Achromatic lens and achromatic telescope

Every lens made of a single glass type bends different colors by different amounts, creating colored fringes around images. Isaac Newton believed this chromatic aberration was fundamental and unavoidable, leading him to develop the reflecting telescope instead. He was wrong. In 1733, Chester Moor Hall discovered that combining lenses of different glass types could cancel out color dispersion—the achromatic lens.

Hall, an English lawyer and amateur optician, reasoned that if different glass types bent colors by different ratios, it might be possible to combine them so their dispersions cancelled while their focusing powers remained. He tested crown glass and flint glass, finding the right combination. To keep his discovery secret, he ordered the two lens elements from different opticians—George Bass for the crown glass element and an optician named Bird for the flint glass component.

Unknown to Hall, both opticians subcontracted the grinding to the same craftsman, John Dollond. When Dollond realized he was making complementary lenses for the same customer, he investigated. After Hall's death, John Dollond and his son Peter patented an improved achromatic lens design in 1758, commercializing what Hall had kept private.

The adjacent possible required two glass types with different dispersive properties. Crown glass—the common window and spectacle glass—had been available for centuries. Flint glass, containing lead oxide for brilliance, had developed for luxury goods and optical instruments. Only when both existed together could someone realize their dispersions might cancel.

The achromatic lens transformed telescopes and microscopes. Objective lenses could now be larger without proportional color aberration, enabling fainter astronomical observations and higher microscope magnifications. The Cassegrain reflecting telescope design, which had competed with refractors, lost its advantage over achromatic refractors for many applications.

Modern achromatic lenses use improved glass formulations, but the principle remains Hall's: combine dispersive differences to cancel while retaining useful refraction. His insight, developed in secret and commercialized by others, opened the adjacent possible for precision optics.