Abstract
MB. M. A. ELLISON has described (J. Brit. Astro. Assoc., 52, Sept. 8, 1942) the celebrated experiments of William Parsons between 1827 and 1845 for the improvement of reflecting telescopes. The earlier workers in speculum metal had jealously guarded their secrets of grinding and polishing, and each step in the process had to be established anew by experimenters. The earliest researches of Parsons were undertaken to determine the composition of speculum metal, which should possess the qualities of whiteness, high reflectivity, and resistance to tarnish. It was found that the conditions were best realized in an alloy of four atoms of copper to one of tin, and specula differing to a slight degree from this proportion of the alloys were found to dim rapidly, though they were quite brilliant when polished. Unfortunately, this alloy was liable to shiver at the slightest blow or sudden temperature changes, and attempts to cast it in disks more than about a foot in diameter were abandoned.
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The Third Earl of Rosse and his Telescopes. Nature 150, 458 (1942). https://doi.org/10.1038/150458a0
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DOI: https://doi.org/10.1038/150458a0