100 LETTERS ON NATURAL MAGIC. 



in the whole. " But why these things should so 

 happen, he did not pretend to determine." 



The best method of observing this deception is 

 to view the engraved seal of a watch with the 

 eyepiece of an achromatic telescope, or with a 

 compound microscope, or any combination of 

 lenses which inverts the objects that are viewed 

 through it.* The depression in the seal will 

 immediately appear an elevation, like the 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, or the 

 light which illuminates the hollow seal LR, whose 

 Fig. U. 



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 hand, and that the light falls in the direc- 

 tion RL, and as everybody with its shaded side 



* A single convex lens will answer the purpose, provided 

 we hold the eye six or eight inches behind the image of the 

 seal formed in its conjugate focus. 



