CONVERSION OF CAMEOS INTO INTAGLIOS. 179 



and thus giving an uncertainty to the direction of the 

 light, we may weaken the illusion in any degree we 

 choose, so as to overpower it by touch or by a process of 

 reasoning. 



I have had occasion to observe a series of analogous 

 phenomena arising from the same cause, but produced 

 without any instrument for inverting the object. If A B, 

 for example, is a plate of mother-of-pearl, and L K a 

 circular or any other cavity (Fig. 19) ground or turned 

 in it, then if this cavity is illuminated by a candle or a 



Fig. 19. 



window at S, in place of there being a shadow of the mar- 

 gin L of the hollow next the light, as there would have 

 been had the body been opaque, a quantity of bright re- 

 fracted 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 consequence 

 of this is, that the cavity will appear as an elevation when 

 seen only by the naked eye, as it is only an elevated sur- 

 face that could have its most luminous side at L. 



Similar illusions take place in certain pieces of polished 

 wood, calcedony, 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 surrounding mass, and the cause of it will be 

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

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

 a section of a knot, more transparent than the rest of the 



