Mirror (Fr. miroir, from Lat. mirari, to wonder, admire), a looking glass or speculum; any bright surface that reflects the rays of light falling upon it. The surface of smooth water is a natural mirror, which the ancient poets sometimes refer to as having been used by persons for viewing their own forms. Metallic mirrors are mentioned in Exod. xxxviii. 8, and job xxxvii. 18. With the ancient Egyptians one of the principal articles of the toilet was the mirror. Wilkinson says it was of j mixed metal, chiefly copper, carefully wrought and highly polished. It was circular, and had an elaborately ornamented handle, the designs of which were sometimes beautiful female figures, and sometimes hideous monsters, whose ugliness contrasted most strongly with the features reflected by its polished surface. The practice of using polished basins for mirrors is alluded to by Artemidorus; and the ancients also had drinking vessels, as mentioned by Pliny, the inside of which was so cut and polished that the image of one drinking from them was reflected many times. The composition of some of the ancient mirrors has been found by William Francis to have been: copper 67.12, tin 24.75, and lead 8'13 parts in 100; and by Klaproth: copper 02, tin 32, and lead 6 per cent.
Their manufacture appears to have been most extensively carried on at Brundusium. Pliny gives to Pasiteles, a native of S. Italy and contemporary of Pompey, the credit of introducing mirrors of silver. They are spoken of by Plautus, ami in the time of the first emperors they became very common among the Romans, so that they were in use. according to Pliny and Seneca, even by maid servants, and the manufacture of them was one of the important trades of Rome. From several statements of Pliny it appears that various stones were employed as mirrors set into the walls as panels, and otherwise used to reflect images of objects. Obsidian appears to have been most employed for this purpose. A similar stone called the itztli, and by the Spaniards gallinazo, was used for the same purpose by the A/tecs, of which hard vitreous stone they also fashioned sword blades and razors. There were other stones of which they made excellent mirrors; but the description of these is too indefinite to determine their names. - Peckmann thinks that the use of the dark obsidian stone for mirrors suggested the use of glass, that this was attempted at the celebrated glass works of Sidon of which Pliny makes mention, and that they were first made of black glass, and afterward of glass covered on the back with black foil.
But from the time of Pliny no certain reference is again found to glass mirrors until the 13th century. In the treatise on optics of Alhazen, the Arabian, of about the year 1000, mention is made of mirrors of iron (steel) and also of silver, but not of glass; and the same thing is remarked of the "Optics" of Yitello, of about the middle of the 13th century. But in the treatise on optics of John Peckham, an English Franciscan monk, who taught at Oxford, Paris, and Rome, and died in 1292, mirrors of iron, steel, and polished marble are spoken of, and also of glass covered on the back with lead. After this time various writers allude to mirrors of this sort, and describe their being made by pouring melted lead over the hot glass plates. In the 14th century glass mirrors were extremely rare in France, while metallic ones were in common use. Beck-mann describes the following method of preparing small convex glass mirrors as common in Germany in the beginning of the 16th century: A hollow ball of glass being blown, while it was still hot a metallic mixture of lead or tin and antimony, with a little resin or salt of tartar, was introduced into it, and coated its inner surface, the resin or salt aiding the fusion of the metal and preventing its oxidation.
The glass, being entirely coated within, and having become cool, was cut into small round mirrors. It is not many years since they were sold in Germany by the name of Ochsenaiigen, ox eyes. They were set in a round painted board, and had a very broad border, and reflected a diminished but very clear image. The coating of glass with an amalgam of tin foil and mercury was practised by the Venetians in the 16th century. The process, as described by Porta, who witnessed it at Murano, consisted in spreading the tin foil smoothly upon a plane surface, and pouring upon it mercury, which was rubbed in with the hand or a hare's foot. The amalgam thus formed was then covered with a sheet of paper, and the glass being laid upon this and pressed down, the paper was drawn out. Weights were then laid upon the glass, and it was left for some time for the excess of mercury to drain off. The introduction of this manufacture into France is noticed in the article Glass. The chief modern improvement in the art consists in the use of very large plates, the process of coating them not differing essentially from that of the Venetians 300 years ago.
The present method is as follows: A large stone table, ground perfectly smooth, is so arranged as to bo easily canted a little on one side by means of a screw set beneath it. Around the edges of the table is a groove, in which mercury may flow and drop from one corner into bowls. The table is first made perfectly horizontal, and then tin foil is carefully laid over it, covering a greater space than the glass to be coated. A strip of glass is placed along each of three sides of the foil to prevent the mercury from flowing off. The metal is then poured from ladles upon the foil till it is nearly a quarter of an inch deep, and its tendency to flow is checked by its affinity for the tin foil and the mechanical obstruction of the slips of glass. The plate of glass, cleaned with especial care, is dexterously slid on from the open side, and its advancing edge is kept in the mercury, so that no air or floating oxide of the metal or other impurities can get between the glass and the clean surface of the mercury. When exactly in its place, it is held till one edge of the table has been elevated 10° or 12° and the superfluous mercury has run off. Heavy weights are placed on the glass, and it is left for several hours.
It is then turned over and placed upon a frame, the side covered with the amalgam, which adheres to it, being uppermost. In this position the amalgam becomes hard, and the plate can then be set on edge; but for several weeks it is necessary to guard against turning it over, as until the amalgam is thoroughly dried the coating is easily injured. - Several serious difficulties attend this process. The health of the workmen is so affected by the fumes of the mercury that they can rarely follow the business more than a few years; for this no remedy has been found so effectual as thorough ventilation and the frequent use of sulphur baths. The glass plates are liable to be broken by the weights placed upon them; and the coating of amalgam is frequently spoiled by the drops of mercury removing portions of it as they trickle down, or by its crystallizing, or by mechanical abrasion. Many methods of silvering have been contrived and patented with the view of obviating these defects, some of which are important. In 1855 a patent was granted in England to Tony Petitjean for a method of precipitating silver, gold, or platinum upon glass, so as to form a coating upon it, by the use of two solutions, the effect of which when mixed upon the glass is to decompose each other.
The solutions he employed were different compounds of ammonio-nitrate of silver, tartaric acid, and distilled water; and they were placed upon the plate while this was at the temperature of 150° F. The precipitated silver within 20 minutes covered the glass, to which it adhered; and the solution being then turned off, all that remained to complete the mirror was to wash the surface, and when dry cover it with a coat of varnish to protect it from injury. The silvering thus obtained is not so white, and is rarely so free from blemishes, as the amalgam coating. In 1849 Mr. Drayton made known a similar method, an improvement upon a process which he patented in 1843. He employed ammonia 1 oz., nitrate of silver 2 oz., water 3 oz., and alcohol 3 oz.; these, being carefully mixed, were all allowed to stand a few hours, when to each ounce of the liquid was added an ounce of saccharine matter, as of grape sugar, dissolved in equal portions of spirit and water. Liebig invented a method of coating glass with silver, in which, after the silver coating is laid on, it is covered with a coating of copper precipitated upon it by the galvanic current, or is protected by varnish.
Silver mirrors are now extensively made in New York. For platinizing glass, R. Bottger recommends the following process: Pour rosemary oil upon the dry chloride of platinum in a porcelain dish, and knead it well until all parts are moistened; then rub this up with five times its weight of lavender oil, and leave the liquid a short time to clarify. The objects to be platinized are to be thinly coated with the preparation and afterward heated for a few minutes in a muffle or over a Bunsen burner. The brilliancy of aluminum has caused the suggestion of its application to the coating of mirrors; but no successful experiments have yet been made with it for this purpose. Large mirrors are made in the United States by coating the imported plates. The old amalgamation method with tin foil and mercury is preferred to any of the more recent inventions, by reason of the greater whiteness and brilliancy of the reflection and the greater permanence of the coating. - For telescopes, philosophical instruments, and lighthouses, various sorts of mirrors are in use, and reference to them may be found under various heads in this work, as Burning Glass, Fresnel, Lighthouse, Optics, Speculum, Telescope, etc.
Concave mirrors serve to concentrate the rays of the sun in one point and produce intense heat.