CONCAVE MIEEORS. 65 



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 pro- 

 perly concealed withdraws the 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 per- 

 son is placed before the concave mirror, and a 

 distinct image of the picture will be seen either 

 in the air or among smoke, in the manner already 

 described. If the background of the picture is 

 temporarily covered with lamp-black, so that 

 there is no light about the picture but what falls 

 upon the figure, the effect will be more complete. 



As in all experiments with concave mirrors, 

 the size of the aerial image is to that of the real 

 object as their distances from the mirror, we may, 

 by varying the distance of the object, increase or 

 diminish the size of the image. In doing this, 

 however, the distance of the image from the 

 mirror is at the same time changed, so that it 

 would quit the place most suitable for its ex- 

 hibition. This defect may be removed by simul- 

 taneously changing the place both of the mirror 

 and the object, so that the image may remain 

 stationary, expanding itself from a luminous spot 

 to a gigantic size, and again passing through all 

 intermediate magnitudes, till it vanishes in a 

 cloud of light. 



Those who have studied the effects of concave 



