The achromatic lens consists of a double convex lens of crown glass combined with a plano-concave lens of flint glass.
If not, the substitution of an achromatic lens will be of no advantage.
A similar argument may be applied to find at what point an achromatic lens becomes sensibly superior to a single one.
It was this accuracy of convergence of the light which led Dr. Draper to prefer the mirror to the achromatic lens.
achromatic lens n.
A combination of lenses made of different glass, used to produce images free of chromatic aberrations.