CONCAVE MIRRORS. 141 



from liis own. By drawing the moveable mirror half up, 

 the spectator at A might see half of his own face joined 

 to half of the face placed at B ; but in the present day the 

 most ignorant persons are so familiar with the properties 

 of a looking-glass that it would be very difficult to employ 

 this kind of deception with the same success which must 

 have attended it in a more illiterate age. The optical 

 reader will easily see that the mirror F and the apartment 

 N C D are not absolutely necessary for carrying on this 

 deception ; for the very same effects will be produced if 

 the person at B is stationed at G, and looks towards the 

 mirror F in the direction G F. As the mirror F, however, 

 must be placed as near to A as possible, the person at G 

 would be too near the partition C N, unless the mirror F 

 was extremely large. 



The effect of this and every similar deception is greatly 

 increased when the persons are illuminated with a strong 

 light, and the rest of the apartment as dark as possible ; 

 but whatever precautions are taken, and however skilfully 

 plane mirrors are combined, it is not easy to produce 

 with them any very successful illusions. 



The concave mirror is the staple instrument of the 

 magician's cabinet, and must always perform a principal 

 part in all optical combinations. In order to be quite 

 perfect, every concave mirror should have its surface 

 elliptical, so that if any object is placed in one focus of 

 the ellipse, an inverted image of it will be formed in the 

 other focus. This image, to a spectator rightly placed, 

 appears suspended in the air, so that if the mirror and 

 the object are hid from his view, the effect must appear to 

 him almost supernatural. 



The method of exhibiting the effect of concave mirrors 

 most advantageously is shown in Fig. 3, where C D is the 

 partition of a room having in it a square opening E F, the 

 centre of which is about five feet above the floor. This 



