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TM 9-254
9-2.
Optical Components - Continued
f.
Mirrors. Mirrors used to reflect the paths of light in fire control instruments are termed front surface mirrors.
The silver coating, on the mirror, is put on the front of the glass instead of on the back of the glass like a common
household mirror. Optical mirrors are sometimes used to reflect light rays in a manner which simulates the same effect
that a prism would have in an optical system (fig. 9-17). Mirrors can also be used in the auto collimations of fire control
test fixtures. Another form of optical mirror, known as a beam splitter (partial reflector) is a flat piece of glass lightly
silvered so that some of the light rays will pass through the glass while others will be reflected off the silvered surface.
Figure 9-17. Using Mirrors to Simulate a Penta Prism
g.
Reticles. Reticles are patterns placed in the focal plane of the objective for a fire control instrument which
appear to the observer to be superimposed upon the field of view (fig. 9-18). They are used as a reference point when
sighting and aiming for the purpose of calculating angular displacement and distance. Examples of some reticles used in
fire control instruments are shown in (fig. 9-19).
9-11
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