Figure B-12. Drift.

TM 9-258

two telescopes not being parallel or not converging to a

cemented together (fig 4-3).

Dove prism - Used as a rotating prism. A form of

point. In minor cases, the eyes will adjust themselves to

compensate for the error of the instrument until the

prism designed by H. W. Dove. It is used to invert the

images are superimposed and only one object is seen.

image in one plane without deviating or displacing the

This may cause eyestrain, eye fatigue, and headaches.

axis of the rays of light. Used as the rotating prism in the

b. An eye disorder, known as diplopia, which

conventional type of optical system of panoramic

results in double vision of a single object. It is usually

telescopes (figs 4-13 and 4-22).

Drift - The amount by which a projectile will deviate

caused by one eye failing to converge or diverge in

unison with the other eye.

horizontally from its proper path, due to rotation caused

Doublet - A compound lens consisting of two

by the rifling in the bore of the gun, and reaction of the air

elements with inner surfaces curved identically and

(fig B-12).

Figure B-12. Drift.

Dynameter- A Ramsden dynameter is a small

contrasted with the entering or incident ray (fig 2-24).

Emmetropia - Normal refractive condition of the

eyepiece or magnifier equipped with a reticle scale and

eyes. A normal eye is termed emmetropic.

used in the precise measurement of the exit pupil and

End correction window - See correction window.

eye distance (eye relief) of other optical instruments.

Eccentric mounting - A type of lens mounting

End play - Movement of a shaft along its axis. A

consisting of eccentric rings which may be rotated to shift

type of lost motion common to worm and wormwheel

the axis of the lens to a prescribed position (fig 4-36).

assemblies. The error lies in looseness in the bearings

Eccentric ray - One of the rim rays which pass

at the ends of the shaft or in the ball cap and socket.

through a lens remote from its center.

The result is that the worm can be rotated a small

Effective aperture of objective - See aperture of

amount without causing rotation of the wormwheel.

Entrance pupil - The clear aperture of the objective.

objective.

Elbow telescope - A refracting optical instrument for

Equilibrium, equipoise - Muscular balance of the

viewing objects in which the line of sight is bent 90

eyes.

Erect - To change an image from an inverted to a

degrees by means of a prism.

Electromagnetic spectrum - A chart or graph

normal position. To both revert and invert.

Erect image - The image produced by an optical

showing the relation of all known electromagnetic wave

forms classified by wavelength.

The visible light

system which is seen with its upper part up. The normal

spectrum occupies an extremely minute portion of the

erect image appears as it is seen by/the normal eye.

The reverted erect image is seen with the right side on

electromagnetic spectrum (fig 2-3).

Elementary lens equation - A law giving the

the left.

Erect type - The image produced by one type of

quantitative relation between the distance of the object,

the image, and the principal focus of the lens (para 2-

coincidence range finder in which the image appears

24.e).

Emergent ray - In optics, the term applied to a ray of

light leaving an optically dense medium as

B-12