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TM 9-258
two telescopes not being parallel or not converging to a
cemented together (fig 4-3).
Dove prism - Used as a rotating prism. A form of
point. In minor cases, the eyes will adjust themselves to
compensate for the error of the instrument until the
prism designed by H. W. Dove. It is used to invert the
images are superimposed and only one object is seen.
image in one plane without deviating or displacing the
This may cause eyestrain, eye fatigue, and headaches.
axis of the rays of light. Used as the rotating prism in the
b. An eye disorder, known as diplopia, which
conventional type of optical system of panoramic
results in double vision of a single object. It is usually
Drift - The amount by which a projectile will deviate
caused by one eye failing to converge or diverge in
unison with the other eye.
horizontally from its proper path, due to rotation caused
Doublet - A compound lens consisting of two
by the rifling in the bore of the gun, and reaction of the air
elements with inner surfaces curved identically and
Figure B-12. Drift.
Dynameter-  A Ramsden dynameter is a small
contrasted with the entering or incident ray (fig 2-24).
Emmetropia - Normal refractive condition of the
eyepiece or magnifier equipped with a reticle scale and
eyes. A normal eye is termed emmetropic.
used in the precise measurement of the exit pupil and
End correction window - See correction window.
eye distance (eye relief) of other optical instruments.
Eccentric mounting - A type of lens mounting
End play - Movement of a shaft along its axis. A
consisting of eccentric rings which may be rotated to shift
type of lost motion common to worm and wormwheel
the axis of the lens to a prescribed position (fig 4-36).
assemblies. The error lies in looseness in the bearings
Eccentric ray - One of the rim rays which pass
at the ends of the shaft or in the ball cap and socket.
through a lens remote from its center.
The result is that the worm can be rotated a small
Effective aperture of objective - See aperture of
amount without causing rotation of the wormwheel.
Entrance pupil - The clear aperture of the objective.
objective.
Elbow telescope - A refracting optical instrument for
Equilibrium, equipoise - Muscular balance of the
viewing objects in which the line of sight is bent 90
eyes.
Erect - To change an image from an inverted to a
degrees by means of a prism.
Electromagnetic spectrum - A  chart or graph
normal position. To both revert and invert.
Erect image - The image produced by an optical
showing the relation of all known electromagnetic wave
forms classified by wavelength.
The visible light
system which is seen with its upper part up. The normal
spectrum occupies an extremely minute portion of the
erect image appears as it is seen by/the normal eye.
The reverted erect image is seen with the right side on
electromagnetic spectrum (fig 2-3).
Elementary lens equation - A law giving the
the left.
Erect type - The image produced by one type of
quantitative relation between the distance of the object,
the image, and the principal focus of the lens (para 2-
coincidence range finder in which the image appears
24.e).
Emergent ray - In optics, the term applied to a ray of
light  leaving  an  optically  dense  medium  as
B-12

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