CONVERSION OF CAMEOS INTO INTAGLIOS. 



175 



immediately appear an elevation, like tlie wax impression 

 which is taken from it ; and though we know it to be 

 hollow, and feel its concavity with the point of our 

 finger, the illusion is so strong that it continues to appear 

 a protuberance. The cause of this will be understood 

 from Fig. 14, where S is the window of the apartment, 



Ffc. 14. 



or the light which illuminates the holloio seal L E, whose 

 shaded side is of course on the same side L with the 

 light. If we now invert the seal with one or more lenses, 

 so that it may look in the opposite direction, it will 

 appear to the eye as in Fig. 15, with the shaded side L 



farthest from the window. But as we know that the 

 window is still on our left Land, and that the light falls 

 in the direction R L, and as every body with its shaded 

 side 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 experi- 



