speculum-metal

So far, it appears only that we have been measuring _heat_, but I have called the curve that of solar "energy," because by a series of independent investigations, not here given, the selective absorption of the silver, the speculum-metal, the glass, and the lamp-black

Scientific American Supplement, No. 363, December 16, 1882 Various

In 1827 experiments on the composition of speculum-metal were set on foot, and the first polishing-machine ever driven by steam-power was contrived in 1828.

A Popular History of Astronomy During the Nineteenth Century Fourth Edition 1874