244 Note on the Magic Mirrors of Japan. [June, 



be altogether smooth and natural, and the first supposition of course was 

 no longer tenable. It then occurred, that the various parts of the 

 Japan mirror might be of different density, supposing the pattern to be 

 mad? by stamping, and that either the rays of light might be more 

 forcibly repelled by the denser metal than by the lighter, or that 

 parts of the surface would acquire different degrees of polish, 

 sufficient to cause the illusion, although imperceptible to the eye. But 

 in such case the thin parts, from being the hardest, should give the 

 stronger reflection. This supposition was also overthrown by experi- 

 ment. A disc of silver having been annealed at a red heat, so as to 

 be quite soft, was stamped on the back with a circular ring, deeply 

 indented, so as to harden the silver in that part only. The opposite 

 surface was then ground and polished, when it was found to give a clear 

 and uniformly reflected spectrum. 



The third and, I believe, the true explanation was suggested by the 

 well known phenomenon of the reflection from a brass button, which 

 every school boy has remarked when sporting his Sunday " blue coat 

 with metal buttons" in the sunshine of his tutor's parlour window. The 

 button throws a radiated irregular image on the wall, exhibiting two 

 bright concentric circles, one on the edge, and another about one-third 

 within it : and there is generally a bright spot in the centre ; all of this 

 seems but the picture of the stamp on the back of the button : the radii 

 resemble, and indeed coincide with the letters of" superfine" or " trebly 

 gilt" inscribed within a double circle, and the central spot represents the 

 shank: there can be little doubt, that the principle is in this case precise- 

 ly that of the Japan mirror, and on cursory view, the surface looks 

 equally smooth and unsuspicious. On minute examination, however, 

 of several buttons, I found them to be by no means plane ; their 

 general surface is slightly convex ; there is a hollow in the centre, and 

 a projection in the position of the inscription behind, caused no doubt 

 by the blow necessary in stamping it — the polish is probably given by a 

 rotatory motion, and consequently does not remove these very small 

 irregularities. To follow up the experimental investigation, I selected 

 one of the buttons, which gave a good image ; ground it on a flat hone, 

 and polished it : all of the magical figures vanished in a moment, and a 

 plain bright disc appeared in their stead ! Here then may be a key to the 

 mystery of the minor : the deception is entirely produced by irregu- 

 larities on the surface, which are rendered the less perceptible to the eye, 

 because the surface is convex instead of being plane. 



But it may be objected, that the two circles which appear bright in 

 the reflected spectium of the button, represent the indented or thin parts 



