64 LETTERS ON NATUEAL MAGIC. 



stands a little farther from a concave mirror thari 

 its principal focus, he will perceive in the air 

 between himself and the mirror an inverted and 

 diminished image of his own person, with the 

 dagger similarly brandished. If he aims the 

 dagger at the centre of the mirror's concavity, 

 the two daggers will meet point to point, and, 

 by pushing it still farther from him towards the 

 mirror, the imaginary dagger will strike at his 

 heart. In this case it is necessary that the direc- 

 tion of the real dagger coincides with a diameter 

 of the sphere of which the mirror is a part ; but 

 if its direction is on one side of that diameter, 

 the direction of the imaginary dagger will be as 

 far on the other side of the diameter, and the 

 latter will aim a blow at any person who is placed 

 in the proper position for receiving it. If the 

 person who bears the real dagger is therefore 

 placed behind a screen, or otherwise concealed 

 from the view of the spectator, who is made to 

 approach to the place of the image, the thrust of 

 the polished steel at his breast will not fail 

 to produce a powerful impression. The effect of 

 this experiment would no doubt be increased by 

 covering with black cloth the person who holds 

 the dagger, so that the image of his hand only 

 should be seen, as the inverted picture of him 

 would take away from the reality of the appear- 

 ance. By using two mirrors, indeed, this defect 

 might be remedied, and the spectator would 

 witness an exact image of the assassin aiming 

 the dagger at his life. 



The common way of making this experiment 

 is to place a basket of fruit above the dagger, so 

 that a distinct aerial image of the fruit is formed 



