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



in the region beyond P. On arranging the light as in fig. 4, and 

 placing the screen successively in the positions 1, 2, 3, J J, and 4, 

 afterwards moving the lens up to the Japanese mirror, until the 

 distance between it and the mirror was less than the focal length of 

 the lens, we found that the experiments bore out, in every detail, 

 the results that must follow from the " inequality of curvature 

 theory." 



Returning now to fig. 3, in which it was first shown that a con- 

 verging beam produced an inversion of the phenomenon, we find it 

 impossible to obtain a distinct dark image of the pattern on a light 

 ground by the employment of one converging lens only. This is 

 partly due to the fact that here we are dealing with diverging pencils 

 of light falling on the screen, so that no true image of the pattern is 

 formed ; and partly caused by the blurring effect arising from a beam of 

 sunlight, consisting of a number of slightly diverging pencils. This 

 latter may be, to a certain extent, corrected, either by allowing a very 

 small beam of sunlight to fall on the single converging lens, or by 

 causing the sunlight to be brought first to a focus by one lens, and 

 then with a second lens at several feet distance, forming another con- 

 vergent pencil of light, in w T hich the convergent mirror is placed. 



Guided by all that proceeds, we are led to the undoubted conclusion, 

 that the third of the proposed explanations is the correct one, namely, 

 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. 



The next question arises, why is there this difference in the curvature 

 -of the different portions of the surface ? The experience that one 

 gains from an examination of a large number of Japanese mirrors 

 supplies, in part at any rate, the answer to the question. 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 we were so fortunate as to be able to experiment with in a 

 darkened room with a bright luminous point at some twelve feet dis- 

 tance, shows 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, fail to reflect any trace of a design, and some old round mirrors 

 without handles, which we have also tried, are, with the exception of 

 one about six inches in radius, and for which the owner asked many 

 pounds, equally unsuccessful. Now this in itself, independently of the 

 erroneous idea regarding stamping, is almost sufficient to negative Mr. 

 Prinsep's idea " that part of the metal was by this stamping rendered 

 in a degree harder than the rest, so that in polishing it was not worn 

 away to the same extent." Again, it is not that the pattern is less 

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



