from
The Collaborative International Dictionary of English v.0.48
eyepiece \eye"piece`\ eye-piece \eye"-piece`\, n. (Opt.)
The lens, or combination of lenses, at the eye end of a
microscope, telescope or other optical instrument, through
which the image formed by the mirror or object glass is
viewed.
Syn: ocular.
[1913 Webster]
{Collimating eyepiece}. See under {Collimate}.
{Negative}, or {Huyghenian}, {eyepiece}, an eyepiece
consisting of two plano-convex lenses with their curved
surfaces turned toward the object glass, and separated
from each other by about half the sum of their focal
distances, the image viewed by the eye being formed
between the two lenses. it was devised by Huyghens, who
applied it to the telescope. Campani applied it to the
microscope, whence it is sometimes called {Campani's
eyepiece}.
{Positive eyepiece}, an eyepiece consisting of two
plano-convex lenses placed with their curved surfaces
toward each other, and separated by a distance somewhat
less than the focal distance of the one nearest eye, the
image of the object viewed being beyond both lenses; --
called also, from the name of the inventor, {Ramsden's
eyepiece}.
{terrestrial}, or {Erecting eyepiece}, an eyepiece used in
telescopes for viewing terrestrial objects, consisting of
three, or usually four, lenses, so arranged as to present
the image of the object viewed in an erect position.
[1913 Webster]