March 30, 2020

Astronomy Video of the Week: An Amazing Piece of Metal

The tarnished mirror used in Herschel’s 40-foot reflector on display at the London Science Museum.

Image credit: geni/Wikimedia Commons

Your backyard Dobsonian has a mirror that far outperforms the best telescopes of yore.

This week, we thought we’d break the “all-Pluto all the time” mantra for July to bring you a classic from our archives.

Those olden mirrors of yore didn’t use silver or aluminized glass like the reflecting telescopes we have today. Instead, they relied on a polished metal alloy known of since antiquity for its reflective properties. This combination of tin, copper with just a pinch of added arsenic was known as speculum. The good folks over at the Periodic Table of Videos show us a fine example of speculum in Newton’s first reflecting telescope at the Royal Society of London:

With a mirror just 2 inches in diameter, you can see just how tiny Newton’s early tabletop reflector was. Polished tin has a blue tint, whereas polished copper is reddish in hue. A optician had to hit a sweet spot to balance out the amalgam to get the desired colorless mirror.

Such metal mirrors of yore had some definite drawbacks, which are the reason they’re no longer in use. They would begin to tarnish and corrode as soon as they were exposed to the damp night air. William Herschel was said to have kept a spare mirror for his massive 49.5” telescope, and was constantly re-figuring one, while the other corroded in the tube.

And such mirrors only reflected 40% of the light falling on them when they were new.  It’s no small wonder that you only see speculum in the museum today. Still, it might be a fun project to construct a speculum metal mirror telescope, just to gain insight into how those astronomers of yore did it. You really had to be part astronomer, part metallurgist,  and part alchemist to make the magic of a speculum metal mirror work.



  1. Wonderful video David! Ernesto & I really enjoyed it. Thanks for sharing.

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