540 



NATURE 



{April lo, 1879 



of course arising from a popular error on the subject. 

 The Roman writer Aulus Gellius, who liv^ed seventeen 

 centuries ago, referred to mirrors that sometimes reflected 

 their backs and sometimes did not. From the great 

 antiquity of these Chinese magic mirrors, the German 

 writer Herr Sterne has concluded that it is probable that 

 the mirrors with secret signs and figures of imps on the 

 back, which formed a portion of the stock-in-trade of the 

 witches of the middle ages, were of Eastern manufacture. 

 The Italian historian Muratori gives an account of the 

 magic mirror found under the pillow of the Bishop of 

 Verona, who was afterwards condemned to death by 

 Martin Delia Scala, as well as of the one discovered in 

 the house of Colla da Rienzi, and on the back of which 

 was the word " Fiorone." But of these magic mirrors, 

 which have played so important a part, not only in the 

 priestcraft of China, but also in the oracles of the Greeks 

 and Etruscans, and in the witchcraft of the middle 

 ages, inquiry has shown that Japanese literature makes 

 absolutely no mention. 



Is it, then, that such mirrors cannot be found in Japan ? 

 Undoubtedly they cannot be bought on inquiry at the 

 shops, but Prof. Ayrton's investigations have shown that 

 if a careful examination with properly arranged light be 

 made of a large number of the ordinary Japanese bronze 

 mirrors, a few, perhaps 2 or 3 per cent., will be found 

 showing the magic property clearly. 



The lecturer then referred to the extracts he had made 

 from a large portion of that which had been written in 

 various languages regarding the explanation of the phe- 

 nomenon. He mentioned that the earliest explanation 

 was given by a Chinaman, Ou tseu-hing, who lived be- 

 tween 1260 and 1341, and who also had the impression 

 that the magic property of the mirror was produced by 

 an artifice ; for he wrote : " 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 orna- 

 ments or the characters which exist in relief on the back, 

 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. After- 

 wards 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 way, 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," 



Ou-tseu-hing adds that he has seen a mirror of this 

 kind broken into pieces, and that he has thus ascertained 

 for himself the truth of this explanation. 



In a paper communicated some years ago to the French 

 Academy of Sciences, the well-known French writer on 

 China, M. Stanislaus Julien, says : " Many famous philo- 

 sophers have for a long time, but without success, endea- 

 voured to find out the true cause of the phenomenon 

 which has caused certain metallic mirrors constructed in 

 China to have acquired the name of magic mirrors. 

 Even in the country itself where tbey are made, no 

 European has up to the present time been able to obtain 

 either from the manufacturers, or from men of letters, 

 the information which is so full of interest to us, because 

 the former keep 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 regard- 

 ing this kind of mirrors, but it was not of a nature to 



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

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



satisfy the very proper curiosity of the philosopher, 

 because sometimes the author gave on his own responsi- 

 bility, an explanation that he had guessed at, and some- 

 times 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." 



The prevalent idea has been that the phenomenon of 

 the magic mirror was caused by a difference of density in 

 the various parts of the surface, either produced intention- 

 ally or accidentally ; and this, the lecturer explained, arose 

 from two causes, first, from the common belief that the 

 patterns on Japanese and Chinese mirrors were, like those 

 on ordinaiy coins, produced by stamping ; the other, 

 because the distinguished European philosophers who 

 had examined into the question had investigated with 

 considerable success, experimentally, how such mirrors 

 might be made, but they had not, the lecturer thought, 

 directed their attention to the examination of the ques- 

 tion — How was the phenomenon in these rare eastern 

 mirrors actually produced ? — obviously a very different 

 question. 



Prof. Ayrton mentioned that he and Prof. Perry were 

 led to take up the investigation from a very remarkable 

 fact pointed out by Prof. Atkinson, of Japan, viz., that a 

 scratch with a blunt iron nail on the back of one of these 

 magic mirrors, although it produced no mark on the face 

 of the mirror which could be seen by direct vision, 

 nevertheless became visible as a bright line on the screen 

 when a beam of sunlight was reflected from the polished 

 face of the mirror. The lecturer mentioned that after 

 trying several experiments with polarised light, &c.. Prof. 

 Perry and himself availed themselves of a very simple 

 method of investigation, but one which had apparently 

 not suggested itself to previous observers. On one occa- 

 sion, when some of their students were using lenses to 

 endeavour to make the exhibition of the phenomenon 

 more striking, it occurred to them that the employment of 

 beams of light of different degrees of convergence or 

 divergence would furnish a test for deciding the cause of 

 the whole action. For while, if the phenomenon were 

 due to the molecular differences in the surface — the com- 

 monly received opinion — the effect would be practically 

 independent of the amount of convergence of the beam 

 of light ; on the other hand, if it by any chance were due 

 to portions of the reflecting surface being less convex 

 than the remainder, a complex inversion of the pheno- 

 menon might be expected to occur, if the experiment, 

 instead of being tried in ordinary sunlight, were made 

 under certain conditions in a converging beam — that is, 

 the thicker portions of the mirror might be expected to 

 appear darker instead of lighter than the remainder. 



[Experiments were then shown of the image cast on the 

 screen : (i) when a divergent beam of light fell on the 

 mirror, (2) when the beam was parallel, (3) when the 

 beam was convergent; and it was seen (i)the pattern 

 appeared as bright on a dark ground, (2) the pattern was 

 invisible, (3) the pattern appeared as dark on a light 

 ground.] 



Again, by allowing a parallel beam of light to fall on 

 it , and interposing a double convex lens between the 

 mirror and the screen, we can make the image show the 

 pattern either as a bright on a dark ground, or as dark on 

 a bright ground, or not at all, merely by causing the screen 

 to be : 1st, nearer the lens than the conjugate focus of the 

 mirror ; 2nd, farther than the conjugate focus ; 3rd, at the 

 conjugate focus. [This experiment was here showri.] Now 

 it can easily be proved by simple geometrical optics that 

 each of these effects would be produced if the thicker 

 parts of the mirror were a little less convex than the 

 remainder. [This was explained by various geometrical 



