Meaning of Ramsden eyepiece

Rams'den eye'piece

Pronunciation: [key]
— Optics. 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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.