Japanese Mirrors

Abstract

MORE than eleven years ago, in February, 1866, I published in The Reader (since extinct), a letter giving, I venture to think, a complete explanation of the phenomena exhibited by certain Japanese mirrors (through a mistake as to their nationality I called them Chinese); and as your readers appear to be unacquainted with this, perhaps I may be allowed to reproduce the substance of my former letter. In order to ascertain whether any variations in the form of the surface of the mirror, which was very slightly convex, affected the question, I looked for any distortions that might be produced in the image of the ground-glass globe of a gas lamp, as the point of incidence moved across that portion of the polished surface on the back of which was a raised figure. Let A and B be two lines on the surface immediately over the two edges of such a figure. Then as the limb of the image approached A, it became flattened; when it had passed A it expanded to more than its original size; indeed between A and B the image was sensibly larger than when viewed from any other part of the mirror. When the limb approached B it was again flattened, and beyond B it resumed its original dimensions. This clearly proves that the portion of the surface of the mirror between A and B was, if not actually plane, at least less convex than the rest of the surface; and as upon this supposition the figures when thrown upon a screen should appear brighter than the rest of the image, which is exactly what occurs in fact, there can be no doubt that this is sufficient to account for the peculiarity in question. In all probability the mirror had warped in cooling, except in the thicker portions where the raised figures existed.