kleptoquark

Achromatic doublets are one of those little innovations that made the modern world, but which you don't usually think about much.

Basically, a single lens can only focus a specific wavelength of light at a time, so if red light is in focus, blue will be out of focus. This is called chromatic aberration and was (apparently) a big problem for early telescopes and microscopes.

The solution is to use a "doublet": two lenses of different material with different refractive indices (in practice, crown glass and flint glass). The chromatic aberration doesn't fully cancel out, but you can make it pretty minimal for visible wavelengths.

Anyway. Here's a fun anecdote about the invention of achromatic doublets, per Wikipedia:

Credit for the invention of the first achromatic doublet is often given to an English barrister and amateur optician named Chester Moore Hall.[1][2] Hall wished to keep his work on the achromatic lenses a secret and contracted the manufacture of the crown and flint lenses to two different opticians, Edward Scarlett and James Mann.[3][4][5] They in turn sub-contracted the work to the same person, George Bass. He realized the two components were for the same client and, after fitting the two parts together, noted the achromatic properties. Hall failed to appreciate the importance of his invention, and it remained known to only a few opticians.

In the late 1750s, Bass mentioned Hall's lenses to John Dollond, who understood their potential and was able to reproduce their design.[2] Dollond applied for and was granted a patent on the technology in 1758, which led to bitter fights with other opticians over the right to make and sell achromatic doublets.