130 Profs. W. E. Ayrton and John Perry. [Dec. 12, 



the reflecting surface corresponding to the raised parts, whereby the 

 amount of light reflected was greater ; or supposing that, of the light 

 which falls upon the surface, a part is absorbed and the rest reflected, 

 those parts corresponding to the raised portions on the back are 

 altered by the pressure in such a way that less is absorbed, and there- 

 fore a bright image appears." 



Professor Atkinson cautiously adds : " This, of course, is not an 

 explanation of the phenomenon, but I put it forward as perhaps in- 

 dicating the direction in which a true explanation may be looked for." 



In vol. i, p. 242, year 1832, of the " Journal of the Asiatic Society 

 of Bengal," Mr. Prinsep gives an account of a Japanese magic mirror 

 which he had seen in Calcutta. He does not appear to have made any 

 direct experiments with this mirror for the purpose of elucidating 

 which of all the possible causes is the real cause of the magic pheno- 

 mena, but rather he concludes " from analogy that the thin parts or 

 tympanum of the Japanese mirror are slightly convex with reference 

 to the rest of the reflecting surface, which may have been caused either 

 by the ornamental work having been stamped or partially carved with 

 a hammer and chisel on its back ; or, which is more probable, that 

 part of the metal was by this stamping rendered in a degree harder 

 than the rest, so that in polishing it was not worn away to the same 

 extent," It does not seem to have occurred to him that Japanese 

 mirrors are cast and not stamped at all. 



In " Nature," June 14th, 1877, Mr. Highley refers to the exhibition 

 of a Japanese mirror by Professor Pepper some years ago at the Poly- 

 technic Institution, London, and to the praiseworthy attempt of an 

 English brass worker, who saw the experiment, and who also was under 

 the false impression that such mirrors were stamped, to solve the pro- 

 blem. " The workman found that taking ordinary brass and stamping 

 upon its 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 unstamped parts, so that images of the pat- 

 tern could be reflected from the finally polished surface, just as with 

 the Japanese specula, though no difference of surface could be detected 

 with the eye." 



To people who have not been in China or Japan, and personally 

 studied mirror-making, this idea of stamping seems very plausible, for 

 Sir David Brewster, on p. 113 et seq. of his " Letters on Natural Magic," 

 published in 1842, describes fully a method, depending on the mole- 

 cular change produced by stamping, by means of which the inscrip- 

 tions on old coins, that have been worn quite smooth, may be deciphered. 

 This method merely consists in heating the coin on a piece of red-hot 

 iron, when the inscription becomes visible from the different rate of 

 oxidation of the part of the coin that has been subjected to great 



