Section V. OPTICAL INSTRUMENTS AND THE EYES

TM 9-258

Section V. OPTICAL INSTRUMENTS AND THE EYES

siderations, when the eyes are seriously affected by

3-12. General. Optical fire-control instruments may be

astigmatism, the observer should not remove his

placed in two general classifications: monocular, for the

spectacles as no compensating adjusting can be

use of one eye and binocular, for the use of both eyes.

secured by focusing the eyepiece of the instrument.

Both types require certain adjustments to accommodate

d. Eyeshields. The proper fit and use of the

them to the eye as the use of any optical instruments

eyeshields should be such that it excludes stray light

affects the functioning of the eye itself.

from the eyes. This is particularly important in night

observation and under conditions of poor illumination so

3-13. Accomodation of Optical Instruments to the

that the pupil may dilate as much as possible. If a

Eyes.

monocular instrument is used, the eye not is use should

a. Conditions for Clear Vision. In order that the

also be shielded from light; if it receives too much light,

eye may function properly in the use of a monocular

the pupil of the eye in use will contract sympathetically.

instrument, the instrument must be focused to permit

Rubber shields at the eyepiece make the most effective

light entering the instrument from an object to form a

seal. Any light used to illuminate the reticle must be held

distinct image on the retina without undue effort on the

to a bare minimum.

part of the eyecontrolling muscles of the observer. The

e. Interpupillary Adjustment.

A binocular

exit pupil (rear opening of the eyepiece) must be

instrument must be adjusted to the eye spacing or

sufficiently large to let the maximum amount of light

interpupillary distance of the observer. If this is not done,

enter the pupil of the eye. Stray light must be kept out of

the lines of sight of both eyes cannot traverse the most

the eye. In addition, the use of a binocular instrument

effective optical paths of the instrument; the observer will

demands that the two optical systems of the unit be

not have full binocular vision, nor view the most distinct

properly alined with each other and conform to the

images, nor be able to seal out unwanted light from the

interpupillary distance of the individual.

eyepieces. Practically all instruments of this type are

b. Focusing.

Precise focusing of the

equipped with means for interpupillary adjustment. They

instrument permits light from the object to form a distinct

are marked with calibrations for such adjustment. The

image on the retina. Focusing changes the position of

observer should determine his interpupillary distance

the eyepiece with relation to the focal plane of the

(para 2-35), memorize it, and adjust any such instrument

objective and the angles at which the light rays are

he may use to conform to the spacing between the pupils

brought to a focus. The eyepieces of focusing-type

of his own eyes.

telescopes generally are designed to accommodate the

refracting qualities of the eyes of individual observers. If

3-14. Eye Tension or Fatigue

the instrument is used consistently by an individual, the

a. Blinking of the eyes is an automatic process

eyepiece setting can be memorized to expedite the

resulting from eye tension or fatigue. It will occur in the

focusing of the instrument. Low-power telescopes have

use of optical instruments because the eyes cannot

a wider range of accommodation without adjustments

focus steadily very long without relaxing. It is muscular

than high-power telescopes. Telescopes in which the

rather than retinal, and is least apparent when the eye is

magnifying power is 4X or less have a sufficiently wide

relaxed as when accommodated for distant objects. For

range of accommodation so that a single focus setting

this reason, in focusing an instrument, it should be

will be satisfactory inasmuch as the eye correction is not

focused for distance and the first distinct focus setting

extremely large. They are generally constructed with a

should be used.

fixed-focus eyepiece which cannot be adjusted during

b. Fatigue of the eye muscles will be

operation. They are called fixed-focus telescopes, and

experienced after comparatively short periods of

usually have a minus 3/4 to minus 1 diopter setting (para

continuous observation. This fatigue usually is greater

10-2).

with low illumination. Inasmuch as the eyes quickly

c. Use of Spectacles. If the proper spectacles

recuperate, frequent rest periods are advisable.

are worn, the corrected eyes will focus at the normal

c. A particular type of fatigue results from the

setting of the instrument.

However, the use of

use of binocular instruments if not set at proper

spectacles prevents the eyes from coming up properly to

interpupillary distance. This is due to the fact that both

the eyeshield of the instrument and the eye may be

eyes involuntarily adjust themselves so that a

placed so far from the eyepiece as to restrict the field of

view. In spite of these con-

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