Converging meniscus lens

TM 9-258

Converging meniscus lens - See meniscus

applies to the process whereby the rods of the retina

take over the major portion of the act of seeing.

converging lens.

Definition - Sharpness of focus: distinctness of

Convex - Rounded and bulging outwardly as the

image.

outer surface of a sphere.

Deflection - A turning aside from a straight course.

Convex lens - See converging lens.

Convexo-concave lens - A lens with one convex

The horizontal clockwise angle from the line of fire to the

line of sight to the aiming point. A small horizontal

and one concave surface.

(traverse) angle by which a gun is aimed slightly away

Cornea - The transparent, slightly bulging front

from its target to allow for factors such as wind and drift.

surface of the eyeball through which all light enters

See drift.

(fig 3-2). It serves as one of the lenses of the eye.

Degree - A unit of measurement equal to the angle

Corrected lens - A compound lens, the various

between radii subtended by 1/360 of a circle.

surfaces of which have been so designed with respect to

Depth perception - The ability to see in depth or

each other that the lens is reasonably free from one or

three dimensions. In addition to stereoscopic vision, light

more aberrations.

Correction wedge - In range finders a rotating

and shade, perspective, convergence of the visual axes

of the two eyes, one object or part concealing another,

wedge-shaped element used to precisely divert the line

atmospheric haze, and apparent size are factors which

of sight to correct errors in the optical system caused by

aid depth perception.

temperature variations. In range finders, serves to

Deviation - Turning aside from a course; deflection;

supply a measuring factor correction for the constant

difference. In optics, the bending of light from its path.

deviation given one of the telescopes. The measuring

In gunfire, the distance from a point or center of impact

factor correction is read on a scale.

Correction window - End correction window. These

to the center of a target.

Dialyte - A type of compound lens in which the inner

are optical wedges of very small angles. They admit

surfaces of the two elements are ground to different

light, seal out dirt and moisture, and are so mounted that

curvatures to correct for aberrations and the dissimilar

they may be rotated to compensate for the accumulated

faces cannot be cemented together (fig 4-3). Gauss

errors in the entire system. Two are used as end

objective (para 4-3b).

windows on some range finders.

Critical angle - That angle at which light, about to

Diaphragm - A flanged or plain ring with a limited

pass from a medium of greater optical density, is

aperture placed in an optical system at any of several

refracted along the surface of the denser medium (fig 2-

points to cut off marginal rays of light not essential to the

field of view. Diaphragms are used as: field stops, to

34). When this angle is exceeded, the light is reflected

reduce aberrations; aperture stops, to limit the aperture

back into the denser medium. The critical angle varies

with the index of refraction of the substance or medium.

or light-gathering power of the instrument; and antiglare

Cross-eye - See strabismus.

diaphragms, to eliminate reflections from the sides of the

Crown glass - One of the two principal types of

tube and consequent glare in the field of view. Lens cells

optical glass; the other type is flint glass. Crown glass is

or the sides of the tube may act as diaphragms. The

harder than flint glass, has a lower index of refraction,

rays eliminated by diaphragms are those which would

and lower dispersion. See compound lens.

cause aberrations or glare and ghost images by

Crystalline lens - The flexible inner lens of the eye

reflection inside the instrument.

Dichromatism - See color blindness.

which provides accommodation for sharply focusing near

Diffusion - The scattering of light by reflection or

and distant objects (fig 3-2).

Curvature of field - See curvature of image.

transmission. Diffuse reflection and transmission result

Curvature of image - An aberration of a lens which

when light strikes an irregular surface such as a frosted

causes an image to be focused in a curved plane instead

window or the surface of a frosted light bulb. When light

of a flat one (A, fig 2-68).

is diffused, no definite image is formed.

Cylindrical lens - A lens having for one of its

Diopter - A unit of optical measurement which

surfaces a segment of a cylinder. Lenses ground in this

expresses the refractive power of a lens or prism. In a

manner are used in spectacles to correct astigmatism of

lens or lens system, it is equivalent to the reciprocal of

the eyes and in some of the coincidence range finders to

the focal length in meters. For example, if a lens has a

cause astigmatism by stretching points of light into lines

focal length of 25 centimeters, this figure converted into

of light.

meters would equal 1/4 meter. In as much as the

Dark-adapted - A term applied to the adjustment of

reciprocal of 1/4 equals 1 divided by 1 which equals 4,

such a lens would be said to have

the visual cells of the retina of the eye for better vision

under

conditions

poor

lighting.

B-10