Old-fashioned Japanese mirrors are circular, and are made of metal,-generally of bronze coated on the front with an amalgam of tin and quicksilver beautifully polished. The back is adorned in relief with flowers, birds, or Chinese characters, and there is a handle on one side, the general appearance being that of a sort of handsome metal fan.
An extraordinary peculiarity characterises some of these Japanese mirrors: sunlight reflected from their face displays a luminous image of the design on their back! So strange a phenomenon has naturally attracted the attention of men of science. After much speculation, it has been clearly proved by Professors Ayrton and Perry to arise from the fact that the curvature of the face of the mirror over the plain part of the back is greater than over the design. The mirror is cast flat, and then rendered convex before polishing, by being so strongly scratched with an iron tool as to cause a buckling of the metal into a convex form, which convexity is afterwards increased by rubbing in mercury repeatedly. The effect of both these processes is greater on the thinner parts of the mirror than on the parts over the raised design. Hence the unequal convexity, which gives the reflection of the design from the face of the mirror.
Books recommended - On the Magic Mirrors of Japan, by Professors Ayrton and Perry, in the "Proceedings of the Royal Society", Vol. XXVII. pp. 127-142. Expansion produced by Amalgamation, by the same authors, in the "Philosophical Magazine," Vol. XXII. p. 327.