Speculum metal is a very hard white alloy of roughly 75-80% copper and 20-25% tin; some compositions contained 1–2% of arsenic. Composition with 45% tin has more resistance to tarnishing.
For reflection, the composition needs to be copper 68.21%, tin 31.7, with more copper making it more yellow and more tin more blue reflection.
An early composition of speculum metal was used in the first reflecting telescope, Newton's Reflector of 1669 (33-mm (1...
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Speculum metal is a very hard white alloy of roughly 75-80% copper and 20-25% tin; some compositions contained 1–2% of arsenic. Composition with 45% tin has more resistance to tarnishing.
For reflection, the composition needs to be copper 68.21%, tin 31.7, with more copper making it more yellow and more tin more blue reflection.
An early composition of speculum metal was used in the first reflecting telescope, Newton's Reflector of 1669 (33-mm (1.3-inch) aperture). The composition of speculum metal was further refined and went on to be used in the 1700s and 1800s in many designs including those by Robert Hooke, William Herschel, and Lord Rosse. The ability to grind parabolic mirrors led to increased use of speculum although advances in the design of refractor telescopes (which use glass lenses), during the same period, resulted in an ongoing competition.
Speculum metal mirrors allowed for very large telescopes; William Herschel's 126-cm (49.5-inch) reflecting telescope of 1789 was...
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