In 1856 and 1857, German astronomer Carl August von Steinheil and French doctor Leon Foucault realized that von Liebig’s method would be perfect for telescope mirrors. Previously, it had been too expensive to make telescope mirrors out of silver, but this chemical technique used so little silver that cost was no longer an issue. Now astronomers had an inexpensive, lightweight, glass mirror that reflected 50 percent more light than metal mirrors had. The silver still tarnished, but it was easier to replace the silver coating than it was to polish a metal mirror.
Foucault also came up with a better way for testing a mirror’s shape. The test worked much like the method used by the makers of early reflectors but was extremely precise. Grinding the mirrors into the correct shape became easier.
Now that astronomers could make giant mirrors, they started to plan and build huge telescopes with mirrors up to 200 inches in diameter. Previous experience with early reflectors taught them about atmospheric distortions that can affect observations. The lights from cities and the pollution produced by their factories also impeded viewing.