light, the lens may be so constructed as to produce a single sharp image at of the same color as the source, but when the prism is introduced the lights of different colors will be differently deviated, and two or three distinct images will be found near If there be many such images, some may overlap, and if there be a great number of kinds of light varying progressively in refrangibility, there will be a great number of overlapping images constituting a continuous spectrum.
358. Dispersive Power.—It is found that prisms of different substances giving the same mean deviation of the light deviate the light of different colors differently, and so produce a longer or shorter spectrum. The ratio of the difference between the deviations of the extremities of the spectrum to the mean deviation may be called the dispersive power of the substance. Thus if represent the extreme deviations, and the mean deviation, the dispersive power is
359. Achromatism.—If in Newton's experiment of recomposition of white light by the reversed prism the second prism be of higher dispersive power than the first, and of such an angle as to effect as far as possible the recomposition, the light will not be restored to its original direction, but will still be deviated, and we shall have deviation without dispersion. This is a most important fact in the construction of optical instruments. The dispersion of light by lenses, called chromatic aberration, was a serious evil in the early optical instruments, and Newton, who did not think it possible to prevent the dispersion, was led to the construction of reflecting telescopes to remedy the evil. It is plain, however, from what has been said above, that in a combination of two lenses of different kinds of glass, one converging and the other diverging, one may correct the dispersion of the other within certain limits, while the combination still acts as a converging lens forming real images of objects. Fig. 133 shows how this principle is applied to
