Object distance

TM 9-258

Object distance - The distance of an object from the

Optical glass - Glass carefully manufactured to

eyes or from an optical system.

obtain purity, transparency, homogeneity, and correct

Objective - The lens in an optical system which

optical properties. See crown glass and flint glass.

Optical properties - In optical glass, those properties

receives light from the field of view and forms the first

which pertain to the effect of the glass upon light such as

image. It is the lens farthest from the eye and nearest

index of refraction, dispersion, homogeneity, and

the object viewed.

Objective cap - A protective device, usually made of

freedom from defects.

Optical surface - A reflecting or refracting surface.

leather, which is placed over the objective end of the

instrument when the instrument is not in 'use (fig 4-37).

Optical system - A number of lenses, or lenses and

Objective prism - A right-angle prism employed in

prisms, or mirrors, so arranged as to refract, or refract

some types of instruments to bend light 90 before it

and reflect light, to perform some definite optical function

enters the objective system (fig 4-22).

(fig B-19).

Oblique Slanting; neither at right angles nor parallel (fig

Optics - That branch of physical science which is

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concerned with the nature and properties of light, related

Ocular -

instruments, and vision.

1. A term sometimes applied to the eyepiece.

Orbit - The socket of the human eye.

2. Pertaining to, connected with, or used for the

Orient - In fire control, to find the proper bearing; to

eyes or eyesight.

fix the position of a gun, with reference to a

Ocular prism - The prism employed in a range finder

predetermined target point.

to bend the line of sight through the instrument into the

Orientation - Determination of the horizontal and

eyepiece.

vertical location of points and the establishment of

Off-carriage instruments - Those instruments used in

orienting lines, adjustment of the azimuth circle of a

artillery which are not mounted on the carriage of the

weapon or instrument to read required azimuths, and

weapon such as the aiming circle, BC periscope, and

adjustment of the vertical quadrant angle of elevation

range finder.

(angle of site plus angle or elevation) to required values.

Offset - See prism offset.

Orthophoria - A term used with reference to the eyes

Oldsightedness - See presbyopia.

indicating that both eyes are equally and properly

On-carriage instruments - Those instruments used in

controlled by eye muscles with the result that, with the

artillery which are mounted directly on the carriage of the

eyes at rest, they focus and converge upon a distant

weapon such as panoramic and elbow telescopes and

object.

Orthoscopic eyepiece - An eyepiece which has been

their associated telescope mounts.

Ophthalmic - Pertaining to the human eye.

corrected for distortion, particularly that type of eyepiece

Optic nerve - Nerve connecting the eye and the optic

having a triple element cemented field lens and a

centers of the brain.

planoconvex or meniscus converging eyelens

Optical - Pertaining to vision and the phenomenon of

(paras 2-24i, 4-4k, and B, fig 4-7). See eyepiece.

Othoscopic lens - A lens which has been corrected

light.

Optical axis - The line formed by the coinciding

for distortion, giving an image in correct or normal

proportions, with a flat field of view (para 2-24i).

principal axes of a series of optical elements comprising

Panoramic telescope - A telescope so designed that

an optical system; in other words, an imaginary line

the image remains erect and the position of the \fs20

passing through the optical centers of the system.

Optical center - Point in a lens on a line connecting

eyepiece is unchanged as the line of sight is pointed in

any horizontal direction.

all focal points, peculiar in that it is a point through which

Parabolic mirror - A concave mirror which has the

the rays of light pass and emerge parallel to their path of

form of a special geometrical surface; a paraboloid of

incidence (fig 2-45). It is in the geometric center of the

revolution. Light rays emanating from a source located

thickest part of a converging (convex) lens and of the

at the focal point of the paraboloid are reflected as

thinnest part of a diverging (concave) lens.

Optical characteristics - Those properties of an

parallel rays. Most searchlights are of this design. All

light rays which are parallel and strike the mirror directly

optical system which it possesses by reason of its optical

from the front are reflected towards the focal point

or visual nature such as field of view, magnification,

(fig 2-21).

brightness or image, and freedom from distortion.

Parallactic angle - In range finding, the angle at the

Optical - element A part of an optical instrument

target subtended by the base length. The

which acts upon the light passing through the instrument

such as a lens, prism, or mirror.

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