CONCAVE MIBEORS. 61 



looks towards the mirror F in the direction GF. 

 As the mirror F, however, must be placed as 

 near to A as possible, the person at G would be 

 too near the partition CN, unless the mirror F 

 was extremely large. 



The effect of this and every similar deception is 

 greatly increased when the persons are illumi- 

 nated with a strong light, and the rest of the 

 apartment as dark as possible ; but whatever pre- 

 cautions 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. 4, 

 where CD is the partition of a room having in it 

 a square opening EF, the centre of which is about 

 five feet above the floor. This opening might be 

 surrounded with a picture-frame, and a painting 

 which exactly filled it might be so connected with 

 a pulley that it could be either slipped aside, or 

 raised so as to leave the frame empty. A large 

 concave mirror MN is then placed in another 

 apartment, so that when any object is placed at 

 A, a distinct image of it may be formed in the 



