Figure B-20. Linear field

TM 9-258

Line of position - See line of site.

Line of site A straight line extending from a gun or

Line of sight - Line of vision; optical axis of a

position-finding instrument to a point, especially a target

(fig B-3). Usually called line of position.

telescope or other observation instrument. Straight line

connecting the observer with the aiming point; the line

along which the sights are set in a gun, parallel to the

axis of the bore (fig B-6).

Figure B-20. Linear field

Lost motion - Motion of a mechanical part which is

Magnetic-grid declination - The angle between

not transmitted to connected or related parts. It is the

Magnetic North and Grid North (Y-North).

Magnetic needle - A magnetized needle used in a

cumulative result of backlash and end play.

Lumen - The amount of light falling on an area of 1

compass. When freely suspended, it will assume a

square foot, at 1 foot distance from a standard candle.

position parallel to the earth's magnetic lines of force

See standard candle.

which connect the magnetic poles.

Luminescence - Giving off light at a temperature

Magnetic North - The direction of the earth's north

below that of incandescence. The radiating of "cold" light

magnetic pole as indicated by the north-seeking end of a

as seen from fireflies or luminous paint.

magnetic needle.

Macula or yellow spot - A small area of the retina of

Magnification - The increase in the apparent size of

the eye which is highly sensitive to light and which is the

an object produced by an optical element or instrument.

Magnifying power - The ability of a lens, mirror, or

zone of the clearest and most distinct vision except for a

much smaller area at its center known as the fovea or

optical system to make an object appear larger (fig B-

fovea centralis. The macula is about 2 millimeters in

21). If an optical element or optical system makes an

diameter, and contains principally cones and only a few

object appear twice as high and twice as wide, the

rods (fig 3-2).

element or system is said to have a magnification of 2-

Magnesium fluoride - See hard coat.

power. The power of an optical instrument is the

Magnetic azimuth - Azimuth measured from

diameter of the entrance pupil divided by the diameter of

the exit pupil, the focal length of the objective divided by

Magnetic North.

Magnetic declination - The angle between True

the focal length of the eyepiece, or the apparent field of

view divided by the true field of view

North and Magnetic North

B-20