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Ramsden eyepiece

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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 of Ramsden eyepiece

1840–50; named after Jesse Ramsden (1735–1800), English maker of astronomical instruments
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018