Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/523

Rh M-I R R R 501 once poured over it, before the heat of the table has time to dry any part of the surface of the glass. The solution used is prepared as follows : in half a litre of distilled water 100 grammes of nitrate of silver are dissolved ; to this there is added of liquid ammonia (sp. gr. 880) 62 grammes; the mixture is filtered, and made up to 8 litres with distilled water, and 7 5 grammes of tartaric acid dissolved in 30 grammes of water are mixed with the solution. About 2 5 litres are poured over the glass for each superficial metre to be silvered. The metal immediately begins to deposit on the glass, which is maintained at about 40 C. (104 F.), and in little more than half an hour a continuous coating of silver is formed. The silvered surface is then cleaned by very cautiously wiping with a very soft chamois rubber, and treated a second time with a solution like the first, but containing a double quantity of tartaric acid. This solution is applied in two portions, and thereafter the glass is once more carefully cleared of all unattached silver and refuse and removed to a. side room for backing up. In silvering by the cold process advantage is taken of the power of inverted sugar to reduce the nitrate of silver. This process has been adopted for the silvering of mirrors for astronomical telescopes, notably for Leverrier s great telescope in the Paris Observatory. For ordinary mirror silvering the following is the process recommended by H. E. Benrath. Two solutions are prepared, the first of which contains the silver salt, and the second the sugar preparation. For the silver solution 800 grammes of nitrate of silver and 1200 grammes of nitrate of ammonium are dissolved in 10 litres of water, and 1 P 3 kilos of pure caustic soda in 10 litres of water, and of each of these solutions 1 litre is added to 8 litres of water, which is allowed to rest till the sediment forms and then decanted. The second solu tion inverted sugar is prepared by dissolving 150 grammes of loaf sugar with 15 grammes of vinegar in 5 litre of water, and boiling the solution for half an hour. After cooling it is made up with water to 4200 cubic centimetres. The silvering is done on horizontal tables in a well-lighted and moderately heated apartment, and the glass is cleaned with scrupulous care. For each square centimetre of glass operated on 15 cubic centimetres of the silver solution above described are measured out, and from 7 to 10 per cent, of the solution of inverted sugar is added, both being quickly stirred to gether and poured rapidly and evenly over the glass. The reduction immediately begins, and the solution exhibits tints passing through rose, violet, and black, till in about seven minutes it again becomes transparent and the deposit of metal is complete. This first deposit is extremely thin, and allows the transmission of bluish rays. The exhausted solution with floating and unattached dust-like granules of silver is carefully wiped off, the silvered surface washed with distilled water and again treated with the mixed solutions to the extent of half the quantity used in the first application. The finished surface is wiped and washed in the most thorough manner, for the least trace of caustic soda left would destroy the mirror. The further processes are the same in both methods of silvering. The deposit of silver on glass is not so adherent and unalter able under the influence of sunlight and sulphurous fumes as the tin-mercury amalgam, and moreover real silvered glass has a slightly yellowish tinge. These defects have been overcome by a process introduced by M. 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. Platinum Mirrors. A cheap process of preparing mirror glass is 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 is 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 especially in Japan and China they con tinue to be the prevalent form of looking-glass. In the former country indeed bronze mirrors are articles of the greatest importance in the generally meagre furnishing of houses, and besides 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 Ise, is an object of the highest veneration in Japan, and an ancient mirror, connected with which is a tradition to the effect that is was given by the sun-goddess at the foundation of the empire, is a principal article of the Japanese regalia. The mirrors of Japan vary in form and size, but in general they con sist of thin disks, from 3 to 12 inches 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 with characteristic Japanese 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 peculi arity which in a few cases they are found to possess. These arc 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 llth century, and mirrors possessed of this property sell among the Chinese at ten or even twenty times the price sought for the ordinary non-sensitive examples. The true explanation of the magic mirror was first suggested by the French physicist Person, who observed that the reflecting surface of the mirrors was not uniformly convex, the portions opposite relief surfaces being plane. There fore, as he says, &quot;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.&quot; That such differences of plane in the mirror surface arise is an accidental circumstance due to the manner in which it is prepared, a process explained by Professors Ayrton and Perry, by whom ample details of the history, process of manu facture, 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 irregu larity of surface is inconspicuous in ordinary light, and does not visibly distort images ; but when the mirror reflects a bright light on a screen the unequal radiation renders the minute differences of surface obvious. The ingenious theory of Person has been estab lished by experiments communicated by M. Govi to the academy of Turin in 1864-65, and more recently by investigations of MM. Bertin and Dubosq. See Annalcs de Chimie et de Physique (5th ser., vol. xx.). (J. PA.) Ancient Mirrors. The mirror of classical antiquity (KaTOTrrpov, speculum) was a thin disk of bronze slightly convex on one side and polished, usually provided with a handle, sometimes mounted on a stand in the form of a female figure (see COSTUME, vol. vi. p. 453, fig. 1), sometimes fixed inside a circular bronze case. The common size is that of an ordinary hand mirror. Examples large enough to take in the whole figure appear to have been rare. Mirrors of glass are mentioned, and though none of them have been found their existence need not be questioned altogether, since the process of silvering occasionally employed on bronze mirrors suggests that an analogous process may have been applied to glass. But the very large number of mirrors still existing from antiquity shows that bronze was the regular material employed. The alloy known as speculum, producing a very hard metal with great reflecting power, is comparatively seldom met with. Silver mirrors are men tioned, but none have as yet been found. The principal feature of ancient mirrors, especially those of Etruria, is the design incised on the back (see ETRURIA, vol. viii. p. 643). While twelve incised specimens are all that are as yet known from Greece, the number found in Etruria must be nearly a thousand. As a rule the subjects incised are taken from Greek mythology and legend, the names of the persons represented being frequently added in Etruscan letters and orthography. 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 im ported from Greece ; but the greater number appear to have been more or less faithfully imitated from such designs as occurred on the almost innumerable printed Greek vases which the Etruscans obtained from Greece. Even where distinctly Etruscan figures are introduced, such as the heroes ^Elius and Caelius Vibenna on a mirror in the British Museum, Greek models are followed. The characteristics of Greek art here referred to date from a little before 400 B.C., and last for some time after. In this period would fall the majority of the Etruscan mirrors, and to this period also belong the Greek incised mirrors, among which may be mentioned for their beauty one representing Leucas and Corinthus, inscribed with their names (engraved, Monuments Grecs, 1873, pi. 3, published by the Association pour I encouragement des Etudes Grecques), and another in the British Museum (Gazette Arch., ii. pi. 27), on the back of which is a figure of Eros which has been silvered over. With this last-mentioned mirror was found the bronze case used to contain it,