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TM 9-258
b. When testing lenses for accuracy of grinding and
elements have been coated produces brighter, effective
images.
polishing, a test lens having the desired curvature is
used. The test lens is placed in contact with the lens to
be tested, care being taken that both surfaces are
2-46.  Parallax.
perfectly clean and dry.  When the lenses have been
a. Parallax is apparent displacement of one object
squeezed together, if the lens being tested is perfect, the
with respect to another due to the observer's change of
air film between the two lenses will be of uniform
position (A, fig 2-72).
thickness and the color will be uniform all over the
b. Parallax in a telescope sight is observed as a
surface. Irregular bands or patches of color show that
relative displacement (apparent movement) between the
the surface of the tested lens is not perfect and point out
reticle and the field of view. Parallax is present when the
clearly the parts of the lens needing attention.
reticle is displaced from the image plane; in other words,
it is present when the image is not formed at the reticle
2-45.  Light Loss.
but is located either in front of or behind the reticle as in
a. Whenever light rays strike the surface of any
B, figure 2-72.  Parallax can be eliminated either by
lens or prism, a certain amount of light is lost by
moving the objective lens, thus shifting the image, or by
absorption and reflection.  Light is absorbed by every
moving the reticle to coincide with the image. In a high-
element it strikes or travels through. The more elements
power instrument (above 4X), it is necessary to provide
in the optical system, the more light is lost by absorption
means of moving the objective to eliminate parallax if the
and reflection.
instrument is to be used at all ranges, for example, from
b. Light loss by reflection from surfaces has been
50 feet to infinity. The engineer's transit has such an
objective-focusing device as do all target-type telescopic
greatly reduced by coating the surfaces of elements
sights used on rifles.
which are intended only to refract light (paras 4-13 thru
4-16). The additional light transmitted by instruments in
which various
2-55

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