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



it a secret when by chance they possess it, and the latter generally 

 ignore the subject altogether. I had found many times in Chinese 

 books details regarding this kind of mirrors, but it was not of a nature 

 to satisfy the very proper curiosity of philosophers, because sometimes 

 the author gave on his own responsibility an explanation that he had 

 guessed at, and sometimes he confessed in good faith that this curious 

 property is the result of an artifice in the manufacture, the monopoly 

 of which certain skilled workmen reserve to themselves. One can 

 easily understand this prudent reticence when we remember that the 

 rare mirrors which show this phenomenon sell from ten to twenty times 

 as dear as the rest." 



M. Julien then gives an elaborate description of one of these mirrors 

 in the possession of the Marquis de La Grange. He further remarks 

 that such mirrors are called in Chinese theou-kouang-Jcien, which means 

 literally " mirrors that let the light pass through them," and that this 

 name has arisen from a popular error on the subject. Chin-kouo, a 

 Chinese writer who flourished in the middle of the eleventh century, 

 speaks with admiration about them in his memoirs called Mong-ki-'pi- 

 tdn, book xix, folio 5. The poet Kin-ma has celebrated them in verse ; 

 but up to the time of the Mongolian emperors nobody could explain 

 the cause of the wonderful phenomenon. Ou-tseu-hing, who lived 

 between 1260 and 1341 under this dynasty, had the honour of: being 

 the first to throw any light on the subject. He says : 



" When we turn one of the mirrors with its face to the sun, and 

 allow it to throw a reflection on a wall close by, we see the ornaments 

 or the characters which exist in relief on the back appear clearly. Now 

 the cause of this phenomenon arises from the employment of two kinds 

 of copper of unequal density. If on the back of the mirror a dragon 

 has been produced while casting it in the mould, then an exactly similar 

 dragon is deeply engraved on the face of the disk. Afterwards the 

 deep chisel-cuts are filled up with denser copper, which is incorporated 

 with the body of the mirror, which ought to be of finer copper, by 

 submitting the whole to the action of fire, then the face is planed and 

 prepared, and a thin layer of lead or of tin spread over it.* 



" When a beam of sunlight is allowed to fall on a polished mirror 

 prepared in this wa} r , and the image is reflected on a wall, bright 

 and dark tints are distinctly seen, the former produced . by the purer 

 copper, and the latter by the parts in which the denser copper is 

 inlaid." 



If, then, we understand this description of Ou-tseu-hing correctly, it 

 would appear that the pattern appears by reflection as a dark image 

 on a bright ground, the opposite of what is experienced in Japanese 

 mirrors. 



* This probably refers to the mercury amalgam which is used in polishing, and 

 which Ou-tseu-hing mistook for lead or tin. 



