Parallax

TM 9-258

impression registered upon the retina and the optic nerve

baseline of the range finder forms the base of a triangle

remaining for about 1/15 second after the stimulus has

and its apex is the target (fig. 8-28). Corresponds to the

been removed.

angle of convergence in stereoscopic vision (fig. 3-8 and

Perspective - Appearance in terms of distance.

3-9).

Parallax - Any apparent displacement of an object

Example, railroad tracks which appear to converge as

due to the observer's change of position such as in A,

they are seen receding in the distance. Appearance in

figure 2-72, where the observer will see either the pole

relief, in three dimensions.

Phosphorescence - The glowing or giving off of light

number 4, or pole 7 positioned opposite the mountain, if

he changes his position. This illusion of a relative

without sensible heat; the property to shine in the dark

displacement of the poles with respect to the mountain is

due to emission of light caused by chemical reaction, as

a deception, inasmuch as neither the poles nor the

fireflies.

Photon - A quantum (elemental unit) of radiant

mountain actually move.

Optically, parallax in a

telescope with a reticle is any apparent movement of the

energy.

reticle in relation to distant objects in the field of view

Plane - A surface which has no curvature; a perfectly

caused by movement of the head of the observer. This

flat surface.

condition exists when the image in the telescope lies in

Planoconcave lens - A lens with one surface plane;

one focal plane and the reticle lies in another

the other concave.

(B, fig 2-72).

Planoconvex lens - A lens with one surface plane;

Paraxial pencil - A narrow group of light rays along

the other convex.

the axis.

Plumbline - A vertical line. When used for adjusting

Paraxial ray - Any ray parallel to the axial ray in a

instruments, it is a weight (plumb bob) hanging on a

pencil of light. A ray in the immediate neighborhood of

string.

the optical axis of a lens or mirror or close to its center.

Plus lens- A converging (convex) lens.

Patina - A film. Surface mellowing or softening. A

Point of fixation - The object on which the optical

film or oxide formed on copper and bronze.

instrument is focused; an object on which the observer's

Pechan or Z-Rotating prism - Used as a rotating

eye is concentrated.

prism. It is used to invert the image in one plane without

Point of principal focus - The point to which parallel

deviating or displacing the axis of the rays of light. Used

rays of light converge or from which they diverge when

as the rotating prism in the conventional type of optical

they have been acted upon by a lens or mirror (figs 2-19,

system of panoramic telescopes (B, fig 4-13).

2-44, and 2-46). A lens has two points of principal focus,

Pencil of light - A narrow group of light rays coming

one on each side of the lens. A lens has many focal

from a point source or converging toward a point. A

points depending upon the distance of the object from

pinhole opening produces a pencil of rays.

the lens (fig 2-47).

Pentaprism - A five-sided prism used to bend light

Polarized light - In optics, the light which vibrates in

through a constant angle, usually 90 degrees, without

one direction only (in a single plane).

producing inversion or reversion if reflection takes place

Plane of polarization is the plane in which the polarized

in the vertical or horizontal plane (fig.4-14).

light vibrates. Light is polarized by natural materials

Peripheral - On the circumference.

Near the

(Iceland spar) or a manufactured product known as

boundary or edge of the field of an optical system; the

"Polaroid" or when being reflected, at a glancing angle

outer fringe.

from a smooth surface.

Periscope - An optical instrument designed to

Polarizing filter - A light ray filter which polarizes the

displace the line of sight in a vertical direction usually

light passing through an instrument.

upwards. Used to permit observation over the top of a

Porro prism - One of two identical prisms (fig B-24)

fortification, a barricade, or out of a tank (fig 8-19).

used in the Porro prism erecting system (fig 4-9). It is a

Persistence of vision - The mental effect of an

right angle prism with the corners rounded to minimize

breakage and simplify assembly. See porro prism

erecting system (fig 4-9).

B-24