122.

Measurement Systems Employed in Optics

METRIC SYSTEM.

a. The meter, which is the basis of the metric system, is intended to be one ten-millionth of the distance from the equator to the pole, measured on the meridan of the earth. All units of linear measurement of the metric system are multiples or divisors of the meter in units of 10. It is a system based on decimals which provides ease of conversion from one to another of the various units of the system. Units of measurement are provided from the extremely small to the very large, inasmuch as they include the physical units of measure of the micron, millimicron, Angstrom Unit, and X-ray Unit.

b. One meter is equal to 39.37 inches. Following is a table of metric units with their equivalents in inches, yards, and miles:

One foot equals 30.48

C. One inch equals 25.4 millimeters. centimeters. One yard equals 0.914 meter. One mile equals 1.609 kilometers. For approximate conversion of millimeters to inches, or vice versa, refer to the conversion table (fig. 140).

d. For more exact conversion and for conversion of large units, use the following factors to convert:

e. For quick, approximate conversion from inches to millimeters or centimeters, or vice versa, refer to figure 140. On this table, locate the measurement to be converted. Directly across the black line is its equivalent.

a.

GENERAL.

This chapter contains specialized definitions of a number of terms found in the study of optics, in the use of sighting and fire control equipment, and with reference to the eyes and vision. It includes definitions of the more unusual words used in this manual. The majority of the italicized words in this manual will be found in this glossary. When in doubt as to the meaning of a word, refer to this glossary.

ABBE PRISM ERECTING SYSTEM. A prism erecting system in which there are four reflections to completely erect the image. It is composed of two double right-angle prisms which bend the path of light 360 deg, displacing but not deviating the path of light (fig. 141).

Figure 141 Abbe Prism Erecting System ABERRATION. Any defect of a lens or optical system which causes the image to be imperfect. See ASTIGMATISM; CHROMATIC ABERRATION; COMA; CURVATURE OF FIELD; DISTORTION; SPHERICAL ABERRATION.

ABSORPTION. The act or process by which an object or substance "takes up" or "soaks up" all the colors contained in a beam of white light except those colors which it reflects or transmits. Because of absorption, objects appear to have different colors; white objects have little absorption; other objects having varying powers

Glossary

RA PD 30536

Figure 142 - Absorption - Selective Transmission

of absorption appear to have different colors. For example, an object which appears red has absorbed all the colors of the spectrum except the color red. This is known as selective absorption. A piece of ruby glass, placed between the eye and a source of white light, allows only red light to pass through because it absorbs all other colors. This is called selective transmission (fig. 142). ACCOMMODATION. Automatic adjustment of the lenses of the human eye for seeing objects at different distances. The process whereby the crystalline lens is adjusted to focus sharp successive images of objects located at various distances from the eye. See LIMITS OF ACCOMMODATION.

ACHROMATIC. Without color.

ACHROMATIC LENS. A lens consisting of two or more elements, usually made of crown and flint glass, which has been corrected for chromatic aberration with respect to two selected colors or wave lengths of light. See COMPOUND LENS.

ACUITY. Keenness; sharpness. See STEREO-ACUITY; VISUAL ACUITY.

ADAPTER. A tube, ring, or specially formed part which serves to fit or connect one part with another.

AIMING CIRCLE. An instrument for measuring angles in azimuth and site and for general topographic work.

AIMING POINT. The point on which the gunner sights when aiming the gun. This point is not necessarily the target itself. AMETROPIA. Any abnormal condition of the seeing power of the eyes, such as farsightedness (hypermetropia), nearsightedness (myopia), or astigmatism. An ametropic eye is one which, in a state of rest, does not form distinct images of objects on its retina.

AMICI PRISM. Also called "roof prism" and "right-angle prism with roof." A form of roof prism designed by G. B. Amici, consisting of a roof edge formed upon the long reflecting face of a right-angle prism (fig. 143). Used as an erecting system in elbow and panoramic telescopes. It erects the image and bends the line of sight through a 90-degree angle.

ANASTIGMAT. A compound lens corrected for astigmatism.

ANGLE. The amount of rotation of a line around the point of its intersection with another, necessary to bring it into coincidence with the second line.

ANGLE OF AZIMUTH. An angle measured clockwise in a horizontal plane, usually from north (fig. 144). (The north used may be True North, Y-North, or Magnetic North.)

Glossary

ANGLE OF CONVERGENCE. Angle formed by the lines of sight of both eyes in focusing on any line, corner, surface, or part of an object. Also referred to as convergence angle.

ANGLE OF DEVIATION. The angle through which a ray of light is bent by a refracting surface; the angle between the subtended path of an incident ray and the refracted ray (fig. 145).

ANGLE OF ELEVATION. The angle between the line of site (line from gun to target) and line of elevation (axis of bore when gun is in firing position) (fig. 146). The quadrant angle of elevation is the angle between the horizontal and the line of elevation. Angle of elevation plus angle of site equals quadrant angle of elevation.