April lo. 1879] 



NATURE 



54» 



Jiagrams.] And lastly, if the phenomenon was, as the 

 previous experiment would lead us to conclude, due not 

 to unequal reflecting power of the different portions of the 

 surface of the mirror, but to minute inequalities on the 

 surface, in consequence of which there is more scattering 

 power of the rays of light falling on one portion than on 

 another, then, since rays of light making very small 

 angles with one another do not separate perceptibly imtil 

 they have gone some distance, it follows that if the screen 

 be held very near to the mirror, the apparent reflection of 

 the back, the magical property, in fact, ought to become 

 invisible. And this also, it was shown, was exactly what 

 happened when the screen was made almost to touch the 

 polished surface. 



The lecturer then proceeded to explain why a divergent 

 beam emitted by a bright luminous point at some fifteen 

 feet distance from the mirror gave the best effects. 



We have therefore strong reasons for favouring the 

 ^'inequality of curvature" theorj-. In order, however, to 

 make the explanation quite certain, the lecturer said he 

 had made a small concavity and a small convexity on the 

 face of one of the mirrors, by hammering with a blunt 

 tool, carefully protected with a soft cushion to avoid 

 scratching the polished surface, and he showed by experi- 

 ment that the concavity reflected a bright image and the 

 convexity a dark one, when the pattern on the back ap- 

 peared bright, but when the light was so arranged that 

 the pattern appeared as dark on a bright ground, it was 

 the convexity which appeared as the bright spot and the 

 concavity as the dark one. 



Guided by all that precedes, we are led to the un- 

 doubted conclusion that the whole action of the magic 

 mirror arises from the thicker portions being flatter than 

 the remaining convex surface, and even being sometimes 

 actually concave. But, in spite of this irresistible con- 

 clusion forced on us by the experiments previously men- 

 tioned, it must be admitted that it seems extraordinary 

 how such small inequalities in the surface of the mirror, 

 so small in fact that the eye quite fails to detect them, 

 can, even with a proper arrangement of the light, produce 

 on the screen an image of ihe pattern on the back as 

 sharp and clear as is seen with a good specimen of the 

 magic mirror. 



The next question arises, why is there this difference in 

 the curv^ature of the different portions of the surface .? The 

 experience that Prof. Ayrton had gained from an examina- 

 tion of a large number of Japanese mirrors supplied, in part 

 at any rate, the answer to the question. No thick mirror 

 reflects the pattern on the back, not one of the many beau- 

 tiful mirrors exhibited at the National Exhibition of Japan 

 in 1877, and which the lecturer was so fortunate as to be 

 able to e.xperiment with in a darkened room with a bright 

 luminous point at some twelve feet distance, showed the 

 phenomenon in the slightest degree ; some good old 

 rnirrors in the museum of the Imperial College of En- 

 gineering, and which belonged to the family of the late 

 Emperor, the Shogun, of Japan, failed to reflect any trace 

 of a design, and some old round mirrors without handles, 

 which he had also tried, were (with the exception of one 

 which was immensely prized, and brought to him 

 wrapped in five distinct silk cases, and the heirloom of 

 the family of a nobleman) equally unsuccessfiil. 



Again, it is not that the pattern is less clearly executed 

 on the backs of these choice mirrors, since the better the 

 mirror the finer and bolder is the pattern, but what is 

 especially noticeable is that every one of these mirrors 

 is, as a whole, far thicker than an ordinary Japanese 

 mirror, and its surface is much less convex. 



This naturally led him to inquire how are Japanese 

 mirrors made convex? Are they cast so, or do they 

 acquire this shape from some subsequent process? 

 His search through all the literature at his disposal — 

 European, Japanese, Chinese— on the subject of mirrors 

 failed to elicit the slightest hint; he was therefore 



compelled to perform the somewhat difficult task of 

 obtaining information from the Japanese workmen them- 

 selves. Eventually he ascertained that while practically 

 all Japanese mirrors were convex, the surface of each half 

 of the mould was flat, and that the cur\ature was given 

 to the mirror after casting in the following way : the rough 

 mirror is first scraped approximately smooth with a hand- 

 scraping tool, and as this would remove any small amount 

 of convexity had such been imparted to it in casting, it is 

 useless to make the mould slightly convex. If, however, 

 a convex or concave mirror of small radius is required, 

 then the surface of the mould is made concave or convex. 

 On the other hand, to produce the small amount of con- 

 vexity which is possessed by ordinary Japanese mirrors 

 the following method is employed, if the mirror is thin, 

 and it is with thin mirrors we have especially to deal, 

 since it is only in these mirrors that the apparent reflec- 

 tion of the back is observed. The mirror is placed face 

 uppermost on a wooden board, and then scraped or 

 rather scratched w ith a rounded iron rod about a third of 

 an inch in diameter, and a foot long, called a rmgebo, " dis- 

 torting rod." so that a series of small parallel scratches is 

 produced, which causes the face of the mirror to become 

 convex in the direction at right angles to the scratches, 

 but to remain straight parallel to the scratches, in fact it 

 becomes very sUghtly cyhndrical, the axis of the cylinder 

 being parallel to the scratches. This effect is ver>^ clearly 

 seen by applying a straight-edge in different ways to 

 the face of an unpolished mirror which has received a 

 single set of scratches only. A series of scratches is next 

 made with the megebo in a direction of right angles to the 

 former, a third set intermediate between the two former, 

 and so on, the mirror each time becoming sUghtly cylin- 

 drical, the axis of the cylinder in each case being parallel 

 to the Une of scratches, so that eventually the mirror 

 becomes generally convex. Some workmen prefer to 

 make the scratches with the megebo in the form of small 

 spirals, others in the form of large spirals, but the 

 general principle of the method employed with their 

 mirrors appears to be always the same — the face of the 

 mirror is scratched with a blunted piece of iron, and 

 becomes slightly convex, the back, therefore, becoming 

 concave. 



[Mirrors were here exhibited, one with its surface flat, 

 although somewhat rough, just as it came from the mould 

 after casting ; a second that had received one set of 

 parallel scratches with the megebo, and which by means 

 of a straight edge was shown to be sUghtly cylindrical ; 

 and a third on the face of which the operation of scratch- 

 ing had been completed, and which was therefore slightly 

 convex.] 



After the operation with the "distorting-rod" the 

 mirror is very slightly scraped with a hand-scraping tool 

 to remove the scratches, and to cause the face to present 

 a smooth surface for the subsequent f>olishing. 



In the case of thick mirrors the convexity is first made 

 by cutting with a knife, and the distorting-rod applied 

 afterwards. But in connection with this cutting process 

 of thick mirrors, there is one very interesting point. If 

 the maker finds, on applying from time to time the face 

 of the mirror to a hard clay concave pattern, and turning 

 it round under a little pressure, that a portion of the 

 surface has not been in contact with the pattern, in other 

 words, that he has cut away this jwrtion too much, then 

 he rubs this spot round and round with the megebo until 

 he has restored the required degree of convexity. Here 

 again, then, scratching on the surface produces convexity-. 



Now, why does the scraping of the "distorting rod" 

 across the face of the mirror leave it convex ? During 

 the operation it is \isibly concave. The metal mast 

 receive, then, a kind of "buckle," and spring back again 

 so as to become convex when the pressure of the rod is 

 removed. It might in such a case reasonably be expected 

 that the thicker parts of the mirror would yield less to 



