MIRROR,. a smooth surface usually of glass, capable of regularly reflecting rays of light. A mirror may be (1) a polished surface of glass; (2) a surface of tin-foil on the fur ther side of a sheet of glass (as in the common looking-glass); (3) the surface of a deposited film of silver or platinum on a polished glass surface, so that rays of to and from the metallic reflecting surface do not pass through the glass i (4), a polished surface of silver, gold, platinum or speculum metal (a bronze composed of about 32 parts of copper to 15 of tin, small quantities of lead, antimony and ar senic being sometimes added).
The use of a reflecting surface would be come apparent to the first person who saw his own image reflected from water; and for primi tive man the Only mirrors were probably his own reflection in the still water of ponds and lakes. The use of mirrors of brass by the He brews is mentioned in the Pentateuch; and bronze mirrors were used by the Egyptians, Greeks and Romans. The use of silver in the manufacture of mirrors was taught by Praxi teles in the 4th century before Christ. Look ing glasses were first made in Venice in A.D. 1300, and in 1673 were introduced into England. They were a great improvement on the more ancient speculum metal mirrors, whose reflect ing surfaces were liable to oxidation from ex posure to the air, but they have the disadvan tage that there are two reflecting surfaces, one of glass, the other of metal. Polished metals reflect nearly all rays of light at all incidences; glass reflects very few rays at the normal inci dence, but the amount of reflection becomes greater and greater as the incidence becomes more and more oblique. Mirrors are usually either °plane* or "spherical.* Spherical mir rors are small portions of the surface of a sphere. In a spherical mirror, concave or con vex, the line through the centre of the spher ical surface of which the mirror is a part and the middle point of the mirror is called "the axis.* From a concave mirror rays parallel to the axis converge after reflection to a point called the "principal focus,' which is half-way between the centre of the sphere and the mirror.
(See LIGHT). Rays from a luminous object outside the spherical centre of a concave mirror form a small, real, inverted image of the ob ject between the centre and the principal focus; when the object is between the centre and the principal focus the image is beyond the centre, and is large, real and inverted.
. Prior to 1835 mirrors were almost univer sally made by applying a coat of tin-foil amal gamated with mercury to the surface of plate glass. In 1835 Baron Liebig observed that on heating aldehyde with an ammoniacal solution of nitrate of silver in a glass vessel, a brilliant deposit of metallic silver was formed on the surface of the glass. To this observation the more recent process of silvering glass is really due. In 1849 Drayton made known a method in which he employed as a backing composition two ounces of nitrate of silver, one ounce of ammonia, three ounces of alcohol and three ounces of water. The defects of these two modern processes are that the deposit of silver on glass is not so adherent and unalterable under the influence of sunlight and sulphurous fumes as the old amalgam of tin and mercury; besides, the newer processes give the glass a slightly yellowish tinge. These disadvantages have been obviated by applying to the silver coating a weak solution of cyanide of mercury, which immediately forms a kind of amalgam and renders the deposit at once much whiter and more adherent. The silvering is protected from mechanical abrasion and the chemical action of gases and vapors by a coating of shellac or copal varnish, which when dry may receive a further covering of red-lead paint. A method of coating glass with platinum has been recently introduced. A solution of bichlo ride of platinum is spread over the surface of the glass with a fine brush, and the metal is precipitated with oil of lavender. As this sum mary process produces a somewhat gray lustre it is used only for cheap mirrors, the lids of ornamental boxes, toys and the like.