172 ANNUAL OF SCIENTIFIC DISCOVERY. 



ring, and dividing double stars well up to a second of space. The ef- 

 fect of such a lens is three-fold : 1st, it brings back the over-correc- 

 tion of the object-glass; 2dly, being of a glass acting somewhat 

 differently on the colored spaces, it causes them to overlap, and gives 

 a greater residuum of white light ; and 3dly, upon the Huyghenian 

 eye-piece, it lessens the spherical aberration by receiving the more re- 

 frangible at a less oblique incidence than the less refrangible rays. 

 It will be obvious, that the nearer the interposed lens is to the object- 

 glass, the less should its curvature be, but in proportion as the object- 

 glass is approached, the difficulty of centering and defects in curva- 

 ture become more felt ; if, again, the interposed lens be brought very 

 near the eye-glass, less effect upon the aberration is produced. Prac- 

 tically, lenses having foci sowewhat greater than the object-glass, 

 and placed at a distance from the eye-glass of from one-fourth to one- 

 fifth of the focal length of the object-glass, will be found to answer ; 

 and the effect being one of degree, no mathematical accuracy in the 

 amount of curvature is required. 



CHINESE MAGIC MIRRORS. 



A great deal of attention has been given in Europe to certain me- 

 talic mirrors fabricated in China in which forms of letters, flowers, 

 and animals are embossed on the back, which is not polished. On 

 looking directly and as closely as possible on the polished face, no 

 trace of these figures is seen ; but if the mirror is made to reflect the 

 rays of the sun upon a wall or screen, the ornaments on the back are 

 plainly seen in the reflected light. Many attempts have been made 

 to explain this phenomenon, but hitherto unsuccessfully. On the 1st 

 of April, however, M. Biot exhibited to the Academy of Sciences in 

 Paris, one of these mirrors, made by M. Lerebours. It appears that 

 in 1847, MM. Arago and Biot suggested an explanation, founded on 

 the fact, that as the embossing of the back surfaces gave different thick- 

 nesses, and therefore different resistance to the metal when the face 

 came to be polished, the surface opposite the raised portions would be 

 more resistant, and would be raised in a convex form, while that op- 

 posite the hollow would, under the same pressure, be slightly concave 

 these effects being so slight as to be invisible to an ocular examina- 

 tion of the surface, but becoming manifest by the deviations impressed 

 on the reflected rays. To test this theory, M. Lerebours took an 

 ordinary daguerreotype p'.ate of copper plated with silver, and on the 

 copper back he engraved a crescent, and then polished the plate. 

 Looking directly on it, and as carefully as possible, nothing is seen ; 

 but when the sun's rays were received on the plate and thrown on a 

 screen, the form of the crescent was clearly defined in the reflected 

 image, darker or lighter than the rest, according to the distance of the 

 mirror from the screen. 



