The New Student's Reference Work/Speculum Metal

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Speculum (spĕk′ ū́-lŭm) Metal is an alloy composed of 126 parts of copper and a little over 58 of tin, which is used for the mirrors or specula of telescopes.  It is very brittle, will take a fine polish, and does not tarnish easily.  A silvered glass is, however, taking its place, being just as good and less expensive.