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



The former amounts to 1,372,500, or nearly three times mine, which is 

 only 494,208. Had I made the estimate for three months of rain instead 

 of four, my average of course would have been larger. There is too 

 another reason why my estimate may be considered as lower than the 

 truth. There is in the rains a small back stream, which forms an island of 

 the opposite shore here. I examined this in the rains, but found the velo- 

 city of it so trifling, that I was induced to pass it over. Neither of these 

 causes could raise mine to within one half of the Benares estimate. 



III. Note on the Magic Mirrors of Japan. By James Prinsep, 



Sec. Ph. Cl. As. Soc. 



[Read before the Physical Class, 13th June.] 



The Japanese have long been celebrated for the manufactory of 

 metallic mirrors, in which they carry on a considerable trade with 

 China. They are ornamented with different devices on the back, and 

 are well polished on the reflecting surface; but what constitutes their chief 

 interest among Europeans is an adventitious property possessed by them, 

 which must have originally been discovered by chance, although it is not 

 certain at what period it became known, or whether the manufacturers, 

 once apprized of the secret, have purposely cultivated its principle in the 

 fabrication of their mirrors to give them an additional value in the 

 market : the fact is known in China and among the English, but I be- 

 lieve no explanation has hitherto been suggested. 



One of these mirrors was lately brought to Calcutta, and most of 

 those whom I have the honor to address have had an opportunity of 

 witnessing its effects ; I have therefore the less cause to regret that its trans- 

 mission to England has prevented my exhibiting it this evening. It has 

 been sent home to a gentleman with whom its mysterious qualities will 

 soon cease to be an enigma, if indeed they excite a moment's curiosity in 

 his mind ; but we have been surely a little hasty in allowing it to go 

 from among ourselves, before we have attempted an explanation of the 

 phenomenon it exhibited, as though we feared to hazard the investigation 

 of so very simple a problem in optics, or have failed to discover its solu- 

 tion. To avert a suspicion of so derogatory a nature either to our zeal 

 or to our optical acumen, I beg leave to offer the following explanation 

 of the phenomenon to the consideration of the Society : 



The Japanese mirror is a slightly convex disc of bell-metal, about 

 six inches in diameter, and a quarter of an inch in thickness on the 

 edge, ground and polished on the convex face ; and covered with a 



