Rutherfurd introduced into common use the reflection grating, finding that speculum metal was less trying than glass to the diamond point, upon the permanence of which so much depends.
His speculum metal is composed of four atoms of copper (126.4 parts) and one of tin (58.9 parts), a brilliant alloy, which resists tarnish better than any other compound tried.
Such speculum metal is exceedingly hard and brittle, takes a fine white polish, and when protected from damp has little liability to tarnish.
The best speculum metal and the greatest care are no guarantee of freedom from tarnish, and, if such a mirror is much exposed, as it must be in the hands of an active observer, frequent repolishing will probably be necessary.
Glass is lighter, stiffer, less costly and easier to work than speculum metal.
