106 LETEBS ON NATUBAL MAGFC. 

 Fig. 20. 



parency of the thin edge at o, opposite to the 

 candle S, the side o is illuminated, while the rest 

 of the knot is comparatively dark, so that, on 

 the principles already explained, the spot m n o 

 appears to be a hollow in the table. From this 

 cause arises the appearance of dimples in certain 

 plates of chalcedony, called hammered chalcedony, 

 owing to its having the look of being dimpled 

 with a hammer. The surface on which these 

 cavities are seen is a section of small spherical 

 aggregations of siliceous matter, which exhibit 

 the same phenomena as the cavities in wood. 

 Mother-of-pearl presents the very same phe- 

 nomena, and it is indeed so common in this sub- 

 stance, that it is nearly impossible to find a 

 mother-of-pearl button or counter which seems to 

 have its surface flat, although they are perfectly 

 so when examined by the touch. Owing to the 

 different refraction of the incident light by the 

 different growths of the shell cut in different 

 directions by the artificial surface, like the annual 

 growth of wood in a dressed plank, the surface 

 has necessarily an unequal and undulating ap- 

 pearance. 



Among the wonders of science there are per- 

 haps none more surprising than the effects pro- 



