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prettygoodwordetalon - n., an optical instrument with two plane parallel reflecting surfaces used to precisely measure distance or the wavelength of light.
The reflecting surfaces being glass plates (you have to allow some light to pass through), and the measured distance being between the plates when the wavelength is known, while the wavelength is measured when the distance is known. Or you can ignore that sentence and take away that it's a doohickey used by astronomers. From French (from around 1900), from older French estalon, standard, ultimately from Old French estal, place, from a Germanic root. (So no, no relation to the Greek letter eta.)
---L.
The reflecting surfaces being glass plates (you have to allow some light to pass through), and the measured distance being between the plates when the wavelength is known, while the wavelength is measured when the distance is known. Or you can ignore that sentence and take away that it's a doohickey used by astronomers. From French (from around 1900), from older French estalon, standard, ultimately from Old French estal, place, from a Germanic root. (So no, no relation to the Greek letter eta.)
---L.
