Figure B-21. Magnifying power .

TM 9-258

Marginal rays - Rays of light near the edge of a lens.

Micrometer scale - An auxiliary scale used to read

Maser - Acronym for microwave amplification by

small angles or dimensions.

Micromicron - A unit of measure equalling one-

stimulated emission of radiation.

Measuring wedge - An optical element employed in a

millionth part of a micron.

Micromillimeter or millimicron - Terms applied to unit

range finder to deviate or displace light entering the

of measure equal to one-millionth part of a millimeter or

variable angle end of the instrument. One type consists

one-thousandth part of a micron.

of a single, perpendicular wedge which is moved in

Microna - A unit of measure equaling one--millionth

alinement with the path of light to shift or displace the

part of a meter or one-thousandth part of a millimeter.

light (fig 4-18). The other type consists of one or two

Mil (Artillery Mil) - A unit of angular measurement

pairs of circular wedges, each pair being capable of

rotating equally and simultaneously in opposite directions

used in Army military calculations. It is 1/6400th of a

about the axis of the path of light to produce variable

complete circle or very nearly the angle between two

deviation (fig 4-19). The wedges of both types are

lines which will enclose a distance of 1 meter at a range

compound elements corrected for color.

of 1,000 meters or 2 meters at a range of 2,000 meters

Medium (plural, media) - Any substance or space.

(fig 10-2). 17.78 mils is approximately equal to 1 degree.

Meniscus - A lens the surfaces of which are curved

NOTE

in the same direction.

The Navy mil and the French infantry mil

Meniscus converging lens - A lens the surfaces of

enclose exactly 1 meter at a range of 1,000

which are curved in the same direction; one surface is

meters. There are 6283.1853 Navy or French

convex, the other is concave. The convex surface has'

infantry mils in a complete circle.

the greater curvature or power (fig 4-1). Spectacle

Minus lens - A diverging lens. A lens with negative

lenses of this type are used to correct farsightedness,

focal length (focal point toward the object).

hypermetropia, or hyperopia (fig 3-11).

Minute - A unit of measurement of an angle equal to

Meniscus diverging lens - A lens the surfaces of

one-sixtieth of a degree.

which are curved in the same direction; one surface is

Mirror - A smooth, highly polished surface for

convex, the other is concave. The concave surface has

reflecting light. It may be plane or curved. Usually a thin

the greater curvature or power (fig 4-1). Spectacle

coating of silver or aluminum on glass constitutes the

lenses of this type are used to correct nearsightedness,

actual reflecting surface. When this surface is applied to

myopia (fig 3-10).

the front face of the glass, the mirror is termed a front

Micrometer - A mechanism for measuring small

surface mirror.

angles or dimensions. On optical instruments it is the

Monochromatic - Composed of one color.

"fine" scale.

Monocular - Pertaining to vision with one eye.

A term applied to optical instruments requiring the use of

only one eye

B-21