Figure B-24. Porro prism .

2. In a telescope, power is the number of

times the instrument megnifies the object viewed. For

example, with a 6-power instrument, an object 600 yards

away is enlarged six times or it appears as it would to the

naked eye if it were at a distance of only 100 yards.

Presbyopia - A defect of vision due to advancing

age, "oldsightedness". The crystalline lens no longer is

capable of accommodating for nearby objects and these

objects cannot be seen distinctly. It is corrected by the

use of convex lenses.

Principal axis - A straight line about which an optical

element or system is symmetrical. A straight line

connecting the centers of curvature of the two refracting

surfaces of a double convex lens. In a mechanical

sense, a line joining the centers of the two surfaces of a

lens as it is placed in the mount.

Principal focal plane - Plane perpendicular to the

axis through the point of principal focus (figs 2-44 and

2-46).

Principal focal point - See point of principal focus.

Principal focus - See point of principal focus.

Figure B-24. Porro prism .

Prism - A transparent body with at least two polished

plane faces inclined toward each other from which light is

Porro prisms erecting system A prism erecting

reflected or through which light is refracted. When light

system, designed by M. Porro, in which there are four

is refracted by a prism, it is deviated or bent toward the

reflections to completely erect the image. Two Porro

thicker part of the prism (fig B-25). See Abbe prism

prisms are employed (fig 4-9). The line of sight is bent

erecting system, Amici prism, coincidence prism, Dove

through 3600, is displaced, but is not deviated.

prism, objective prism, Pechan prism, pentaprism, Porro

Positive lens - See converging lens.

prism, rhomboidal prism, right-angle prism, and triple

Power -

mirror.

1. In a prism or a lens, power is a measure of

ability to bend or refract light. It is usually measured in

diopters. See diopter and prism diopter.

Figure B-25. Prism .

Prism diopter A unit of measure of the refracting

mounting of the prism erecting system in the body of the

instrument.

power of a prism. One diopter is the power of a prism to

Prismatic - Pertaining to a prism or the effects

deviate a ray of light by one centimeter at a distance of

produced by prisms.

one meter from the prism.

Prism erecting system - See erecting system.

Probable error - An allowance based on average

Prism offset - The term applied to certain telescopes

errors made by a large group of range finder

having a characteristic offset due to the

B-25