visual blurriness that comes with old age, you would have to have more than one pair of glasses to account for them. Benjamin Franklin, getting annoyed at having to switching between his pairs, invented the first pair of bifocals in 1784. His design, which had a more convex lens on the bottom for reading and a less convex lens on the top for his myopia, became a very convenient invention going forward. Although now we have all sorts of lenses which can account for this (progressive lenses) without being a truly ugly bifocal set of glasses, we can thank Ben Franklin for this invention.
What’s interesting is that Benjamin Franklin was not, in fact, the first to invent bifocal lenses, as natural selection devised a pair several million years ago. It turns out that the larvae of the beautiful sunburst diving beetle (Thermonectus marmoratus) are equipped with bifocal lenses of their own.1 According a 2015 paper published in Current Biology, the six eyes of these larvae are adjusted for two separate optical distances via adaption in two separate retinas stacked one behind the other in the same lenses. Presumably, this is to help the beetles focus on far and near objects in their visual field, presumably to find prey. The authors hypothesize that when a larva focuses in on its prey, the receptors in the rear retina are focused in on the closer object, only blurring the background.
Even so, this isn’t the first time this has occurred in nature. I have written about trilobite vision before, but have only recently become aware that the vision of Dalmanitina socialis may have been bi-focal, as well.2 In this case, the trilobite’s bifocal vision was caused by a central bulge in its calcite lens which allowed for near-sighted focus. At an age of roughly 450 million years, it seems Mother Nature beats Mother Necessity at almost every turn.
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1Stowasser, A., Rapaport, A., Layne, J.E., Morgan, R.C. and Buschbeck, E.K., 2010. Biological bifocal lenses with image separation. Current Biology, 20(16), pp.1482-1486.
2Gál, J., Horváth, G., Clarkson, E.N. and Haiman, O., 2000. Image formation by bifocal lenses in a trilobite eye?. Vision research, 40(7), pp.843-853.