The Modern Era of the Telescope

The method of chemically depositing silver on glass8 discovered about 1840 by Justus von Liebig, of Nuremberg, was successfully applied to a small glass telescope mirror in 1856 by Karl Steinheil, a German physicist, and independently in the following year by Jean Foucault, the famous French physicist.

 Then, in 1858, Foucault announced the development of his amazingly delicate and simple test for a concave reflecting surface, using an illuminated pinhole and a straightedge placed in the vicinity of the center of curvature of the mirror. The pinhole and straightedge were the outgrowth of earlier experiments in which simultaneous microscopic comparison was made of a pin point,

8Various processes of plating glass with metal for the making of mirrors had
been known and practiced for centuries, but for one reason or another the
coatings were unsuited for front-surface reflection.

likewise placed at the center of curvature of a mirror, and its reflected image, which was caused to fall alongside.

Modern era of the telescope continued