Figure 4-36. Eccentric objective mounting.

TM 9-258

caused by sunlight striking the outer face of the

objective, but they protect from extreme heat the

thermosetting cement used to cement the elements of

the objective.

Figure 4-36. Eccentric objective mounting.

b. Prisms were formerly often placed in mountings

designed to hold them firmly and yet afford movement for

exact adjustment. Screws bear upon two faces of the

Figure 4-37. Sunshade and objective cap.

prism to lock it in place in such mountings. By letting out

on one screw and taking up on the other, the prism may

b. Objective caps (fig 4-38) are leather covers

be positioned exactly. Extremely fine adjustments can

which are fitted over the sunshade or objective end of

be made. Care must be exercised in tightening screws

the instrument to protect the objective when the

to insure that prisms will not chip or crack from excessive

instrument is not in use. The caps are usually attached

pressure.

to the instrument by strips of leather to prevent loss.

c. In modern instruments, mirrors and prisms are

4-21. Focusing Devices. The distance between the

bonded to metal brackets or holders. Adjustment of the

reticle and the eyepiece must be adjusted to the

various components is accomplished by the screws

observer's eye so that the reticle and image of the object

bearing against the metal holder.

will be sharply defined and to eliminate eye fatigue. To

4-19. Eyeshields. Eyeshields or eyeguards are fitted

provide this adjustment, the two or more lenses of the

to fire-control instruments to maintain proper eye

eyepiece are mounted in a single tube or lens cell and its

distance and to protect the eyes of the observer from

distance from the reticle (and the focal plane of the

stray light, wind, and injury due to the shock of gun fire or

objective) can be adjusted by a rack and pinion, by a

similar disturbances. Eyeshields are made of rubber,

simple draw tube, or by rotating the entire eyepiece when

plastic, and metal but only those made of soft rubber can

adjusting the diopter scale (fig 4-38) causing it to screw

most effectively meet all of these requirements.

in or out. This is referred to as the diopter movement

4-20. Sunshades and Objective Caps.

(para 5-14). The knurled ring of the "screw out"

a. Sunshades are tubular sections of metal, usually

adjustment type is provided with a scale reading in

with the lower portion cut away, which are fitted into slots

diopters by which this adjustment can be made directly if

around the objective cells of many instruments to protect

the correction required by the eye is known.

against rain and the direct rays of the sun (fig 4-37).

They not only reduce glare which would be

4-36