3601 x 2800 px | 30,5 x 23,7 cm | 12 x 9,3 inches | 300dpi
Date de la prise de vue:
2011
Informations supplémentaires:
Speculum metal is a mixture of around two-thirds copper and one-third tin making a white brittle alloy that can be polished to make a highly reflective surface. It is used primarily to make different kinds of mirrors including early reflecting telescope optical mirrors. Speculum metal can also be used as the metallic coating on glass mirrors (as opposed to silver or aluminium) giving a reflectivity of 68% at 6000 angstroms when evaporated onto the surface. Speculum metal mixtures usually contain two parts copper to one part tin along with a small amount of arsenic, although there are other mixtures containing silver, brass, lead, or zinc. The knowledge of making very hard white high luster metal out of bronze type high tin alloys may date back more than 2000 years in China although it could also be an invention of western civilizations as well. Such metals were used in sculpture and to make more effective mirrors than the more common yellow easily tarnishing bronze mirrors. Mirrors of speculum metal or any precious metal were rare and only owned by the wealthy. Speculum metal found an application in early modern Europe as the only known good reflecting surface for mirrors in reflecting telescopes. Such telescopes needed first surface mirrors that could be ground and polished into complex shapes such as parabolic reflectors. For nearly 200 years speculum metal was the only mirror substance that could perform this task. One of the earliest designs, James Gregory’s Gregorian telescope could not be built because Gregory could not find a craftsman capable of fabricating the complex speculum mirrors needed for the design. Isaac Newton was the first to successfully build a reflecting telescope, his 1668 Newtonian reflector with a 33-mm (1.3-inch) diameter speculum metal mirror of his own formulation.
