Figure B-8. Centers of curvature

TM 9-258

Case I pointing (or firing) - Direct pointing, laying, or

Blind spot - The part of retina, inner coat of eyeball,

not sensitive to light where the optic nerve enters.

fire; gun pointing in which direction and elevation are set

Bore sight - A sighting device consisting of a breech

with sight of telescope pointed at the target.

Case II pointing (or firing) - Combined direct and

element and a muzzle element which, inserted in a gun,

indirect pointing, laying, or fire; gun pointing in which

is used to determine the axis of the bore and the

direction is set with a sight or telescope pointed at the

alinement of other sighting equipment with the axis of the

target and the elevation with an elevation quadrant,

bore. A bore sight may consist of a metal disk with a

range quadrant, or range disk.

peephole for the breech and a pair of crosslines for the

Case III pointing (or firing) - Indirect pointing, laying,

muzzle (fig B-6).

Boresight - To adjust the line of sight of the sighting

or fire; gun pointing in which direction is set with an

instrument of a gun to the axis of the bore (fig B-6).

azimuth circle or with a sight or telescope pointed at an

Brightness of image - A term used to denote the

aiming point other than the target; the elevation is set

with an elevation quadrant, range quadrant, or range

amount of light transmitted by an optical system to give

disk.

definition to the image seen by the observer.

Cataract - A diseased condition of the human eye in

Burnishing - The process of turning a thin edge of

which the cornea or crystalline lens becomes opaque

metal over the edge of a lens to hold it in place in its cell

resulting in blindness.

(fig B-7). This eliminates the use of a retaining ring.

Cell - A tubular mounting used to hold a lens in its

Burnished optics are usually procured as assemblies,

proper position. The lens may be held in the cell by

inasmuch as it is difficult to replace their component

burnishing or by a retaining ring.

parts.

Center of curvature - The center of the sphere of

which the surface of a lens or mirror forms a part. Each

curved surface of a lens has a center of curvature

(fig B-8). The curved surfaces may be convex or

concave.

Figure B-7. Burnishing

Figure B-8. Centers of curvature

Centric Pencil - Oblique pencil or cone of light which

Calcite - See Iceland spar.

Canada balsam - A clear, practically colorless sap

passes through the center of a lens at a considerable

angle to the principal axis.

of fir, solidifying to a transparent resin used in cementing

Choroid - The opaque middle coat or tunic of the

optical elements together particularly the components of

human eye. It is a deep purple layer made up of blood

a compound lens.

It is used because it has

vessels; it supplies nourishment to the eye tissues and

approximately the same index of refraction as glass.

Candlepower - A unit of measure of the brightness

shuts out external light (fig 3-2).

Chromatic aberration - An aberration (deviation from

or power of any light source. One candlepower is the

normal) of a lens which causes

rate at which a standard candle, initially made of sperm

whale oil, 7/8 inch in diameter, burning at the rate of 120

grains per hour, emits light. Current standards of

candlepower are electric lamps.

B-7