Magic Mirrors - Page 4
On the 12th December 1878, Professors W.E Ayrton and John Perry of the Imperial College of Engineering in Japan published a detailed paper in the Proceedings of the Royal Society which really got down to the optics of Magic Mirrors. They reviewed the literature and also referred to an exhibition of the Japanese mirror by Professor Pepper ( he of the "Ghost") at the Polytechnic Institute in London. As a result of seeing Professor Peppers experiment an English brassworker, who was under the false impression that the mirrors were stamped, attempted to solve the problem. "The workman found that by taking ordinary brass and stamping it's surface with any suitable die, not once, but three times in succession, upon exactly the same spot, grinding down and polishing between each act of stamping, a molecular difference was established between the stamped and the unstamped parts, so that the images of the pattern could be reflected from the polished surface, just as with the Japanese specula, though no difference of the surface could be detected with the eye." Clearly this was not the authentic method of producing magic mirrors but it seems to have produced a working result.
Ayrton and Perry tried to find references to the magic mirror in Japanese literature but could not find any. (Have any come to light since?) But there are many in Chinese books. Needham (Science and Civilisation in China, Cambridge 1963) refers to a magic mirror owned by Wang Tu, a Chinese official in the 5th century AD.
An article in No.29 of "Gartenlaube", Heft 8, 1877 by Carus Sterne remarks that the Roman writer Aulus Gellius who lived in the 3rd century AD refers to "mirrors that sometimes reflect their backs and sometimes do not". Sterne also refers to an account by the Italian historian Muratori of the magic mirror found under the pillow of the Bishop of Verona, who was afterwards condemned to death by Martin della Scala.
Ayrton and Perry conducted a series of optical experiments to determine the nature of the surface of the mirrors. They used converging, parallel and diverging beams of light, a variety of lenses and a polariscope. They could detect no differences in the polarisation of the light from different parts of the surface, so they eliminated Sir Dave Brewster’s suggestion. "We have strong reasons for favouring the inequality of curvature theory..... not only do we think that the thicker portions of the convex mirror are flatter than the remainder but in some instances the thicker portion is actually concave with a radius of 3 to 4 metres."
In his book "Light visible and invisible" (1897) Professor Silvanus P. Thompson explains how this variation on curvature comes about. The mirrors are cast in moulds. To polish their faces they are laid on their backs by the workman, who scrapes them violently with a round iron rod about a half an inch in diameter and a foot long, called a "megebo" or distorting rod. During the process the mirror surface becomes slightly convex. As the work progresses the thinner parts of the mirror bend and give to the tool more than the thicker parts which lie over the pattern on the back. When the pressure of the scraper is relieved the thinner parts spring back and rise slightly above the average level of the mirror surface.
More recent research by Herbert Maryon, published in the Archives of the Chinese Art Society of America (1963) suggested that the English Brassworker's method of stamping should not be so lightly dismissed. Perhaps there are two methods of producing Magic Mirrors, one by Casting, scraping and polishing, and another by casting, scraping, punching or stamping, followed by more scraping and polishing and punching. Maybe the punching produces localised hardening of the metal so that it reacts slightly differently to the scraper. When the bumps on the face produced by punching on the back are removed some of the forces caused by the hardened particles of the metal spring back.
More research by Cyril Stanley Smith of M.I.T. also published in "Chinese Art of American Society" showed the punched grooves in photomicrographs of magic mirrors. This method could account for the fact that certain mirrors show an image of the relief picture on the back and some show another image altogether. If you can find a small convex mirror you can use it to produce a real image on a screen simply by fixing a small piece of opaque adhesive tape to the front surface. If you want to be more subtle use a wax crayon or a transparent water-soluble gum.
The front of a magic mirror is generally extremely shiny, and forms an excellent mirror.
We would be interested to hear from anyone with magic mirrors and about any experiments you have tried to recreate their effects.