Section III. GENERAL CONSTRUCTION FEATURES

TM 9-258

pounds from getting on the coated surfaces of lenses.

d. Every precaution should be taken to prevent

The removal of such compounds undoubtedly would

inexperienced personnel from misunderstanding the

result in injury to the coated surfaces.

function of the coating and attempting to remove it.

Section III. GENERAL CONSTRUCTION FEATURES

thin edge of the cell (burnishing). A setscrew locks the

4-17.

Lens Cells, Separators, Lens Retaining

retaining ring in position in the lens cell. When two or

Rings, and Adapters.

more lenses are mounted in a cell, the different elements

a. A lens cell (fig 4-35) is a tubular mounting or

are maintaintained at the proper distance from one

frame made of metal, plactic, or hard rubber which holds

another by spacers of separators. The cell mounting

a lens or a number of lenses in the proper position within

permits the entire assembly to be handled and mounted

an optical instrument. The lens usually is secured into

as a unit.

the cell by a lens retaining ring or by turning over a

Figure 4-35. Lens cell, separators, lenses, and retaining ring.

b. Separators or spacers are smooth or threaded

lens or prism. When the center of a lens is moved it can

be made to cause the light to be bent in a different

tubular sections which separate or space the elements of

direction. In cases where extreme exactness is required,

a lens system in the proper relation with one another (fig

proper adjustment of the center of the objective is

4-35). Adjoining faces are usually beveled to fit the faces

accomplishing by mounting it in a pair of eccentric rings

of lenses snugly.

c. Lens retaining rings are generally threaded

(fig 4-36). By rotating the inner ring about in the outer

ring, or by rotating the outer ring in the lens cell, the axis

about their outer diameters and are screwed in over

of the objective may be moved to any point in a relatively

lenses to secure the lenses to their cells.

large area. When properly positioned, the rings may be

d. Adapters may be used to mount an element or

locked in position. Skill is required to set the rings in the

part of smaller diameter into a part of the instrument

proper manner.

body which is of larger diameter.

4-18. Centering Devices.

a. Light always bends toward the thickest part of a

4-35