CONVEKSION OF CAMEOS INTO INTAGLIOS. 101 

 Fig. 15. 



farthest from the light must necessarily be convex 

 or protuberant, we immediately believe that the 

 hollow seal is now a cameo or bas-relief. The 

 proof which the eye thus receives of the seal 

 being raised, overcomes the evidence of its being 

 hollow, derived from our actual knowledge, and 

 from the sense of touch. In this experiment the 

 deception takes place from our knowing the real 

 direction of the light which falls upon the seal ; 

 for if the place of the wmdow,-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 LR, Fig. 14, to be illuminated with a 

 candle S, the place of which we can change at 

 pleasure. If we invert LR, it will rise into a 

 cameo, as in Fig. 15 ; and if we then place 

 another candle S on the other side of it, as in 

 Fig. 16, the hollow seal will be equally illumi- 

 Fig. 16. 



nated on all sides, and it will sink down into a 



