The fix: the achromatic doublet

Cement a converging crown-glass element to a diverging flint one and the second undoes the first's color spread while keeping the focusing power. Red and blue return to a common focus. Left is one lens, two foci. Right is the doublet, one.

One lens: two focal points

blue focuses short, red focuses long

Doublet: one focal point

crown plus flint: both land together

Left: one lens, and blue lands short of red. Right: crown plus flint, and both colors land together.