Focusing sleeve

TM 9-258

usually carries a diopter scale to permit presetting of the

graduated arc, used in range adjustment. It measures

the angle of elevation of the gun.

instrument (fig 4-38).

Halving line - The line which divides the two half

Focusing sleeve - A knurled sleeve which is rotated

images in a coincidence type range finder. The two

to shift the positions of the erectors with relation to the

halves of the images produced by the two objectives of

objective and eyepiece to focus the instrument or to

the instrument must be brought to a point where they

change its magnification.

Foot-candles - A unit of measurement of the intensity

match or coincide above and below the halving line.

Hard coat - The term applied to the coating of

of illumination. A foot-candle is the amount of light

magnesium fluoride on coated optics to differentiate

intensity received on a surface 1 foot away from a light

between other softer coatings which are less durable.

source of one candle power, such as a standard candle

Height of image adjuster - A glass plate with plane

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

surfaces which is tipped one way or the other in the line

rate of 120 grains per hour.

Fovea or fovea - centralis A tiny area of cones in the

of sight in one of the internal telescopes of a range

finder. It deviates the light slightly upward or downward

center of the macula or the yellow spot of the retina of

to adjust the image of one eyepiece to the same height

the eye. It is the area of the eye responsible for the

as the image visible in the other eyepiece.

clearest vision. It is about 0.25 millimeter in diameter,

Heterophoria or muscular imbalance - Any

and contains cones only.

Freedom from defects - Optical glass which is

disturbance of the power, strength, or nervous system of

the eye muscles which does not permit both eyes to

homogeneous and free from bubbles or striae (localized

function together in a normal way. It may occur in any of

variations in the index of refraction).

Freedom from distortion - An image that is a true

a number of different forms such as esophoria in which

the lines of sight tend to turn inwardly, esophoria in which

reproduction of the object.

they tend to turn outwardly, and hyperphoria in which the

Frequency - In light or other wave motion, the

line of sight of one eye tends to be above that of the

number of crests of waves that pass a fixed point in a

other.

given unit of time, usually 1 second.

Homogeneity - Being uniform throughout.

Front surface mirror - An optical mirror on which the

Homogeneous - Uniform throughout; composed of

reflecting coating is applied to the front surface of the

similar parts.

mirror insead of to the back.

Horizontal travel - Rotation of an instrument (or the

Fuse - In stereoscopic vision, the action of seeing as

line of sight of an optical instrument) in a horizontal

one image the images seen by the two eyes.

plane; traverse; movement in azimuth.

Fusion - The mental blending of the right and left eye

Hypermetropia - An eye defect in which the image

images into a single, clear image by stereoscopic action.

tends to be focused beyond the retina with the result that

Galilean telescope - One of the first telescope

the image is blurred and indistinct, farsightedness. It is

models, utilizing a positive objective lens and diverging

caused by the refracting surfaces of the eye being too

eyelens (figs 5-13 and 5-14).

slightly curved, by the eye being of insufficient depth, or

Gauss - objective See dialyte.

by the decline in the power of accommodation in viewing

Gear train - Two or more gears meshed together so

nearby objects. It is corrected by a converging lens (fig

that rotation of one causes rotation of the others.

3-11).

Geometrical center - As applied to lenses, the

Hyperopia - See hypermetropia.

physical center of the lens as determined by

Hyperphoria - A condition of the eyes in which the

measurement; sometimes referred to as the mechanical

line of sight of one eye tends to be above that of the

center to distinguish it from the optical center.

other.

Ghost image - A hazy second image caused by

Hyperphoric displacement - Action of the eyes necessary

reflection sometimes seen in observing through a

to obtain fusion in observing through a stereoscopic or

telescope.

binocular instrument when the image seen by one eye is

Gimbal joint - A mechanism which permits rotation

above that seen by the other eye.

around two perpendicular axes or for suspending an

Iceland splar, calcite - A mineral that has the faculty

object so that it will remain vertical or level when its

of polarizing light.

support is tipped.

Illuminated - Lighted by another source than

Grid declination - The angle between True North

and Y-North.

Grid North - See Y-North.

Gunner's quadrant - An instrument with a

B-16