APPENDIX B GLOSSARY

APPENDIX B

GLOSSARY

system which causes the image to be imperfect. See

B-1. General. This glossary contains definitions of a

astigmatism, chromatic aberration, coma, curvature of

number of specialized terms found in the study of optics,

image, distortion, and spherical aberration.

in the use of sighting and fire-control equipment, and

Absorption - The act of process by which an object

with reference to the eyes and vision. It includes

or substance "takes up" or "soaks up" all the colors

definitions of most of the more unusual words used in

contained in a beam of white light except those colors

this manual. The majority of the italicized words in this

which it reflects or transmits. Because of absorption,

manual will be found in this glossary. When in doubt as

objects appear to have different colors; white objects

to the meaning of a word, refer to this glossary.

have little absorption; other objects having varying

B-2. Glossary.

powers of absorption appear to have different colors and

Abbe prism - Two right-angle prisms joined to form

different shades of brightness. For example, an object

one prism (fig 4-16). Used as component of one form of

which appears red has absorbed all the color of the

the Abbe prism system (figs 4-10 and 4-21).

spectrum except the color red. This is known as

Abbe prism erecting system - A prism erecting

selective absorption or selective reflection. A piece of

system in which there are four reflections to erect the

ruby glass placed between the eye and a source of white

image. It is composed of two double right-angle prisms

which bend the path of light 360ƒ, displacing but not

light allows only red light to pass through because it

absorbs all other colors.

This is called selective

deviating the path of light (fig 4-21).

absorption or selective transmission (fig B-1).

Aberration - Any defect of a lens or optical

Figure B-1. Absorption-Selective transmission.

Accommodation - Automatic adjustment of the

glass, which has been corrected for chromatic

aberration. See compound lens.

crystalline lens of the human eye for seeing objects at

Acuity - Keenness; sharpness. See stereo-acuity

different distances. The process whereby the crystalline

and visual acuity.

lens is adjusted to focus successive images of objects

Adapter - A tube, ring, or specially formed part

located at various distances from the eye. See limits of

accommodation.

which serves to fit or connect one part with another, or

Achromatic - Without color.

mount an element of smaller diameter into a part of

Achromatic lens - A lens consisting of two or more

larger diameter.

Aiming circle - An instrument for measuring

elements, usually made of crown and flint

B-1