Manchester Afeniozrs, Vol. xlvii. (1903), No. Tf. 



VII. On the Production of Polished Metallic Surfaces 

 having the Properties of Japanese " Magic " 

 Mirrors. 



By Thomas Thorp, F.R.A.S. 



Read January 20th. Received Februa7y lyih, igoj. 



On pp. 51 to 53 of Light, Visible and Invisible, by 

 Prof. Silvanus Thompson, the following passages occur : — 



" For many years it was supposed that these mirrors 

 "were produced by some trick. But the extraordinary 

 " fact was discovered by Professor Ayrton in Japan that 

 " the Japanese themselves were unaware of the magic 

 "property of the mirrors. It results, in fact, from an 

 " accident of manufacture. Not all Japanese mirrors show 

 " the property : those that show it best are generally thin, 

 " and with a slightly convex face. It was demonstrated 

 " by Professor Ayrton, and I have since accumulated some 

 "other proofs, that the effect is due to extremely sh'ght 

 " inequalities of curvature of surface. These arise acci- 

 " dentally in the process of polishing. The mirror.s are 

 " cast in moulds. To polish their faces they are laid down 

 " on their backs by the workman, who scrapes them 

 " violently with a blunt iron tool, using great force . . . 

 " During this process they become slightly convex. The 

 " polishing is completed by scouring with charcoal and 

 "scrubbing with paper, after which they are "silvered " by 

 " application of an amalgam of tin and mercury. Now 

 " during the violent scraping with the iron tool the mirror 

 " bends, but the thin parts yield more under the pressure 

 " than the thick parts do ; hence the thick parts get worn 



April 23rd, jgoj. 



