4i8 



SCIENCE. 



when exposed to the sun. But Brewster was not aware 

 that the surface of the mirrors was amalgamated, and 

 we may safely say that he was wrong to attempt a' solu- 

 tion of the mystery, without ever having seen a magic 

 mirror. 



Nevertheless, before rejecting his idea completely, let 

 us seek to verify it. I myself have had an engraving made 

 upon copper, then caused it to be effaced. When it was 

 no longer visible in a diffused light, it was, unfortunately 

 no longer visible when exposed directly to the sun. Per- 

 haps I went badly to work, and very likely a more care- 

 ful and delicate operator might have succeeded better. 

 We might have explained in this way the effects of an 

 extraordinary mirror mentioned by M. Ayrton, which, 

 instead ot reproducing the image engraved upon the re- 

 verse, disclosed to the astonished spectators the gro- 

 tesque features of a Buddhist saint. 



Brewster's theory, fortunately, was not known in 

 France when public attention was directed upon magic 

 mirrors. I say fortunately, because his opinion, being 

 that of a celebrated man, might have led us astray. The 

 first French physician who examined a magic mirror, 

 Person, immediately discovered the true solution of the 

 problem. In the first place, he found that the polished 

 surface of the mirror was not perfectly convex, but only 

 so in certain parts, corresponding with the lines of the 

 figure upon the reverse. In the portions corresponding 

 to the relief, it was almost flat. Rays of light reflected 

 upon the convex parts diverge, and produce but a faint 

 image. On the contrary, rays reflected upon the flat 

 portions retain their parallelism, and produce an image 

 which is very intense. This is why the ornaments in 

 relief appear brilliant upon a dark background. 



This irregularity of the surface depends of course upon 

 the method of fabrication. When taken out of the cast, 

 the mirror presents the appearance of a flat disk. Before 

 being polished it is scratched in every sense of the word 

 with a pointed instrument, to which it naturally offers 

 more resistance in the thick parts than in the thin. This 

 operation makes it at first slightly concave, and by the 

 elastic reaction of the metal it becomes convex. This 

 convexity is more sensible in the thin places than in those 

 corresponding to the relief of the design. The mirror is 

 finally polished with a whet-stone, then with charcoal, 

 which must frequently destroy the irregularities which 

 produce the magical effect. The surface thus becomes 

 perfectly smooth, but generally one or two cavities can 

 be found. The manufacturer fills these in with balls of 

 copper which he has ready prepared and of all dimen- 

 sions, and which he afterwards rubs and polishes until 

 he thinks they are invisible to the naked eye. An ex- 

 pectation, however, which is but imperfectly realized, 

 generally speaking. The entire surface is then rubbed 

 by hand with an amalgam composed of equal portions of 

 mercury and tin. 



Such are the details of the manufacture of magic mir- 

 rors. It is easy to see that they quite agree with Person's 

 explanation, but the latter has one objection. How, it 

 will be asked, can the surface of the mirror be irregular 

 without this being apparent in the images it reflects in a 

 diffused light ? 



However, this objection is removed when we come to 

 consider the facts attentively. 



A mirror with a perfectly regular surface is an exceed- 

 ingly rare object. Here, for instance, is a flat, metallic 

 mirror employed in astronomical observations. The re- 

 flections it gives are very good. 



Here also is a silver plaque which reflects in a manner 

 equally perfect. If however, it is made to reflect an 

 electric light, we can see clearly that the surface is not 

 uniform, for we are able to perceive, so to speak, every 

 blow of the hammer which it received during the process 

 of manufacture. Here is one of those little round mir- 

 rors which we buy for a few pennies at the bazaars. It is 

 excellent and extremely serviceable if you desire to comb 



your beard, but detestable if you wish it to reflect light. 



By means of these examples you can easily see that all 

 our common mirrors are irregular and reflect light imper- 

 fectly, although forms can be reflected by them very 

 well. These are true magic mirrors, only the image re- 

 flected is as irregular as the mirror itself, while that of 

 the Japanese mirror is regular like the curves of the sur- 

 face which produces it. 



But are we quite certain that the flat and the convex 

 parts of an irregular mirror reflect a sufficiently variable 

 amount of light to make them quite distinct, one from 

 the other. Let us see : 



Here is a convex mirror, the summit of which has 

 been planed off, making a flat mirror in the middle of a 

 convex one. I will now reflect it upon the the screen by 

 means of an electric light. You see the central portion 

 is a very brilliant disk which shows that the flat mirror 

 reflects the cylindric portion. Around it is a black circle 

 in which there is no light at all. This is the space be- 

 tween the cylinder and a sort of funnel which contains 

 the light reflected by the convex mirror. This light 

 forms around the black circle a grayish ring of feeble in- 

 tensity and quite distinguishable from the white central 

 disk. The difference would be much more marked if 

 the two lights were closer together. 



Here I have another mirror which is flat. To the middle 

 of it has been attached a convex lens. The whole has then 

 been silvered. A reflecting light you see, shows us up- 

 on the screen, a large spot nearly black surrounded by 

 a brilliant ring which has another ring around it of a gray 

 color. The centre and the rings about it are produced 

 by the reflection upon the central convex lens, across 

 which comes the cylinder formed by the reflection upon 

 the flat mirror. 



The variation in intensity of the two reflections is enor- 

 mous, particularly in the centre, which only appears 

 black by contrast. For, of course, there is just as much 

 light upon the central disk as upon the edges. We 

 know that it is really luminous for we can cast upon it 

 the shadow of an opaque body. 



It has been, I hope, clearly demonstrated to you, there- 

 fore, that the curves upon the surface of the mirror pro- 

 duce inequalities of marked intensity when reflected. 

 You have, however, a perfect righf to remain in doubt as 

 to Person's theory, because in all magic mirrors, these 

 irregularities are very faint, being almost invisible to the 

 naked eye. Although Person has endeavored to sustain 

 his theory by direct observations upon the surface of his 

 mirror, it was necessary to support it still further by 

 means of new experiments. 



An Italian savant, M. Govi, has undertaken this task, 

 and in 1864 and 1865 presented two papers upon the sub- 

 ject to the Academy of Turin. 



The first one contains several experiments made for 

 the purpose of upholding Person and utterly demolishing 

 Brewster. But Brewster was determined not to give in, 

 and after having translated M. Govi's article for the 

 Scientific Review, he followed it up with a quantity of 

 remarks and objections which he certainly never would 

 have made had he ever been fortunate enough to hold 

 a magic mirror in his hands. 



The stupidity of the illustrious Scotch physician had a 

 very good result, for it incited M. Govi to seek new proofs 

 and obtain a surer ground than ever for his opinion. In 

 this way he conceived the idea of making the most im- 

 portant and most curious experiment which had yet been 

 seen in regard to metallic mirrors. He thought that by 

 heating ihe mirrors on the back, the warmth would take 

 effect sooner upon the thin parts than upon the thick ; that 

 the former would become more convex, and thus the 

 magic property would increase in such mirrors as already 

 possessed it in some degree, and might possibly be pro- 

 duced in those which were not magic. 



Here is General Teissier's mirror — you have already 

 seen that it was slightly magic — I shall now have it heated 



