180 LETTERS ON NATUEAL MAGIC. 



mass. Owing to the transparency 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 appear- 

 ance of dimples in certain plates of calcedony called 

 hammered calcedony, 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 



Fig. 20. 



phenomena as the cavities in wood. Mother-of-pearl 

 presents the very same phenomena, and it is indeed so 

 common in this substance 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 appearance. 



Among the wonders of science there are perhaps none 

 more surprising than the effects produced upon coloured 

 objects by illuminating them with homogeneous light, or 

 light of one colour. The light which emanates from the 

 sun, and by which all the objects of the material world 

 are exhibited to us, is composed of three different colours, 

 red, yellow, and blue, by the mixture of which in different 



