Double Right-Angle Abbe Prism.

TM 9-258

j. Double Right-Angle Abbe Prism.

The

such slight deviations by means of reflection surfaces.

Wedges are used in the measurement systems of range

double right-angle Abbe prism consists of two right-angle

finders and to correct or adjust the alinement of paths of

prisms joined as shown in figure 4-16. This prism

light. They are of two general shapes--rectangular and

reflects light twice at right angles to the direction of the

incident ray. The image is rotated 90ƒ two times.

round.

b. The angle at which a wedge will deviate the

path of light is dependent upon the relative slant of its

two faces which are inclined toward each other like the

surfaces of a common house shingle. Some wedges

used in fire-control instruments appear to be plates or

disks of glass with parallel surfaces, because the angle

between the surfaces is so slight it cannot be detected

except by careful examination or by actual

measurement.

c. Because deviation of rays of light by wedges

is produced by refraction instead of reflection, a certain

amount of dispersion or separation of colors results from

the use of wedges. For this reason achromatic wedges,

composed of two different kinds of glass to neutralize the

dispersion, must be used where the angles through

which the rays are bent are relatively large or where the

work performed by the wedges is of the most exacting

nature.

d. All wedges cause a certain deviation of the

path of light.

Instruments employing wedges are

sometimes designed so that the path of light entering the

wedge has a certain amount of initial deviation which is

termed the constant deviation. The constant deviation

makes it possible for the wedge to neutralize the

deviation in the path of light or divert the light at a minus

angle.

e. A wedge may be caused to change the path

of light in various directions by rotation of the wedge (fig

4-17). The extent to which a wedge may be made to

divert the path of light may be varied, likewise, by

changing the position of the wedge with relation to the

other elements of the optical system (fig 4-18).

Figure 4-16. Double right-angle Abbe prism.

4-6.

Wedges.

a. The optical wedges used in fire-control

instruments are prisms with two plane surfaces at slight

angles which divert the paths of light through small

angles by refraction instead of by reflection. They are

used where the angle of deviation required may be a

matter of fractions of seconds and it would not be

Figure 4-17. Rotation of wedge changes direction of

practical to produce

path of light.

4-19