Achromatic Lens (Gr. a, without, and color). When light is refracted by any transparent medium, dispersion always takes place; that is, the rays of different color contained in white light are not equally refracted or deviated from their path. It would seem that the amount of this dispersion must always be proportional to the amount of refraction, but experiments have shown that diverse refracting substances differ considerably in this respect. Their dispersing and refracting properties are determined by passing a ray of light through solid prisms of different material, or liquid prisms enclosed between glass plates. • The refracting power is then measured by the amount of deviation of the ray, and the dispersive power by the length of the colored spectrum produced. So it has been found that if the relative amounts of refraction of water, crown glass, flint glass, and oil of cassia are expressed by the numbers 133, 152, 162, and 159, the amounts of dispersion or the lengths of their spectra are in ratio of 145, 203, 433, and 1,080. If the angle of a prism is increased, the refracting and dispersing power both increase in the same ratio; and it is evident that two prisms of different material may be made at such angles that they produce the same length of spectrum, or possess the same dispersion, but that then their refracting powers will not be the same.
In figs. 1 and 2 two such prisms are represented, the first refracting more than the second, but giving equal lengths of spectra. If now two such prisms are joined in opposite directions, as represented in fig. 3, they will cause a neutralization of the equal spectra, but not of the unequal refraction, and therefore they will produce a deviation or refraction of the rays without dispersion of the light; no colored spectrum will be produced, but only a pure white spot will be the result of such a combination, which is called an achromatic prism. This is the principle on which the lenses in all our modern telescopes, microscopes, photographic and other optical apparatus are constructed. A convex lens of crown glass brings the rays together to a number of differently colored foci, of which the red rays will be the furthest from the lens, fig. 4. (See Aberration, Chromatic.) A concave lens will throw the red rays nearer to the axis, fig. 5; but if this concave lens is made of flint glass (a material having a slightly greater refracting but a much greater dispersive power), and ground to such a curve as completely to neutralize the dispersion or coloring of the first lens, while it affects its refraction only so far as to lengthen its focal distance, the combination will bring the rays to a focus without separating the luminous rays into their colored constituents; see fig. 6. Such a lens is said to be corrected for chromatic aberration.
Sometimes the concave correcting lens of flint glass does not quite accomplish the purpose, and then the combination is said to be under-corrected; but sometimes the opposite is the case, when the combination is said to be over-corrected. In this case the chromatic aberration will be the reverse of what it is with a single convex lens. As the different parts of the colored spectra produced by different media have not an exact proportionality toward one another, an absolute achromatism is impossible; but successful attempts have been made to cure it in some degree by the addition of a third lens of plate glass. Attempts to make achromatic lenses by enclosing fluids of different diffractive powers between glass lenses have all failed, by reason of the variability in such fluids; in the course of time portions of higher refractive power will accumulate at the lower sides, and by changes of temperature currents will be set up which disturb the images seen. As the manufacture of flint glass for large achromatic lenses is a very difficult and uncertain operation, and therefore very expensive, their size has been reduced by placing an over-corrected combination of half the size in the middle of the telescope; such an instrument is called a dialitic telescope.
Recently the plan of the elder Her-schel has been revived, namely, to use no large achromatic objective lenses at all, but reflectors, which of course can have no chromatic aberration, which is the result of refraction.
Refraction and Dispersion by Prisms.
Refraction and Dispersion by Lenses.