CONVERSION OF CAMEOS INTO INTAGLIOS. 105 



but produced without any instrument for invert- 

 ing the object. If AB, for example, is a plate 

 of mother-of-pearl, and LR a circular or any 

 other cavity (Fig. 1 9) ground or turned in it, then 

 if this cavity is illuminated by a candle or a 

 window at S, in place of there being a shadow 



Fig. 19. 



of the margin L of the hollow next the light, as 

 there would have been had the body been opaque, 

 a quantity of bright refracted light will appear 

 where there would have been a shadow, and the 

 rest of the cavity will be comparatively obscure, 

 as if it were in shade. The necessary conse- 

 quence of this is, that the cavity will appear as 

 an elevation when seen only by the naked eye, 

 as it is only an elevated surface that could have 

 its most luminous side at L. 



Similar illusions take place in certain pieces of 

 polished wood, chalcedony, and mother-of-pearl, 

 where the surface is perfectly smooth. This arises 

 from there being at that place a knot or growth, 

 or nodule, of different transparency from the sur- 

 rounding mass, and the cause of it will be under- 

 stood from Fig. 20. Let m o be the surface of a 

 mahogany table, m A o B a section of the table, 

 and m n o a section of a knot more transparent 

 than the rest of the mass. Owing to the trans- 



