Wedge

TM 9-258

from crest to crest (fig 2-9). Wavelength determines the

ten-millionth part of a micron, or one-ten thousand

nature of the various forms of radiant energy which

millionth part of a millimeter.

Yaw - The angle between the axis of the projectile at

comprise the electromagnetic spectrum; it determines

the color of light (fig 2-3).

any moment and the tangent to the trajectory.

Wedge - An optical element with plane inclined

Y-azimuth - The azimuth rading taken from the Y-

surfaces, Usually the faces are inclined toward one

North direction. See Y-North.

Yellow spot - See macula.

another at very small angles. Wedges divert light toward

their thicker portions. They may be circular, oblong, or

Y-North - Grid North.

The north direction as

square. See correction wedge and measuring wedge.

determined by the grid lines on a map. The United

White light - Light, such as sunlight and daylight,

States is laid out in seven grid zones. The central grid

which is composed of all the different wavelengths of the

line of each of these zones points to True North. The

visible spectrum.

other grid lines being parallel to the central grid line will,

Window - A piece of glass with plane parallel

due to the spherical shape of the earth's surface, not lie

surfaces used to admit light into an optical instrument but

in a true northsouth line. The deviation between True

exclude dirt and moisture. Also see correction window.

North and grid north at any point due to this curvature,

Worm - A shaft with a gear in the form of a screw

grid declination will never exceed.39. Arbitrarily selected

which meshes with a worm-wheel. The axes of the

North direction on a drawing or a map, different from

worm and worm-wheel are generally-perpendicular to

True North, usually parallel to or at right angles to some

each other.

objects shown, and used for reference in that area only.

Wormwheel - A gear designed to mesh with a worm.

Z-rotating prism - See Pechan or Z-rotating prism.

X-ray unit - A unit of measure equaling one

B-31