ACHRO'MATISM. Because the several components of a beam of ordinary light are of different refrangibilities, it follows that they are not brought to a common focus by a simple convex lens. The violet rays meet at a point nearer the lens than that at which the red rays unite, and the optical image is confused and fringed with prismatic colors.
The difficulty is greatest with lenses of short focus, whence the early practice of constructing telescopes of enormous length. Sir Isaac New ton, misled by a really remarkable series of petty accidents, concluded that this difficulty could not be obviated, and that large refracting telescopes were therefore impracticable. He therefore gave his attention to the development of the reflector.
In 1757, however, John DoUond, a Spkal fields weaver, discovered that different sub stances separate the colors of light, for a given mean refraction, to a different degree. He therefore constructed double lenses of two dif ferent kinds of glass,— crown glass and flint glass. A concave lens of flint glass brings the
colors together while not entirely destroying the refraction caused by a convex lens of crown glass. The correction is far from perfect, how ever, and even the best telescope lenses produce a blue halo surrounding the stellar images. This outstanding color may be reduced by combina tions of three or more lenses; hut such devices greatly increase the mechanical difficulties of the optician. The present practice is to bring to gether such portions of the light as most power fully affect the eye or the photographic plate, leaving the other tints uncorrected. The intro duction of new kinds of glass, especially the Jena glass, so called, has somewhat improved the chromatic correction of smaller objectives. See DISPE'RSION; LENS; MICROSCOPE; Tpi FSCOpE LIGHT.