In many ways, reflecting telescopes were the ideal instrument — easier and cheaper to build than the large refractors that were the most popular kind of telescope. But reflectors had a major problem: the metal mirror.
The telescopes’ tin and copper mirrors would tarnish, reflecting less and less light as time passed. Unless the telescope had a backup mirror that could be moved into place, observations would come to a halt until the mirror in place could be polished.
Even in the early years of telescopes, glass mirrors with a metal backing — like the mirrors typically found in homes — were being made and used. But those mirrors wouldn’t work in telescopes. The light would bend as it traveled through the glass, bounced off the metal backing and traveled through the glass again, blurring the image.
In the 1850s, a German chemist named Justus von Liebig made a new kind of mirror. He used a newly discovered chemical reaction to cover the surface of a piece of glass with a thin film of silver. The silver could easily be polished to create a mirror.