1911 Encyclopædia Britannica/Mirror
MIRROR (through O. Fr. mirour, mod. miroir, from a supposed Late Lat. miratorium, from mirari, to admire), an optical instrument which produces images of objects by reflection. In its usual forms it is simply a highly polished sheet of metal or of glass (which may or may not be covered, either behind or before, with a metallic film); a metallic mirror is usually termed a speculum. The laws relating to the optical properties of mirrors are treated in the article Reflection of Light.
Ancient Mirrors.—The mirror (κάτοπτρον, ἔσοπτρον, ἕνοπτρον, speculum) of the Etruscans, Greeks and Romans consisted of a thin disk of metal (usually bronze) slightly convex and polished on one side, the other being left plain or having a design incised upon it. A manufactory of mirrors of glass at Sidon is mentioned by Pliny (Nat. Hist. xxxvi. 66, 193), but they appear to have been little used (one has been found at San Remo). Glass mirrors were coated, but with tin; some silver mirrors have also been found. They are said to have been in use as early as the time of Pompey, and were common under the empire. Homer knows nothing of mirrors, but they are frequently mentioned in the tragedians and onwards. The usual size was that of an ordinary hand-mirror, but in imperial times some appear to have been large enough to take in the whole figure (Seneca, Nat. quaest., i. 17, 8), being either fixed to the wall or working up and down like a window sash. The first specimen of a Greek mirror was not discovered till 1867, at Corinth, and the number extant is comparatively small. They are usually provided with a handle, which sometimes took the form of a statuette (especially of Aphrodite) supported on a pedestal, or consist of two metallic circular disks (the “box” mirrors) fitting in to each other, and sometimes fastened together by a hinge. The upper disk or cover was ornamented on the outside with a design in low relief; inside it was polished to reflect the face. The lower disk was decorated inside with engraved figures. The best specimens of both kinds of mirrors date from a little before 400 B.C. and last for some time after that. Of the reliefs, one of the best examples is “Ganymede carried away by the eagle”; amongst the incised mirrors may be mentioned one representing Leucas and Corinthus, inscribed with their names (both the above in Collignon, L'Archéologie grecque, 1907, figs. 212, 213), the Genius of the Cock-fights (Revue archéologique, new ser. xvii., 1868, Pl. 13). A bronze mirror-case, found at Corinth, has attached on the outside a relief representing an Eros with two girls; on the inside is incised a design of a nymph seated on a bench and playing with Pan at a game resembling the Italian mora (Classical Review, Feb. 1889, p. 86). On the back of another mirror in the British Museum (Gazette archéologique, ii. Pl. 27) is a figure of Eros which has been silvered over. With this was found the bronze case used to contain it, on the back of which is a group of Aphrodite and Eros in repoussé. It was found in Crete; but most of the Greek mirrors and mirror-cases having designs are from Corinth.
The principal feature of the Etruscan mirrors, the extant examples of which far outnumber the Greek, is the design incised on the back. Belonging chiefly to the 4th and 3rd centuries, they mostly resemble the Greek disk-mirrors in form, box-mirrors being rare. As a rule the subjects incised are taken from Greek mythology and legend (Trojan War, birth of Athena, Aphrodite and Adonis), the names of the persons represented being frequently added in Etruscan letters and orthography (Apul = Apollo, Achle = Achilles, Achmemrum = Agamemnon). Scenes from daily life, the toilet, the bath, the palaestra, also occur. In most cases the style of drawing, the types of the figures, and the manner of composing the groups are true to the characteristics of Greek art. Some may have been imported from Greece, but the greater number appears to have been more or less faithfully imitated from such designs as occurred on the Greek vases which the Etruscans obtained from Greece. Even where distinctly Etruscan figures are introduced, such as the heroes Aelius and Caelius Vibenna on a mirror in the British Museum, Greek models are followed. Although the work is frequently rough and careless, certain very fine and beautiful specimens have been found: the famous Semele-mirror, and the healing of Telephus, in which Achilles is shown scraping the healing rust from the lance with a crescent-shaped knife (Baumeister, Denkmäler, figs. 557, 1774). Roman mirrors are usually disk-mirrors, the back of the disk, if engraved, being generally ornamented with decorative patterns, not with any subject design.
Plain mirrors are found wherever Greek and Roman civilization spread, and a specimen found in Cornwall (now in the British Museum) shows that the Celtic population of England had adopted the form and substance of the mirror from their conquerors. This specimen is enriched with a Celtic pattern incised. The shape of the handle exhibits native originality. Mirrors were sometimes used in Greece for purposes of divination (Pausanias vii. 21, 5). The mirror was let down into a well by means of a string until it grazed the surface of the water with the rim; after a little while it was pulled up, and when looked into showed the face of the sick person, alive or dead, on whose behalf the ceremony had been performed. This took place at Patrae.
See J. J. de Witte, “ Les miroirs chez les anciens,” in Extrait des annales de l'académie, xxviii. (Antwerp, 1872); Mylonas, Ἑλληνικὰ κάτοπτρα (Athens, 1876); M. Collignon, L'Archéologie grecque (new ed., 1907; Eng. tr. by J. H. Wright, 1886); E. Gerhard, Etruskische Spiegel (1840–1867), continued by K. Klugmann and G. Körte (1884–1897); article in Smith's Dictionary of Greek and Roman Antiquities (3rd ed., 1891). (J. H. F.)
Medieval and Modern Mirrors.—Small metallic mirrors with a highly polished surface were largely used during the middle ages: pocket mirrors or small hand mirrors carried at the girdle being indispensable adjuncts to ladies’ toilets. The pocket mirrors consisted of small circular plaques of polished metal, usually steel or silver, fixed in a shallow circular box covered with a lid. Mirror-cases were chiefly made of ivory, carved with relief representations of love or domestic scenes, hunting and games, and sometimes illustrations of popular poetry or romance. Gold and silver, enamels, ebony and other costly materials were likewise used for mirror cases, on which were lavished the highest decorative efforts of art workmanship and costly jewelling. The mirrors worn at the girdle had no cover, but were furnished with a short handle. In 625 Pope Boniface IV. sent Queen Ethelberga of Northumbria a present of a silver mirror; and in early Anglo-Saxon times mirrors were well known in England. It is a remarkable fact that on many of the sculptured stones of Scotland, belonging probably to the 7th, 8th or 9th century, representations of mirrors, mirror-cases and combs occur.
The method of backing glass with thin sheets of metal for mirrors was well known in the middle ages, at a time when steel and silver mirrors were almost exclusively employed. Vincent of Beauvais, writing about 1250, says that the mirror of glass and lead is the best of all, “”; and a verre à mirer is mentioned in the inventories of the dukes of Burgundy, dating from the 15th century. A gild of glass-mirror makers existed at Nuremberg in 1373, and small convex mirrors were commonly made in southern Germany before the beginning of the 16th century; and these continued to be in demand, under the name of bull’s-eyes (Ochsen-Augen), till comparatively modern times. They were made by blowing small globes of glass into which while still hot was passed through the pipe a mixture of tin, antimony and resin or tar. When the globe was entirely coated with the metallic compound and cooled it was cut into convex lenses, which formed small but well-defined images. As early as 1317, a “Magister de Alemania,” who knew how to work glass for mirrors, broke an agreement he had made to instruct three Venetians, leaving in their hands a large quantity of mixed alum and soot for which they could find no use. It was, however, in Venice that the making of glass mirrors on a commercial scale was first developed; and the republic enjoyed a much prized monopoly of the manufacture for about a century and a half. In 1507 two inhabitants of Murano, representing that they possessed the secret of making perfect mirrors of glass, a knowledge hitherto confined to one German glass-house, obtained an exclusive privilege of manufacturing mirrors for a period of twenty years. In 1564 the mirror-makers of Venice, who enjoyed peculiar privileges, formed themselves into a corporation. The products of the Murano glass-houses quickly supplanted the mirrors of polished metal, and a large and lucrative trade in Venetian glass mirrors sprang up. They were made from blown cylinders of glass, which were slit, flattened on a stone, carefully polished, the edges frequently bevelled, and the backs “silvered” by an amalgam. The glass was remarkably pure and uniform, the “silvering” bright, and the sheets sometimes of considerable dimensions. In the inventory of his effects, made on the death of the French minister Colbert, a Venetian mirror, 46 by 26 in., in a silver frame, is valued at 8016 livres, while a picture by Raphael is put down at 3000 livres.
The manufacture of glass mirrors, with the aid of Italian workmen, was practised in England by Sir Robert Mansel early in the 17th century, and about 1670 the duke of Buckingham was concerned in glass-works at Lambeth where flint glass was made for looking-glasses. These old English mirrors, with bevelled edges in the Venetian fashion, are still well known. The Venetians guarded with the utmost jealousy the secrets of their manufactures, and gave exceptional privileges to those engaged in such industries. By their statutes any glass-maker carrying his art into a foreign state was ordered to return on the pain of imprisonment of his nearest relatives, and should he disobey the command emissaries were delegated to slay him. In face of such a statute Colbert attempted in 1664 to get Venetian artists transported to France to develop the two great industries of mirror-making and point-lace working. The ambassador, the bishop of Béziers, pointed out that this was to court the risk of being thrown into the Adriatic, and, further, that Venice was selling to France mirrors to the value of 100,000 crowns and lace to three or four times that value. Nevertheless, twenty Venetian glass-mirror makers were sent to France in 1665, and the manufacture was begun in the Faubourg St Antoine, Paris. But previous to this the art of blowing glass for mirrors had been practised at Tour-la-Ville, near Cherbourg, by Richard Lucas, Sieur de Nehou, in 1653; and by the subsequent combination of skill of both establishments French mirrors soon excelled in quality those of Venice. The art received a new impulse in France on the introduction of the making of plate glass in 1691. The St Gobain Glass Company attribute the discovery to Louis Lucas of Nehou, and over the door of the chapel of St Gobain they have placed an inscription in memory of “Louis Lucas qui inventa en 1691 le méthode de couler les glaces et installa la manufacture en 1695 dans le chateau de Saint Gobain.”
Manufacture.—The term “silvering,” as applied to the formation of a metallic coating on glass for giving it the properties of a mirror, was till quite recently a misnomer, seeing that till about 1840 no silver, but a tin amalgam, was used in the process. Now, however, a large proportion of mirrors are made by depositing on the glass a coating of pure silver, and the old amalgamation process is comparatively little used.
The process of amalgamation consists in applying a thin amalgam of tin and mercury to the surface of glass. A sheet of thin tin-foil, somewhat larger than the glass to be operated on, is spread out on a flat table, and after all folds and creases have been completely removed a small quantity of mercury is rubbed lightly and quickly over the whole surface, and the scum of dust, impure tin and mercury is taken off. Mercury is then poured upon the “quickened” foil until there is a body of it sufficient to float the glass to be silvered (about 1 in. deep), and the glass (scrupulously cleaned simultaneously with the above operations) is slid over the surface of the mercury. Weights are placed over the surface until the greater part of the amalgamated mercury is pressed out, and the table is then tilted so that all superfluous mercury finds its way to the gutter. The glass is left twenty-four hours under weights; it is then turned over, silvered side up and removed to a drainer, where it dries and hardens. This process, when elaborated, yields excellent results, producing a brilliant silver-white metallic lustre, which is only subject to alteration by exposure to high temperatures or by contact with damp surfaces; but the mercurial vapours to which the workmen are exposed give rise to the most distressing and fatal affections.
The “silver on glass” mirror may be regarded as a discovery of J. von Liebig, who in 1835 observed that by heating aldehyde with an ammoniacal solution of silver nitrate in a glass vessel a brilliant deposit of metallic silver was formed on the surface of the glass. In practice the process was introduced about 1840; and it is now carried on, with several modifications, in two distinct ways, called the hot and the cold process respectively. In the former method there is employed a horizontal double-bottomed metallic table, which is heated with steam to from 35° to 40° C., and the reduction of the ammoniacal silver solution is effected with tartaric acid.
In silvering by the cold process advantage is taken of the power of sugar to reduce the silver nitrate. This method has been generally adopted for the silvering of mirrors for astronomical telescopes. G. W. Ritchey (“The Modern Reflecting Telescope,” Smithsonian Contributions to Knowledge, xxxiv. 40) used the process devised by Brashear in 1884. The glass disk is mounted on a rocking-table, and most carefully cleaned with nitric acid, potash, and finally with distilled water. The reducing solution (which improves on keeping) is made up from 200 parts of water, 20 of loaf sugar, 20 of alcohol and 1 of nitric acid (commercial pure). The silver solution is prepared as follows: 2 parts of silver nitrate are dissolved in 20 parts of water, and strong ammonia added until the brown solution becomes clear. A solution of 11 parts of potash (pure by alcohol) in 20 of water is now added, and then ammonia until the solution is again clear. A solution of 1 part of silver nitrate in 16 of water is added until the liquid is straw-coloured; it is then filtered. Quantities of the solutions, such that the sugar equals one half the nitrate, are taken, then diluted, mixed, and poured on to the plate, which is gently rocked. The liquid goes muddy-brown, and in 3 to 4 minutes it begins to clear, a thick deposit being formed in about 5 minutes. The solution is poured off, and water run on, the streaks of precipitate being removed by lightly held cotton wool. The washing is repeated, and then water is allowed to remain on the film for one hour. The water is then run off, and the plate is washed several times with alcohol, and then dried by an air fan. The film is now burnished with a chamois leather pad, and finally with the finest jewellers’ rouge, the silver surface being the reflecting surface of the mirror.
The deposit of silver on glass is not so adherent and unalterable under the influence of sunlight and sulphurous fumes as the tin-mercury amalgam, and, moreover, real silvered glass has in many cases a slightly yellowish tinge. These defects have been overcome by a process introduced by Lenoir, which consists of brushing over the silvered surface with a dilute solution of cyanide of mercury, which, instantaneously forming a kind of amalgam, renders the deposit at once much whiter and more firmly adherent than before. To protect the thin metallic film from mechanical injury and the chemical action of gases and vapours it is coated with shellac or copal varnish, over which, when dry, are applied two coatings of red-lead paint or an electrolytically-deposited film of copper. This precaution only applies when the silver forms the back of the mirror.
Platinum Mirrors.—A cheap process of preparing mirror glass was to some extent prosecuted in France, whereby a thin but very adherent deposit of platinum is formed on the glass. A solution of chloride of platinum with a proportion of litharge and borate of lead dissolved in essential oil of spike is applied with a brush to well-cleaned glass, which is then placed on edge in a muffle furnace, and the platinum is thus burned in, forming an exceedingly thin but brilliant metallic backing having a somewhat grey lustre. It was used only for the lids of cheap boxes, toys, ornamental letters, &c.
Magic Mirrors.—Hand mirrors of metal are still in common use in Oriental countries, and in Japan bronze mirrors possess a religious significance. They have been known and used from the most remote period, mention of them being found in Chinese literature of the 9th century. The (reputed) first made Japanese mirror, preserved at Isé, is an object of the highest veneration in Japan, and an ancient mirror, connected with which is a tradition to the effect that it was given by the sun-goddess at the foundation of the empire, is a principal article of the Japanese regalia. The mirrors of Japan in general consist of thin disks, from 3 to 12 in. in diameter, of speculum metal with handles, cast in one piece. The polished face of the mirror is slightly convex in form, so that a reflected image is seen proportionately reduced in size; the back of the disk is occupied wit ornamentation and inscriptions in bold relief, and its rim is also raised to the back. Much attention has been attracted to these mirrors by a singular physical peculiarity which in a few cases they are found to possess. These are known as magic mirrors from the fact that when a strong beam of light is reflected from their smooth and polished surface, and thrown on a white screen, an image of the raised ornaments and characters on the back of the mirror is formed with more or less distinctness in the disk of light on the screen. This peculiarity has at no time been specially observed by the Japanese, but in China it attracted attention as early as the 11th century, and mirrors possessed of this property sell among the Chinese at ten or even twenty times the price sought for the ordinary nonsensitive examples. The true explanation of the magic mirror was first suggested by the French physicist Charles Cléophas Person in 1847, who observed that the rejecting surface of the mirrors was not uniformly convex, the portions opposite relief surfaces being plane. Therefore, as he says, “the rays reflected from the convex portion diverge and give but a feebly illuminated image, while, on the contrary, the rays reflected from the plane portions of the mirror preserve their parallelism, and appear on the screen as an image by reason of their contrast with the feebler illumination of the rest of, the disk.” Such differences of plane in the mirror surface are accidental, being due to the manner in which it is prepared, a process explained by W. E. Ayrton and J. Perry (Proc. Roy. Soc., 1878, vol. xxviii.), by whom ample details of the history, process of manufacture and composition of Oriental mirrors have been published. A preliminary operation in polishing the surface consists of scoring the cast disk in every direction with a sharp tool. The thicker portions with relief ornament offer more resistance to the pressure of the tool than the thin flat portions, which tend to yield and form at first a concave surface, but this by the reaction of its elasticity rises afterwards and forms a slightly convex surface, while the more rigid thick, portions are comparatively little affected. This irregularity of surface is inconspicuous in ordinary light, and does not visibly distort images; but when the mirror redirects a bright light on a screen the unequal radiation renders the minute differences of surface obvious.