ABSTRACT
Mirrors are probably the oldest optical instruments known to humans. Mostly, they consisted of a smooth reflecting surface that exhibited the natural reflectance of whatever material was involved. Then the fifteenth century with its development of outstandingly clear glass saw the introduction of the essentially modern mirror consisting of a glass plate with a tin amalgam coating. Initially, this was a product of the island of Murano, where the Venetian glass was manufactured, but despite the greatly enforced secrecy, the invention spread. However, it was not until the middle of the nineteenth century that the reflecting astronomical telescope exchanged its metallic speculum mirrors for glass chemically coated with silver. Astronomical telescopes require front surface mirrors where the reflecting coating is carried on the front surface rather than on the rear. The second surface mirror with its coating on the rear of the substrate yields a reflecting surface with essentially the quality of the substrate, while the front surface mirror possesses the properties of the front surface of the metal film rather than that of the substrate. With chemical deposition, this was not of the same high quality as the substrate, and so the silver coatings had to be hand polished, a difficult and somewhat uncertain process and even more uncertain when a beam splitter rather than a mirror was involved. The revolutionary development came in the early twentieth century with the discovery that vapor deposition under vacuum resulted in a metallic film exhibiting a surface of exactly the same quality as the supporting substrate.
