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This allows to make, say, a two-element lens with less chromatic aberration.

We make a too-strong convex (positive) lens using low-dispersion material, then a concave (negative) lens behind, of lesser power but higher dispersion. If you choose the two materials correctly, you can (partially) cancel out the spread of colors. The two different dispersions can be adjusted to compensate perfectly while allowing a positive optical power to remain.

Do the same with more lenses for better correction.