ACHROMATIC GLASSES are so named from being specially constructed with a view to prevent the confusion of colours and distortion of images that result from the use of lenses in optical instruments. When white light passes through a lens, the different-coloured rays that constitute it are refracted or bent aside at different angles, and so converge at different foci, producing a blurred and coloured image. To remedy this compound lenses have been devised, which present a well-defined image, unsurrounded by coloured bands of light. To instruments fitted with lenses of this kind has been given the name achromatic, from ἀ privative, and χρώμα, colour. The celebrated optician, John Dollond, was the first to surmount this practical difficulty, about the year 1757, by the use of a combination of crown and flint glass. See Optics, Microscope, &c.