Now the coloured primary rays can only be made to coincide in direction, so that the light parts of an object may appear white, by refraction. And the correction is produced by the same plano-concave lens as that which corrects the spherical aberration. But in this case the relative dispersive powers of the media composing the convex and the concave lenses form the point to be considered. If the dispersive power of the media of which the convex and concave lenses are composed were the same, the dispersive power of the convex lens would be in excess, and the coloured rays in each compound ray could not become parallel. But by forming the concave lens of a more highly dispersive medium, with a less proportional mean refraction than the convex, when the curves of the surfaces and the relative thickness of the lenses are properly adjusted, the dispersive action of the concave lens may be made equal to that of the convex; and being exerted in the opposite direction, the coloured rays will become parallel and meet at a single focus. This may be elucidated by considering the lenses as composed of prisms. Thus, let fig. 28 represent the compound lens, the two halves of the doubly convex lens acting as two triangular prisms (fig. 19) with their bases opposed, converging the compound white rays _w w_, and dispersing the coloured elementary rays, which would form spectra at _s s_. In the plano-concave lens the triangular prisms may be considered as placed with their apices towards each other, and so would tend to disperse the coloured rays in the opposite direction, to form spectra at _t t_. Then, supposing the dispersions to be equal and in opposite directions, the coloured rays would become parallel and meet at a definite focus, the colour being destroyed. At the same time, the spherical action of the concave lens being opposite to that of the convex, the converging action of the latter will be diminished, so that the focus of the compound lens will be longer than that of the convex alone; but as the dispersive power of the concave is greater relatively than that of the convex, the mean refraction is less altered than the refraction or dispersion of the separate coloured rays; so that the concave wholly opposes or corrects the dispersion produced by the convex, while it only partially corrects its mean refraction. A lens in which the chromatic and spherical aberrations are corrected or destroyed is commonly called achromatic; although the term properly applies to the correction of the colour only.