Aberration, SPHERICAL

When a beam of light is incident upon a spherical mirror or lens, the rays after reflection or refraction do not all come together at a single point on the principal axis. This deviation from exact concurrence is called spherical aberration, by reflection in the case of the mirror and by refraction in the case of the lens. Rays incident upon the circumferential parts of the mirror or lens cro the principal axis after reflection or refraction nearer to the instrument than those rays nearer the axis, and the distance between the point of cro ing for the outside rays and that for rays nearest the axis is called the longitudinal spherical aberration. If now the rays be received on a screen, the image of a luminous point will be a circle of a radius varying according to the position of the screen; the diameter of the least image that can be formed is called the lateral spherical aberration. The envelope of a system of rays after reflection or refraction is called a "caus Parabolic reflectors and refractors produce no caustics, and hence no aberration.