Whether out of curiosity, vanity, or a motive as yet unexplored, people throughout the ages have wanted to see their own reflection.
As early as 2500 B.C. the Egyptians had mirrors of highly polished metal, usually of bronze, occasionally of silver or gold.
The first commercial glass mirrors were made in Venice in 1564; these were made of blown glass that was flattened and coated with an amalgam of mercury and tin. The Venetians proceeded to supply Europe with mirrors for centuries.
It wasn’t until 1840 that a German chemist named Justus Liebig came up with the method of silvering that we use today. By this technique, silver ammonia compounds are subjected to the chemical action of a reducing agent, such as invert sugar, Rochelle salt, or formaldehyde, and the resulting metallic silver is spread evenly over the back of a smooth pane of plate glass.
Although scarcely noticeable in everyday use, plane mirrors actually produce multiple images: a slight reflection from the front as well as the stronger reflection from the back. The distortion, caused by small amounts of light passing through the glass, becomes extremely significant in scientific use, for which precision is imperative.
The mirrors used in telescopes, therefore, are coated on the front as well as on the back. Aluminum, or aluminum and chromium, has replaced silver, which tarnishes rather easily. The coating is formed by vacuum deposition, a method in which the metal is heated on a coil in a vacuum chamber.
The resulting vapor deposits a very thin film, a few millionths of an inch thick, onto glass ground to the proper spherical or paraboloidal shape.