Ye Magick Mirrour of Old Japan

by Silvanus P. Thompson

“The material used for making the mould is a mixture of a special kind of clay (found near Tokio and Osaka), with water and straw-ash. Two suitable slabs having been formed from this plastic compound with the aid of wooden frames, a thick layer of half-liquid mixture of powdered old crucibles, or of a fine powder called to-no-ko, made from a soft kind of whetstone, is spread on them. The design for the back of the mirror is then cut directly on one half of the mould, or a sketch drawn on paper is first stuck on and used as a guide in cutting the design in the clay. Sometimes, but rarely, the design is stamped in the clay with a pattern wood-block cut in relief like the proposed back of the mirror. After the design is complete, a rim of the same material as that used in the construction of the mould, and having a thickness equal to that desired for the mirror, is attached to one half of the mould. The two halves are then dried in the smoke of a pine-tree fire, pressed and tied together, and laid in the casting-box at an angle of 80° to the horizon, the half of the mould on which the design has been cut being uppermost. Finally the molten speculum metal is run into a number of moulds at the same time, which, when cold, are broken up, and the castings removed. Mirrors cast in a mould, in which the design has been cut by hand, are called ichi mai buki, ‘mould used once,’ and are regarded as artists’ proofs, as the design on the back is well defined. To form subsequent moulds the two halves are pressed, when the clay is wet, on an ichi mai buki mirror, and the pattern is in this way transferred, but the designs on the backs of the mirrors cast in such moulds are not as clear as on an ichi mai buki mirror, which therefore sells for a much higher price.” Flaws in the face of the casting are filled by inserting little balls of copper—giving rise, perhaps, to the idea that inlaying was used to produce the illusions. The handle is not cast along with the disk of the mirror, but is attached afterwards. Mirror castings when they are removed from the mould are roughly flat on the face, and need to be subjected to several processes to finish their reflecting surfaces: during these processes they acquire their characteristic convexity and their high polish. The mirror is laid down on its back on a wooden board, and then scraped or scratched with a rounded iron rod about a foot long, called a megebo (“distorting rod”). The process of scoring over with the blunt tool is called mege. After being scored all over with scratches in every direction, it is found to be convex. Next the face is scraped with a hand-scraping tool, then rubbed down with a whetstone, then polished over with a piece of magnolia-charcoal; and lastly, when quite smooth, the amalgam of tin and mercury is rubbed in with a stiff straw brush, and polished off with soft paper. Thicker mirrors are sometimes pared down with a knife to the convex shape: they seldom or never show magic properties. The knife is also used to pare down any portion which in the operation mege may have become too convex. The convexity is tested from time to time by applying a concave wooden form. Professor Ayrton was of opinion that the magic properties were conferred during the operation of scraping with the megebo, the thicker parts of the mirror yielding less, and therefore being polished down more than the thinner parts. He also noted that if the face was scored by the megebo with parallel lines in one direction only, the convexity acquired was cylindrical. His conclusion was: “It appears then that the magic of the Eastern mirror results from no subtle trick on the part of the maker, from no inlaying of other metal, or hardening of portions by stamping, but merely arises from the natural property possessed by thin bronze of buckling under a bending stress, so as to remain strained in the opposite direction after the stress is removed. And this stress is applied partly by the ‘distorting rod,’ and partly by the subsequent polishing, which, in an exactly similar way, tends to make the thinner parts more convex than the thicker.” The researches of Ayrton and Perry may be said to have determined once and for all the main cause of the magic properties. No one has since disputed the principal propositions of their memoir, though it is certain that the differences of curvature may be produced in several varieties of operation.

Bertin, who wrote two years later, confirmed the conclusions arrived at, and repeated on other mirrors the experiments of Ayrton and Perry, as well as those of Person and Govi. Finding that the distortions produced by heating were particularly efficacious in bringing out the magic qualities, he sought to imitate them mechanically, and, with the aid of the optician Duboscq, constructed an apparatus for conferring temporary convexity upon mirrors by mounting them against an air-tight back, with a cavity behind into which air could be driven by a force-pump. With this apparatus he examined the effects that are produced by drilling cavities, cutting grooves, and etching depressions in the backs of mirrors. Laurent, taking up this line of suggestions, produced magic mirrors of thin glass, etched into patterns at the back, and silvered on the front face. These, when mounted on an air-tight back, could be made to show either positive or negative figures by diminishing or increasing the pressure of the air behind the mirror by means of a simple india-rubber pear attached by a flexible tube to an aperture in the back; the pressure of the hand sufficing to bring out the optical effects. Laurent further showed that by taking an ordinary piece of mirror glass (patent plate glass silvered at the back) magic effects could be produced by lightly pressing against the back hot pieces of metal on which a raised pattern had been engraved. In this case the portions of the mirror nearest in contact with the hot metal became hotter than the other parts, and expanding more, set up minute differences of curvature sufficient to concentrate the reflected rays of light from the heated parts.

About the same time Mendenhall, then in Japan, made further observations, which were communicated to the American Association for the Advancement of Science at its meeting at Cincinnati. Mendenhall was followed by two Japanese observers,—both trained in physical research in Europe,—Goto and Muraoka, the latter of whom, while confirming the main propositions that the effects are due to differences of convexity, and that the convexity is acquired during the process of mege or scoring over with the megebo, gave some further details learned from the mirror-makers of Tokio. He showed that any plate, if thin enough, whether of bronze, brass, copper, lead, zinc, iron, or glass, acquires the property of convexity on being scored over with scratches; and he came to the conclusion that this surface-expansion on the scored side of the metal arose from some sort of release of molecular tension across the lines so scored. Like Ayrton, he held that this production of convexity on the scored surface explained the phenomenon that a scratch made by a file or pointed tool on the back causes a corresponding bright line to be reflected from the face. In a second article Muraoka sought to prove that the differences of curvature are not really due to differences of pressure during the operations of scoring and surfacing, but are due to the unequal rising-up of the metal in the thick and thin parts when the surface is caused to expand by scoring it over with scratches.

Plate IV.

In May, 1886, Professors Ayrton and Perry, having discovered that the act of amalgamating with mercury the surface of a thick brass bar produces a powerful expansion of the amalgamated surface and bends the bar convexly, made the further suggestion that the employment by the Japanese mirror-polishers of a mercury-amalgam might assist the operation of the megebo in producing the differences of curvature between the thinner and thicker parts.

The author having from time to time picked up a number of Japanese mirrors from dealers in oriental curiosities, and having repeated all the researches enumerated above and added some others, is able to demonstrate very completely the phenomena in question.

Plate I. depicts a mirror in the author’s possession, about 7 inches in diameter, the back of which bears in high relief a crest in the form of a bird (hoho). The casting has evidently been tooled up to give greater sharpness, and the part in high relief has been ground and polished. The pattern which this mirror casts from its face is shown beside the mirror, both being photographically reproduced. This mirror shows the pattern with sunlight, or with the light of the electric arc, or with lime-light. It can also be shown to a few persons at a time by means of the flame of a paraffin lamp twenty feet away, or even by the light of an ordinary candle a couple of feet away. In the case of these weaker sources of light the mirror must be held close to the white screen or card upon which the image is to be received.

Plate II. is taken from a mirror upon which there is, in high relief, polished, a crest consisting of the imperial kiri within a circlet, together with a landscape of storks and bamboos in lower relief. In this case only the ornament in high relief produces any effect: and it is interesting to notice that, whereas the circlet on the back is a simple flat band, the circlet in the image (which is distorted in consequence of a general distortion of the mirror face as a whole) presents double lines. The author is disposed to think that the ornament on the back must in this instance have been subjected to polishing subsequently to the front face.

Plate III. represents a very thin mirror, about 9¼ inches in diameter and not more than 0·04 inch thick in its thinnest parts. The central character in high relief is surrounded with the seven precious objects in lower relief. Most of these may be seen more or less distinctly in the luminous pattern cast from the front.