Six and a half inch concave mirror of enamel (white glass or Tassies compound) prescribed at back:- "no.8, 7 feet enamel mirror (F)" in cardboard round box ("Pl ) with lid, exact focal length 7ft, 3.25 in
The astronomer William Herschel constructed his own reflecting telescopes using mirrors made of speculum metal. A bronze alloy with arsenic added for a more lustrous finish, the hard metal was very brittle, difficult to polish and tarnished quickly. To avoid these shortcomings, Herschel experimented with trying to produce mirrors from glass. The mirror shown here is made of a white enamel glass, known as Tassie's compound. First used in London by James Tassie in the 1760s, the moulded material was polished to made cameo glasses. Although this 6 ½-inch concave mirror has been fully polished, its low reflectivity made it unsuitable for use in an astronomical telescope.