Figure 4-19. Principles of rotating measuring wedges.

TM 9-258

length in the optical system is a distinct advantage as in

4-7. Erecting Systems.

periscopes. This system also is employed in instruments

a. General.

with variable magnification because different degrees of

(1) The image generally is reverted and

magnification can be obtained by changing the relative

inverted by the objective.

positions of the lenses.

(2) The function of the erecting system is to

(2) Where compactness of instrument is a

bring the image upright and correct reversion.

desirable feature, a prism erecting system is used. Such

b. Types. Erecting systems are of two general

a system may consist of a single element or several.

type, lens and prism. While most lens erecting systems

The great reduction of length resulting from the use of a

are of the same general design, differing only in the

prism system is due to the doubling back of the path of

types of lenses used and the spacing of the elements,

light. The prism erecting systems in most general use

the prism erecting systems employ elements of a wide

employ Porro and Abbe prisms, the Amici or roof prism,

variety of shapes causing the rays of light to be diverted

and single and double right-angle prisms.

in different paths in the different systems.

c. Lense Erecting Systems.

(1) A lens erecting system greatly increases

(1) The simplest form of lens erecting system

the length of an instrument. It is often used where

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