minated, throw back the light in all directions, and this is known as irregular reflection or diffusion. The effect of regular reflection, which depends upon the polish of surfaces, is not to make those surfaces visible, but to exhibit images of surrounding objects; it is by the light irregularly reflected upon their surfaces that objects are seen. In looking into a mirror, the image of the face is seen by regular reflection; the surface of the mirror is recognized by irregular reflection. "The mirrors of the ancients were of metal, usually of the compound now known as speculum-metal. Looking-glasses date from the twelfth century. They are plates of glass, coated at the back with an amalgam of quicksilver and tin, which forms the reflecting surface. This arrangement has the great advantage of excluding the air, and thus preventing oxidation. It is attended, however, with the disadvantage that the surface of the glass and the surface of the amalgam form two mirrors; and the superposition of the two sets of images produces a confusion which would be intolerable in delicate optical arrangements. The mirrors, or specula as they are called, of reflecting telescopes, are usually made of speculum-metal, which is a
bronze composed of about thirty-two parts of copper to fifteen of tin. Lead, antimony, and arsenic, arc sometimes added. Of late years specula of glass coated in front with real silver have been extensively used; they are known as silvered specula. A coating of platinum has also been tried, but not with much success."
It is well known that the effect of plane mirrors or of any polished plane surface is to produce behind them images exactly similar both