“Is lens a Latin word?” asked an observant student. “You know, like lens, lentis, masculine?”
Yes. But actually it is feminine. It means a lentil.
The metaphora of using this word to mean the piece of glass with one flat surface and one curved is a useful illustration for pupils of how language can proceed: how so many words start with a physical meaning, and then later are used to denote something more abstract; or, in this case, the transference can be from one physical object to another, as with tibia and fibula—which occurs when a word is needed for a physical object as yet unnamed.
The Romans were familiar with lentils (the best ones were supposed to come from Egypt—Georgics I.228), and Europeans have known them ever since. The shape of a lentil makes it a good metaphor for the optical glass, although the latter is so much larger.
The diminutive lenticula, which becomes the English word, was also apparently the normal Latin word for a freckle.
It seems uncertain who first used the word lens in its optical sense. The OED records Sir Isaac Newton using it in 1704; but Sir Edmond Halley (the second Astronomer Royal, who identified the comet) used the word in the Royal Society’s Philosophical Transactions in 1693.
Both men were, of course, members of the Royal Society, which, having met in various forms in Oxford and London, became an official body, and was granted its title by Charles II, in 1660. It is the oldest scientific society in the world.