Speculum, an alloy of copper and tin, was the same metal
that William Herschel used for the 6.2-inch mirror for the
telescope that he made and set up in his garden where he
discovered Uranus in 1781.
The mirror for Herschel’s 40-foot telescope was
49.5-inches in diameter, but compared to Herschel’s 6.2-inch
mirror, William Parsons’ 72-inch mirror could make visible
astronomical features that were over a hundred times fainter.
However, this was still not powerful enough to distinguish
individual stars, even though Parsons believed the spiral nebulae
were composed of stars and not gas.
We will always, it seems, struggle against our limitations
to understand the intricacies of our universe, and of our own
See also in The book of science:
Readings on wikipedia: