Dispersion, in Optics, means the separation of rays of different refrangibilities when passed through a prism or lens. If a beam of light be composed of different colours, it will be split up into its constituents when passed through a prism; the rays will be deviated from their original direction, but each colour will be deviated differently. Thus, although at first the rays are all coincident, their ultimate directions are dispersed. Another prism of different transparent material may produce the same amount of dispersion between any two coloured rays, but with less deviation. It is by combining two such prisms that we are able to get a resultant deviation without any dispersion, and so to obtain more or less complete achromatism (q.v.). Newton thought that the deviation depended simply on the dispersion, and that no such combination could be effected.