Achromatism

Achromatism, the property in virtue of which certain combinations of lenses, &c. refract a beam of light without producing coloured fringes. Any arrangement of lenses or prisms which refract light without dispersion (see under LIGHT) is achromatic. Newton, misled by certain imperfect experiments, concluded that dispersion could not be annulled without annulling refraction. Hall, in 1733, and later, Dollond (independently), found that certain media give large refraction with small dispersion, while others give small refraction with large dispersion; so that the dispersion produced by one medium can be made to annul that due to another, while its refraction is not entirely annulled. For example, by properly combining a convex lens of crown-glass with a concave one of flint-glass, a compound achromatic lens can be produced. The achromatism in the above arrangement, and in every other arrangement yet tried, is not absolutely perfect. The reason is that such media do not give exactly similar spectra (see SPECTRUM)—i.e. the ratio of the distances between any two pairs of rays is not quite the same for the different media. A combination of three lenses, or prisms, gives a better approximation to absolute achromatism than a combination of two. Blair, in 1791, constructed an achromatic telescope giving far better definition for high magnifying power than has since been obtained. He used a compound lens consisting of two glass lenses in-losing a liquid.