His first telescope, completed in 1874, had a ( glass, not speculum metal ) reflector, and his second, which took 11 years to complete, had a diameter reflector, the largest privately owned telescope in Australia.
12.
Although this introduces geometrical aberrations, Herschel employed this design to avoid the use of a Newtonian secondary mirror since the speculum metal mirrors of that time tarnished quickly and could only achieve 60 % reflectivity.
13.
Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the speculum metal mirrors being used at the time meant it took over 100 years for them to become popular.
14.
No further practical advance appears to have been made in the design or construction of the reflecting telescopes for another 50 years until John Hadley ( best known as the inventor of the parabolic speculum metal mirrors.
15.
In 1856-57 Karl August von Steinheil and L�on Foucault introduced the process of depositing an ultra-thin layer of silver on the front surface of a piece of glass, making the first optical-quality first surface glass mirrors, replacing the use of speculum metal mirrors in reflecting telescopes.
16.
Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by the use of fast tarnishing speculum metal mirrors employed during the 18th and early 19th century a problem alleviated by the introduction of silver coated glass mirrors in 1857, and aluminized mirrors in 1932.
17.
A new illustrated dictionary edited by Lucy Trench, " Materials and Techniques in the Decorative Arts " ( University of Chicago Press, $ 60 ), states that until medieval times, mirrors were still usually made of polished bronze, silver and speculum metal ( bronze composed of more than 20 percent tin, as well as copper and lead ).
