Ciliary body or muscle

TM 9-258

Color blindness - An eye condition of not being

different colors of wavelengths of light to be focused at

different distances from the lens resulting in colored

capable of proper discrimination of colors. In the most

common type, dichromatism, red-green blindness, these

fringes around the borders of objects seen through the

lens (fig 2-60).

two colors appear gray.

Ciliary body or muscle - The muscle in the eyeball

Coma - An aberration of a lens which causes

which is capable of increasing or decreasing the

oblique pencils of light from a point on the object to be

curvature of the crystalline lens to decrease or increase

imaged as a comet-shaped blur instead of a point. This

the focal length of the lens system of the eye. Used in

aberration is caused by unequal refraction through the

accommodation (focusing) of the eye (fig 3-4).

different parts of the lens for rays coming from a point

Circle of least confusion - A focal plane between the

which lies a distance off the axis (fig 2-65).

Compass North (Magnetic North) - The direction

two focal points of a lens affected by astigmatism where

the most clearly defined image can be obtained. See

indicated by the north-seeking end of the needle of a

astigmatism.

magnetic compass. This direction is different usually

Coated optics - Optical elements which have been

from either True North or Y-North (Grid North),

coated with a thin chemical film, such as magnesium

depending upon one's position upon the earth's surface.

Compensator - See measuring wedge.

fluoride, to reduce reflection and thereby increase light

transmission.

Compound lens - A lens composed of two or more

Coincidence - Occupying the same place; agreeing

separate pieces of glass. These component pieces or

as to position; corresponding. In the coincidence range

elements may or may not be cemented together. A

finder, two optical systems are focused into a single

common form of compound lens is a two element

eyepiece assembly. When the range knob is rotated, the

objective; one element being a converging lens of crown

left and right fields of view are superimposed and

glass and the other being a diverging lens of flint glass.

coincidence of the two images of the selected target is

The surfaces of the two elements are ground to eliminate

established.

aberrations which would be present in a single lens (fig

Coincidence prism - A compound prism, consisting

4-3).

Concave - Hollowed to form a shallow cavity and

of a system of small prisms cemented together, used as

an ocular prism assembly in a coincidence range finder

rounded like the inside of a sphere.

to bring the image from the two objectives to a single

Concave lens - See diverging lens.

eyepiece for viewing.

Concave-convex lens - A lens with one concave

Coincidence range finder - A self-contained

surface and one convex surface.

distance measuring device operating on the principle of

Concentric - Having the same center. Circles

triangulation. Images, observed from points of known

differing in radius but inscribed from a single center

distance, are alined to determine the range. See

point.

coincidence.

Cone - One of the two types of light-sensitive

Collective lens - A convex or positive lens used in

elements or visual cells in the retina of the eye which

an optical system to collect the field rays and bend them

permit sight. The cones are associated with daylight

to the next optical element. It prevents loss of rim ray

vision and give clear sharp vision for the seeing of small

light. Sometimes used to denote a convergent or convex

details and the distinguishing of color. The other type of

lens.

visual cell is termed the rod (fig 3-5).

Collimating telescope - A telescope with an outer

Conjugate focal points (conjugate foci) - Those pairs

cylindrical surface that is concentric with its optical axis.

of points on the principal axis of a concave mirror or a

It contains a reticle, usually straight crosslines. It is used

convergent lens so located that light emitted from either

to aline the axis of the optical elements to the

point will be focused at the other as shown in figure B-9

mechanical axis of the instrument (collimation) in other

where D and D- are the conjugate focal lengths of a lens.

telescopes, to focus eyepieces, and to set diopter scales

Shifting the source of light along the axis will cause a

at infinity focus.

shifting of the second corresponding conjugate focal

Collimation - The process of alining the axis of the

point. Likewise related points in object and image are

optical elements to the mechanical axis of an instrument.

located optically so that for every point on the object

Collimator - A sighting instrument occupying a

there is a corresponding point in the image.

position between the open sight and the telescopic

instrument (fig 8-27). It has an eyelens, and reticle of

cross or vertical line pattern.

B-8