Figure 4-15. Triple mirror prism.

TM 9-258

(2) In the obsolete long base height finders, the

pentaprism that would be required would be so large that

it would be difficult to find blocks of glass sufficiently

large and homogeneous for the construction of such

prisms. Two mirrors were used to replace the two

reflecting surfaces of a pentaprism (C, fig 4-14). These

mirrors were fastened in a rigid mount at precisely the

same angles as the reflecting surfaces of the pentaprism

of the same size and produced the same effect. The

effects of temperature changes were eliminated insofar

as possible to hold the reflecting surfaces in the proper

relation to each other.

(3) This prism is known as a constant deviating

prism because it deviates light constantly 90ƒ regardless

of slight deviation of incident rays from the perpendicular.

i. Triple Mirror Prism. The triple mirror prism

(fig 4-15) has four triangular faces, any three of which

can be reflecting surfaces. It may be considered as

consisting of two triangular roofs, one above and one

below, with the crests of the roofs at right angles to one

another. This prism has the unique property of deviation,

through an angle of 180ƒ, any ray of light entering it. If

the angles of this prism are accurately made, incident

light will be returned back along a course parallel to its

original path. It produces an inversion of the image.

Figure 4-15. Triple mirror prism.

4-18