Focal = Fire ? Does anyone know of the specific reference?

Started Apr 24, 2016 | Discussions thread
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TacticDesigns Veteran Member • Posts: 8,395
Focal = Fire ? Does anyone know of the specific reference?

Ok. This may be a bit off topic, but it's in response to a post I posted here . . .

. . . where I mentioned that . . .

"And I read somewhere that focal actually means fire and that focal length got its name because if you take a simple lens, hold it up to use the sun's rays to light a fire . . . the distance from the middle of that simple lens to the point where the sun's rays concentrate is called the fire length . . . or focal length."

Well. I was curious . . . and did a bit more digging. I figured, if it was actually true, then that would be pretty cool.

I was able to track some stuff down.

I think it may have been Kepler that coined the phrase. (Or perhaps there is an earlier reference?)

Here on . . .

When you look up the history of Focus . . . it says . . .

"1640s, "point of convergence," from Latin focus "hearth, fireplace" (also, figuratively, "home, family"), which is of unknown origin. Used in post-classical times for "fire" itself; taken by Kepler (1604) in a mathematical sense for "point of convergence," perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before Kepler, but it is not recorded). Introduced into English 1650s by Hobbes. Sense transfer to "center of activity or energy" is first recorded 1796."

So not exactly a smoking gun, but . . . in another reference I read that Kepler used someone's book called "Magia Naturalis" (Giambattista della Porta).

And in that book, Della Porta speaks a lot about lenses to burn things. (Burning glasses).

Ok. Here's the question . . .

Is there a reference in Kepler's work (from 1604?) that is the first time he uses the term focal or focus?

Oh, yeah . . . does anyone have a copy of this book?

A History of Optics from Greek Antiquity to the Nineteenth Century? Maybe there is a reference in there?

Take care & Happy Shooting!

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