Figure B-13. Lens erecting system.

TM 9-258

normal erect when in coincidence except for the

objective. An erecting system may consist of one or

presence of the halving line.

more lenses (figs B-13 and 4-20), each of which is called

Erecting system - Lenses or prisms, the function of

an erector, or of one or more prisms (figs 4-21 and 4-11).

which is to erect the image, that is, to bring the image

upright after it has been inverted by the

Figure B-13. Lens erecting system.

Erector - One of the lenses of a lens erecting

Extra-foveal vision - Vision in parts of the retina

other than fovea.

Esophoria - A condition of the eyes in which the

Eye distance or eye relief - The distance from the

lines of sight tend to turn inward.

rear surface of the eyelens to the plane of the exit pupil

Etching - The marking of a surface by acid, acid

(fig 2-55). The center of rotation of the observer's eye

should be situated in this plane. The center of rotation of

fumes, gas, or a tool. A process extensively used in the

the eye is about 6 millimeters behind the vertex of the

manufacture of reticles.

Exit pupil - The diameter of the bundle of light

cornea.

Eyeguard - See eyeshield.

leaving an optical system. The small circle or disk of

Eyelens - The lens of an eyepiece which is nearest

light which is seen by looking at the eyepiece of an

to the eye (fig B-14). See eyepiece. Various types of

instrument directed at an illuminated area (fig 2-53). Its

lenses are used for this purpose.

diameter is equal to the entrance pupil divided by the

magnification of the instrument.

Exophoria - A condition of the eyes in which the

lines of sight tend to turn outwardly.

B-13