Standard letters

TM 9-258

Standard letters - Letters of specific size which are

about by improper cooling or annealing during

manufacture of the glass.

viewed at specified distance (20 feet) to test the

Straie - Localized variations in the index of refraction

keenness of sight.

Stereoacuity - Keenness or sharpness of sight in

of a piece of optical glass, usually occurring in streaks. It

is caused by improper mixing of the ingredients during

discerning depth or three dimensions.

Stereovision See stereoscopic vision.

manufacture.

Sunshade - A tubular projection provided to protect

Stereogram - A card containing two pictures taken

the objective from the direct rays of the sun (fig 4-37).

from different angles (corresponding to the spacing

Surface plate - A plate having a very accurate plane

between the pupils of the eyes), which when viewed

surface used for testing other surfaces or to provide a

through a stereoscope present a single image with the

true surface for accurately locating testing fixtures. It is

illusion of depth or three dimensions.

Stereopsis - See stereoscopic vision.

usually mounted on three adjustable legs so that it can

be accurately leveled in a horizontal plane.

Stereoscope - A binocular instrument used to view

Tangent - A straight line which touches a curve or a

stereograms. See stereogram.

curved surface but does not cut in.

Stereoscopic contact - A term applied to the action of

Telescope - An optical instrument containing a

bringing the target into the same apparent distance plane

system of lenses, usually with magnifying power, which

as the central measuring mark (pip) of the reticle in the

renders distant objects more clearly visible by enlarging

use of a stereoscopic range finder.

their images on the retina of the eye, or which

See stereoscopic range finder.

superimposes a reticle pattern on the image of the object

Stereoscopic effect - The sense of depth, relief, or

to assist in aiming or in the gaging of distance (figs 5-6,

solidity resulting when an object is viewed by both eyes,

6-8, 5-9, and 5-13).

due to the fact that each eye views the object from a

Thermoplastic cement - Cement capable of being

slightly different angle (fig 3-7).

repeatedly softened by increase of temperature and

Similar effect produced when viewing a stereogram.

hardened by lowering of temperature, such as resin

See stereogram.

derived from Canada balsam. It is used because it has

Stereoscopic power - The gain in stereoscopic effect

approximately the same index of refraction as glass.

afforded by a magnifying binocular instrument as

See Canada balsam.

compared with the ability of the naked eyes. This power

Thermosetting cement - A clear, colorless cement

will vary with the separation of the objectives and the

used in cementing optical elements together, particularly

power of the instrument.

the components of a compound lens. It is used because

Stereoscopic range finder - A self-contained distance

it has approximately the same index of refraction as

measuring device operating on the principle of

glass. It changes into a substantially infusible product

triangulation, Images, observed from points of known

when cured under application of heat.

distance by the eyes, result in a sense of depth, and are

Toroidal (toric) lens - A lens having for one of its

made to appear in the same distance plane as the

surfaces a segment of a tore (the surface described by a

central measuring mark (pip) of the stereo reticle. See

circle or a curve rotating about a straight line in its own

stereoscopic contact.

plane). Used in spectacles to correct astigmatism of the

Stereoscopic vision - Vision in depth or three

eyes. See astigmatism.

dimensions due to the spacing of the eyes. This spacing

Total internal reflection - Reflection which takes

of the eyes permits them to see objects from slightly

place within a substance because the angle of indidence

different angles. An impression of the shape, depth, and

of light striking the boundary surface is in excess of the

position of an object with relation to other objects is

critical angle. See critical angle.

received when the brain fuses the two separate pictures

Tourmaline - A mineral that has the faculty of

seen by both eyes into a single image (fig 3-7).

Strabismus - Affection of the eye in which the lines of

polarizing light.

Trajectory - The curved path through the air

sight cannot be directed to the same object at once

described by a projectile in its flight (fig B-3).

because of abnormal contraction, relaxation of, or injury

Traverse - Movement in azimuth; rotation in a

to one or more of the muscles controlling the movement

horizontal plane. See azimuth.

of the eyeball. A condition of the eyes in which the lines

of sight tend to turn inwardly is known as convergent

Triple mirror prism - A prism with four triangular

strabismus, esophoria, crosseye, or squint. A condition

surfaces arranged like two triangular roofs placed at right

angles to one another. Three of the four surfaces are

of the eyes in which the lines of sight tend to turn

outward is known as divergent strabismus, exophoria, or

used as mirrors in reflecting light. This prism has the

property of deviating,

wall eye.

Strain - Distortion or fault in optical glass caused by

internal stress or tension and brought

B-29