Achromatic doublet lens
Achromatic doublet lens with a focal length of 52 inches, maker unknwon, England, c. 1780.
Achromatic doublet lenses reduce the chromatic aberration inherent in the images of single spherical lenses. They combine two types of glass with different dispersive powers, so that the ratio of the dispersive powers is equal to the ratio of their focal lengths. If this is the case, two colours will be brought to the same focus.