144 LETTERS ON NATURAL MAGIC. 



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 direction 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 other- 

 wise 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 appearance. 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 in the focus of the mirror. 

 The spectator having been desired to take some fruit from 

 the basket, approaches for that purpose, while a person 

 properly concealed withdraws he real basket of fruit 

 with one hand, and with the other advances the dagger, 

 the image of which, being no longer covered by the fruit, 

 strikes at the body of the astonished spectator. 



The powers of the concave mirror have been likewise 

 displayed in exhibiting the apparition, of an absent or 

 deceased friend. For this purpose a strongly-illuminated 

 bust or picture of the person is placed before the concave 



