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TM 9-258
g. Ramsden. The Ramsden eyepiece consists
h. Huygenian.
This eyepiece employs a
of two planoconvex lenses made of ordinary crown glass
collective or field lens moved away from the eyelens so
of equal focal lengths separated by a distance equal to
that the real image lies between the two lenses. The
two-thirds of the numerical value of the focal length of
Huygenian eyepiece (A, fig 4-6) is not suitable for use
either. The reticle is located in front of the field lens at
with reticles in telescopes as distortion of the reticle
one-quarter of its focal length as illustrated in B, figure 4-
would result since the individual components are not
5. The field lens thus contributes to the magnification
corrected for aberration. It is sometimes used however
and quality of the image. Dirt on its principal plane is not
with a small reticle in the center of the field in some
in focus and, therefore, not visible. The convex surface
microscopes and it is useful in observation instruments.
of each lens faces the other.  This eyepiece has the
Eyepiece focal length usually used is over 1 inch for
disadvantage of considerable lateral color which can be
sufficient eye relief. Both elements are normally of the
defined as a difference in image size for each color. It is
same type of crown glass with convex surfaces facing
widely used with reticles, however. This type of ocular
forward.  This type of ocular is of negative type in
can be used as a magnifier and it is therefore called a
contradistinction to the Ramsden or positive type (g
positive ocular.
above).
4-7

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