CONVEBSION OF CAMEOS INTO INTAGLIOS. 103 







careless observers cannot but know that the 

 shadow of a hollow is always on the side next 

 the light, and the shadow of a protuberance on 

 the side opposite to the light ; but if the object 

 is the raised impression of a seal upon wax, I 

 have found that, when inverted, it still seemed 

 raised to the three youngest of six persons,, while 

 the three eldest were subject to the deception. 



This illusion may be dissipated by a process of 

 reasoning arising from the introduction of a new 

 circumstance in the experiment. Thus, let RL, 

 Fig. 17, be the inverted seal, which consequently 



Fig. 17. 



appears raised, and let an opaque and unpolished 

 pin, A, be placed on one side of the seal. Its 

 shadow will be of course opposite the candle as 

 at B. In this case the seal, which had become a 

 cameo by its inversion, will now sink down into 

 a cavity by the introduction of the pin and its 

 shadow; for as the pin and its shadow are in- 

 verted, as shown in Fig. 18, while the candle 

 retains its place, the shadow of the pin falling in 

 the direction AB is a stronger proof to the eye 

 that the light is coming from the right hand, than 

 the actual knowledge of the candle being on the 

 left hand, and therefore the cameo necessarily 

 sinks into a cavity, or the shadow is now on the 

 same side as the light. This experiment will 



