1879. J on tlic Mirror of Japan and its Magic QiiaUhj. HI 



oven being sometimes actually concave. But, in sj^ito of tliis irre- 

 sistible conclusion forced on us by the experiments previously 

 mentioned, it must bo 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 arrange- 

 ment of the light, produce on the screen an imago of the pattern on 

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

 magic mirror. 



The next question arises. Why is there this difference in the curva- 

 ture of the different portions of the surface ? The experience that 

 Professor Ayrton had gained from an examination of a large number of 

 Japanese mirrors supplied, in part at any rate, the answer to the ques- 

 tion. No thick mirror reflects the pattern on the back, not one of the 

 many beautiful mirrors exhibited at the National Exhibition of Japan 

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

 ment 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 mirrors in the museum of the Imperial College of 

 Engineering, 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, ancl the heirloom of the 

 family of a nobleman) equally unsuccessful. 



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 tlie 

 slightest hint, he was therefore compelled to perform the somewhat 

 difficult task of obtaining information from the Japanese workmen 

 themselves. Eventually he ascertained that while practically all 

 Japanese mirrors were convex, the surface of each half of the mould 

 was quite flat, and that the curvature was given to the mirror after 

 casting in the following way. 



The rough mirror is first scraped aj^proximately 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 convexity 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 



