Possibly a poor analogy, but imagine making a particle accelerator with the goal that objects of different mass put into it are accelerated and leave it going t

Possibly a poor analogy, but imagine making a particle accelerator with the goal that objects of different mass put into it are accelerated and leave it going the same velocity. The current state of technology allows that the best we can do is make one that accelerates lighter objects slightly more than heavier objects. However, we've found a way to make an accelerator using a different design that accelerates heavier objects more than lighter objects. If we pass objects through the first accelerator, then the second, the second accelerator reverses out some of the non-linearity of the first. Unfortunately this second accelerator is very, very expensive to make. This is how lenses of different refractive indexes are used: one element partially corrects the dispersion produced by earlier elements. Fluorite has the right refractive index to correct aberrations created in long focal length lenses.