Sir William Herschel's white enamel glass mirror, 1780-1800.
© Science Museum / Science & Society Picture Library
The British astronomer Sir William Herschel (1738-1822) constructed his own reflecting telescopes using mirrors made of speculum metal. A bronze alloy with arsenic added for a more lustrous finish, this 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 make cameo glasses. Although this 6 1/2 inch (165mm) concave mirror has been fully polished, its low reflectivity made it unsuitable for use in an astronomical telescope.