Spherical surfaces are easy to grind and they do not bring all colours, or all zones, to the same place. An asphere can fix the zones; two glasses of different dispersion can fix the colours. Putting them on one board is the old teaching trick: same lamp, same meter, different glass.
Newton thought a colour-free lens was impossible — the argument behind the prism demo. Hall and Dollond later showed it was not. This gallery is that argument in three columns — move a lens and the fringe on the meter is the whole point.