ABSTRACT
The refractive index of any medium other than vacuum varies with wavelength. Thus the Gaussian and aberrational properties of any refracting optical system are functions of wavelength, that is, chromatic aberrations exist. It is traditional to consider chromatic variations of Gaussian properties as aberrations as well as chromatic variations of true aberrations. This usage, although perhaps inaesthetic, has ample practical justification; in fact, many modern refracting systems intended for use over an appreciable range of wavelengths are ultimately limited in performance by chromatic effects, both Gaussian and higher order, rather than by the monochromatic aberrations which we have so far been considering. The history of astronomical telescope design provides a useful example. There is in addition an interesting formula which can be applied to finite rays to give an approximation to the chromatic aberration which is linear with wavelength. It is usual to talk of the chromatic effects on Gaussian properties as the primary or first order chromatic aberrations.
