ACHRO'MATISM (colorlessness, from Gk.
a prix. xp(7)/(a, chrOina, color). The prop erty by virtue of which certain combinations of lenses and prisms refract a beam of white light without producing dispersion of certain colors. (See DiseERsiox.) Newton, misled by imperfect experiments, concluded that dispersion could not be annulled without annulling refraction. Hall, in 1733, and later. Dollond ( independently ) , found that certain media have large powers of refrac tion with small dispersion, while others give small refraction with large dispersion; so that the dispersion of two colors produced by one medium can be corrected by that due to another, while the deviation of the light from its orig inal direction is not entirely annulled. For ex ample, by properly combining a convex lens of crown-glass with a concave one of flint-glass an "achromatic lens" can be produced which will have the same focus for the two selected colors, while the foci for the other colors are at neigh boring points along the axis of the lens. It is thus seen that the achromatism in the above ar rangement is not perfect. In Fig. 1 a beam of white light having the direction c d meets the crown-glass prism and is refracted. Dis persion also takes place, and the beam as it emerges is separated into its component col ors. Adjacent to the prism of crown-glass is one of flint-glass, whose action is to bring together the rays so that they emerge parallel, with the desired deviation. The
reason is that prisms of different media do not give exactly similar spectra, the colors being dis persed according to different laws for different media. Fig. 2 shows achromatic combinations of lenses where the flint and crown glasses are combined with the same effect as in the achromatic prism il lustrated. A combi nation of three lenses, or prisms, gives a bet ter approximation to absolute achromatism than a combination of two.
If a lens is to be used for visual servations, it is "corrected" generally for a nite wave-length in the yellow and one in the bluish-green, i.e. these two colors are brought to the same focus; but if it is to be used for photographic purposes, it is "corrected" for two wavelengths, which include those radiations pos sessing the greatest photographic action. There are two defects which a lens may have, owing to chromatic aberration (q.v.), in that the colored images may he at different distances from the lens and that they may he of different sizes. The second of these defects is insignificant if the lens is thin; and the first may be "corrected," as just described, by votubining two thin lenses. If the lens is thick, or if the lenses of the lens-system are some distance apart, the second of the alcove mentioned errors becomes serious. It may, how ever, he corrected.