Figure B-26. Protractor .

TM 9-258

operators. It will be exceeded as frequently as it is not.

semicircle, sometimes containing a radial arm (fig B-26).

Protractor An instrument for measurng or laying off

The graduations can be Artillery mils, Navy mils, or

degrees. See mil.

angles. It consists of a graduated arc or

Figure B-26. Protractor .

Pupil - The dark center of the eye. It is the aperture

triangulation which is performed mechanically and

optically.

through which light enters the eye.

Range quadrant - An instrument used to set range or

Quadrant -

measure vertical angles of elevation in laying a gun.

1. A quarter of a circle: a sector, arc, or angle

Ray - See light ray.

of 90 degrees.

Ray filter - See filters.

2. An instrument for measuring or setting

vertical angles such as a gunner's quadrant or a range

Real image - See image.

quadrant.

Rectilinear - In a straight line. When applied to a

Quadrant angle of elevation - The angle between the

lens, it indicates that images of straight lines produced by

horizontal and the line of elevation or axis or bore

the lens are not distorted.

(fig. B-3). See angle of elevation and angle of site.

Rectilinear propagation - Straight line travel; refers to

Quanta or Quantum - See photon.

the fact that light travels in a straight line when traveling

Radian - An angle included within an arc equal in

through a medium of constant optical density.

length to the radius of a circle. It is equal to 57, 17

Reduction - The decrease in the apparent size of an

minutes, and 44.8 seconds.

object produced by an optical element or instrument.

Radiant energy - Energy that is radiated or thrown off

Reflected ray - The ray of light leaving a reflecting

in all directions by its source. Light is a form of radiant

surface representing the path of light after reflection (fig

energy. Energy, by definition, is capacity for performing

2-13).

work.

Reflection - Light striking a surface and returning or

Ramsden dynameter - See dynameter.

"bouncing back" into the medium whence it came.

Range Distance. - The horizontal distance from a

Regular reflection (figs 2-13 and 2-15) from a plane

gun to its target; the distance from an observer to a

polished surface, such as a mirror, will return the major

designated object.

Range usually is measured in

portion of the light in a definite direction lying in the plane

meters.

of the incident ray and

Range finder - An optical instrument used to

determine the distance of an object or target by

B-26