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Ramsden eyepiece

noun, Optics.
1.
an eyepiece consisting of two plano-convex crown-glass lenses of equal focal length, placed with the convex sides facing each other and with a separation between the lenses of about two-thirds of the focal length of each.
Origin
1840-1850
1840-50; named after Jesse Ramsden (1735-1800), English maker of astronomical instruments
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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