176 LETTERS ON NATURAL MAGIC. 



merit the deception takes place from our knowing the 

 real direction of the light which falls upon the seal ; for 

 if the place of the window, with respect to the seal, had 

 been inverted as well as the seal itself, the illusion could 

 not have taken place. 



In order to explain this better, let us suppose the seal 

 L R, Fig. 14, to be illuminated with a candle S, the 

 place of which we can change at pleasure. If we invert 

 L R it will rise into a cameo, as in Fig. 15 ; and if wo 

 then place another candle D on the other side of it, as in 

 Fig. 16, the hollow seal Avill be equally illuminated on 

 all sides, and it will sink down into a cavity or intaglio. 

 If the two candles do not illuminate the seal equally, or 



Fig. 16. 



if any accidental circumstance produces a belief that the 

 light is wholly or principally on one side, the mind will 

 entertain a corresponding opinion respecting the state of 

 the seal, regarding it as a hollow if it believes the light 

 to come wholly or principally from the right hand, and as 

 a cameo if it believes the light to come from the left 

 hand. 



If we use a small telescope to invert the seal, and if 

 we cover up all the candle but its flame, and arrange the 

 experiment so that the candle may be inverted along 

 with the image, the seal will still retain its concavity, 

 because the shadow is still on the same side with the 

 illuminating body. 



If we make the same experiments with the raised im- 

 pression of the seal taken upon wax, we shall observe the 



