MIRROR

(Fr. miroir.) A speculum or looking gla , or any other polished body capable of reflecting the images of objects, rays of light from which fall upon them. Silver is considered to be the most powerful reflector; but the speculum metal, as now prepared, is scarcely inferior, if at all so, and in some cases even better. In the very ^arly ages of the world, polished metallic specula were employed as mirrors by the Jewish and Egyptian women, especially of bra ; but in modern times, quicksilvered plates of gla are alone used as mirrors. Concave mirrors are used to concentrate the rays of the sun in a single point, and thereby produce intense heat. The surfaces formed by the revolution of the ellipse, parabola, and hyperbola, are such as reflect them accurately to one point; provided they emanate from one point, are parallel to one another (as the solar -rays), or would converge to a more remote point than the one which it is desirable to Use. The great diflnculty of constructing these has led to the employment of spherical segments, which, though not accurate, yet, under proper restrictions are approximately so. For the mathematical theory, see Reflection j see also Burning Gla , Speculum Telescope. [s. 768]