M. TECHNOLOGY. 543 



are manifold, as has already been stated. They are much 

 more durable than the mercurial mirrors, as the coating can 

 not be rubbed off"; they are not affected by heat, and do not 

 readily crack. There is no double reflection from the sur- 

 face, nor is the quality of the glass of any importance. Curved 

 surfaces, whether cylindrical or spherical, can be coated as 

 readily as plain, and no injury can accrue to the health of 

 the workmen. The time required to produce a given surface 

 of mirror is very much less than by the old process, the es- 

 tablishment where this new manufacture is prosecuted turn- 

 ing out ten thousand square feet of mirror daily with a very 

 small force, each man being capable of coating fifty square 

 feet in an hour. The expense is very much less than by the 

 old method, as, while the mercurial amalgam costs about nine 

 cents to the square foot, the platinum costs only about two 

 and a half cents. A finished mirror can be sold at the rate 

 of fifty cents per square foot, and of the very best quality. 

 By an improved but secret process, the grinding and polish- 

 ing of the surface of the glass is accomplished in three hours' 

 time, instead of the twenty-four usually required in other es- 

 tablishments. 



A curious application of this new mirror is based upon a 

 property of metals, to which we called the attention of our 

 readers some months ago ; namely, that gold, silver, plati- 

 num, and other metals, in very thin layers, applied to glass, 

 are transparent when looked through toward the light, while 

 they form an opaque reflecting surface under other circum- 

 stances. Screens are now made of platinized glass, inclosing 

 the back part of offices and stores, which to persons entering 

 the front part of the room have the appearance of highly- 

 polished mirrors and of being entirely opaque. To one stand- 

 ing behind them, however, they are almost as transparent as 

 pure glass, permitting every movement of those outside to be 

 observed without the slightest difficulty. Placed as panes in 

 the windows of houses, though what is behind them is en- 

 tirely invisible to the passer-by, external objects are perfectly 

 visible to persons in the room. 



The same article is used in the construction of what is 

 called "Surprise Mirrors," which consists in placing a pict- 

 ure behind the platinum mirror and in front of a window, 

 with a dark curtain interposed. So long as the curtain re- 



