Figure B-4. Apertures.

TM 9-258

Aperture of objective - The diameter of that part of

is contained between the cornea and the crystalline lens

of the eye (fig 3-2).

the objective which is not covered by the mounting.

Arc - A part of the circumference of a circle.

Aperture stop - The diaphragm which limits the size

Artillery - mil See mil.

of the aperture. See diaphragm.

Aplanatic lens - A lens which has been corrected for

Asthenopia - Weakness or rapid fatigue resulting

spherical aberration, coma, and chromatic aberration.

from use of the eyes indicated by headache or pain in

Apochromatic lens - A lens, usually consisting of

the eyes. Often referred to as weak sight or eyestrain.

Astigmatism

three components of different kinds of glass (two crown

a. An aberration or defect of a lens which

glass elements, one flint glass element), which has been

corrected for chromatic aberration with respect to three

causes a point of the object off the axis to be imaged as

selected colors or wavelengths of light.

a short line or pair of short lines. When two lines are

Apparent field of view - The angular size of the field

formed, each is at a different distance from the lens and

of view of an optical instrument, as seen through the

is at an angle to the other, and the lens has two points of

instrument by the eye (fig 2-51). See field of view.

principal focus (B, fig B-5). A sharp image cannot be

Aqueous humor - The transparent liquid which

secured at either focal point and the best results are

obtained at a point between the two focal points in a

plane known as the circle of least confusion (fig 2-64).

B-4