Figure 4-11. Amici or roof-angle prism.

TM 9-258

(2) When employed as the erecting prism of an

elbow telescope (fig 8-11), one of its uses, the light ray

enters One short face (fig 4-11) where it strikes the

reflecting surface of one side of the roof. It is reflected to

the other side of the roof where it is reflected outwardly

at right angles to the direction in which it entered the

prism.

Usually, the parts of the prism not used by light rays are

removed affecting the appearance of the prism but not

its fundamental shape or function.

(3) The Amici prism is compact and it transmits

a great amount of light but is one of the most difficult of

all prisms to manufacture because the angle between

the two inclined faces of the roof cannot differ from 90

by more than 2 seconds if the prism is to function

satisfactorily.

e. Rhomboidal Prism. The Rhomboidal or

Rhomboid prism (fig 4-12) may be considered as made

up of two right-angle reflecting prisms built in one piece,

or as a block of glass with the upper and lower or two

other opposite faces cut at an angle of 450 and parallel

to each other. This prism has two parallel reflecting

surfaces providing two reflections in the same plane and

transmits the image unchanged. It does not invert or

revert the image nor change the direction of the light rays

but displaces the light rays parallel to themselves.

Figure 4-12. Rhomboidal prism.

This is due to double reflection without reversal of the

f. Rotating Dove Prism. The rotating Dove

direction of light. This result is obtained from mirrors in

prism (A, fig 4-13) resembles the rhomboidal prism

certain periscopes (fig 8-19). Rhomboidal prisms are

except that the angles of its ends are opposite to one

employed to shift the lines of sight to secure

another

and

this

prism

serves

interpupillary distance adjustment. It is used in the range

finder.

4-13