[NickNameNick]

Isn't that covered in the 7th paragraph?

'the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass. Using a convex fluorite lens element alongside a high-dispersion glass concave lens element therefore eliminates residual chromatic aberration'

[lambdasquirrel]

I thought the reason underlying that is that fluorite is not a glass, technically. It is a crystal. But I’m not a MatSci person so that doesn’t leave me any bit more informed.