French

TM 9-258

l. French. This eyepiece is illustrated in A,

eye, the second eyelens (C, fig 4-8).

Variable

magnification is obtained by changing the distances

figure 4-8. It employs elements substantially resembling

between elements of the eyepiece and the erecting

those of the orthoscopic eyepiece in the reverse order. It

system of the telescope. A collective lens in the body of

was generally indicated on French drawings furnished.

the instrument is often one of the elements of the optical

the United States of America during World War I.

system of such an instrument.

4-5.

Prisms.

General.

(1) A prism, unlike a lens, is bounded by plane

surfaces and can be designed to deviate, displace, and

reflect light in numerous ways. The introductions of

prisms into optical instruments permits design variations

otherwise impossible.

(2) In fire-control instruments, it frequently is

desirable to bend the rays of light through an angle in

order to make a shorter, more compact instrument, to

bring the eyepiece into a more convenient position or to

erect an image. Plane mirrors are sometimes used to

change the angles of the light rays but the silvered

surfaces tend to tarnish and cause a loss of light which

grows more serious as the instrument becomes older. A

prism used for the same purpose can be mounted in a

simpler and more permanent mount, the angles of its

surfaces cannot be disturbed, and it can produce more

numerous reflection paths than would be practical with

mirrors.

(3) Optical prisms are blocks of glass of many

shapes especially designed and ground to permit them

to perform the various functions. They are used singly or

in pairs to change the direction of light from a few

seconds of arc (measuring wedges) to as much as 3600

(Porro prism system).

(4) When the incident rays strike the reflecting

surface of a prism at an angle greater than the critical

angle (para 2-16) of the substance of which the prism is

composed, the reflecting surface does not need to be

silvered. This surface must be silvered, to insure total

reflection, if the incident rays strike it at an angle less

than the critical angle.

b. Right-Angle Reflecting Prism. Depending

upon which reflecting surface is struck by the incident

ray, the right-angle reflecting prism will bend the light

rays through an angle of 900 or 1800. The manner in

which this prism reflects rays through an angle of 900 is

illustrated and described in paragraph 2-16e (A and B, fig

2-36) and through an angle of 1800 in paragraph 2-

Figure 4-8. French, Plossl, and variable magnification

21a(1) (fig 2-43). When used to reflect rays through an

eyepieces.

angle of 900, the rays are reflected only once with the

m. Plossl.

This eyepiece employing an

result that the image is reverted with reflection in a

achromatic doublet for each lens is illustrated in B, figure

horizontal plane or inverted with the reflection in a

4-8. The converging components of the doublets face

vertical plane. When used to reflect rays through an

each other.

angle of 180, the rays are reflected twice and a

n. Eyepiece With Variable Magnification.

These eyepieces generally employ two eyelenses in

addition to the field lens. The forward eyelens is called

the first eyelens and the eyelens nearest the

4-11