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A ABBE CONSTANT Ration of refractivity of a material to its dis- persion. Also called the Nu or Vee value. A calculated value from known indices of refraction. ABERRATION (1) Generally, any systematic deviation from an ideal path of the image-forming rays passing through an optical system that causes the image to be imperfect. Specific aberrations are spherical aberrations, coma, chromatic aberration, and distor- tion. (2) The phenomenon caused by rotation of the earth in its orbit that is sufficient to cause the light from a star to appear to shift for- ward. For those starting at right angles to the direction of the earth's travel, the maximum effect is 20.5 seconds of arc. ABERRATION, CHROMATIC Image imperfection caused by light of different wavelengths following different paths through an optical system due to dispersion in the optical elements of the sys- tem. ABERRATION, CHROMATIC, LATERAL A variation in the size of images for lights of different colors or wavelengths pro- duced by an optical system. It is measured as the radial displace- ment of the image in the first color, from the image of the same point in the second color. A direction away from the axial image point is taken as positive, and a direction toward the image point is of difference in magnification for light of different colors, and is measured as the angular separation in apparent field between the images of the same point in two colors. ABERRATION, CHROMATIC, LONGITUDINAL The dis- tance between the foci for light of different colors measured along the optical axis. ABERRATION, SPHERICAL A symmetrical optical defect of lenses and spherical mirrors in which light rays that come from a common axial point, but strike the lens at different distances from the optical axis, do not come to a common focus. 1