The separation, by an optical or other system, of a single ray of polychromatic light in object space into distinct monochromatic rays in image space, without recombining them. In the absence of other kinds of optical aberration, chromatic aberration causes white light from a point in object space to focus into a continuous sequence of differently coloured points in image space. Two kinds of chromatic aberration (c. A. ) are distinguished: longitudinal c. A. (also called axial c. A. ) and transverse c. A. also called lateral c. A. or off axis c. A. ). Longitudinal c. A. produces a sequence of differently coloured points along a line parallel to the optical axis of the system. Transverse c. A. produces a similar sequence of points along a line perpendicular to the optical axis. Chromatic aberration can be overcome over a moderate range of wavelengths by using a lens system composed of optical elements having different dispersive powers. (An optical system made achromatic for visual observation is not strictly achromatic for photographic work. )