The achromatic lens
About 1733 (Chester Moore Hall); 1758 (Dollond patent)
Two lenses of different glass, crown and flint, cemented together so that red and blue light focus in the same place. It removed the colour fringes of simple lenses.
How it began
Newton had decided that colour fringes could never be removed from a lens. Around 1733 the English barrister Chester Moore Hall showed otherwise, combining two kinds of glass, but he did not publish. The optician John Dollond patented the idea in 1758 and built a business on it.
How it works
Glass bends blue light more than red, so a simple lens focuses each colour at a slightly different distance (chromatic aberration). Flint glass spreads colours more than crown glass. Pairing a strong convex crown lens with a weaker concave flint lens cancels the spread while keeping most of the focusing power.
Why it mattered
Achromats gave sharp, clean images in telescopes and, from 1812, in camera lenses such as Chevalier’s, which Daguerre used. Without them, photographs focused by eye would have been blurred, because early plates were most sensitive to blue light the eye barely sees.
Limits and what replaced it
An achromat brings only two colours together. Apochromats correct three, and modern lenses use special low-dispersion and fluorite glass.
Today
Almost every camera lens contains achromatic pairs, and special low-dispersion glass extends the idea.