True field of view

TM 9-258

through an angle of 1800, any ray of light entering it (fig

projecting point on the axis of a convex or concave

4-15).

mirror or lens.

Triplet - A three-element compound lens with inner

Vertical travel - Movement of the line of sight of an

surfaces curved identically and cemented together

optical instrument or gun in a vertical plane.

Virtual base - The actual base or baseline of a range

(fig 4-3).

True field of view - See field of view.

finder multiplied by the power of magnification of the

True North - Geographic north; the direction along the

instrument.

Virtual image - The image formed by a diverging

geographical meridian of the geographic north pole from

a point on the earth's surface.

lens. It is formed by rays which come from the direction

Unit of correction - An arbitrary scale graduation as

of the image but do not originate in it.

laid off on the correction wedge scale of a range finder.

It cannot be thrown upon a screen. It exits only in the

Unit of error (UOE) - Unit of measurement

mind of the observer (fig 2-50). Similar image formed by

a converging lens if the object is inside one focal length

corresponding to 12 seconds of arc. When checking the

(fig 2-48). Apparent position of reflected point which

accuracy of an observer's readings of range, as

appears to be located behind the mirror (figs 2-40 and

determined by a range finder, it is necessary to have

2-41).

some unit of measure. It has been determined by test

Vision persistence - See persistence of vision.

that the mean error of the normal, well-trained observer

Visual acuity - Acuteness or sharpness of sight.

under good conditions of observation should not exceed

12 seconds of arc for a series of readings, regardless of

It is measured by the ability to distinguish letters of

the range involved. This mean error is taken as the

specified size at a given distance.

Visual purple - A substance surrounding the light-

basis for determining the accuracy of range finder

readings. The unit of error (UOPE) is the unit of

sensitive visual cells in the retina of the eye and

measure arbitrarily selected and all errors in range

stimulating the rods for keener sight under poor light

readings are converted to this unit during analysis.

conditions.

Velocity of light - The speed of light. It is numerically

Vitreous humor - A transparent, jelly-like substance

equal to the frequency multiplied by the wavelength. The

with which the rear and greater portion of the eye is filled

speed of light in vacuum equals 186,330 miles per

and which is part of the refracting mechanism of the eye

second.

(fig 3-2).

Vertex - Highest or lowest, the most or least

Walleye - See strabismus.

Wave - Vibration; a form of movement by which all

radiant energy of the electromagnetic spectrum is

assumed to travel (fig B-29).

Figure B-29. Waves and have fronts .

Wave front - A surface normal (at right angles) to a

plane; for rays diverging from or converging toward a

point, the wave front is spherical (fig B-29).

bundle of rays as they proceed from a source. The wave

Wavelength - Length of a wave measured from any

front passes through those parts of the waves which are

point on one wave to the corresponding point, in the

in the same phase and go in the same direction. For

same phase on the next wave; usually measured

parallel rays, the wave front is a

B-30