CONCAVE MIRRORS. 145 



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 tempo- 

 rarily 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 dis- 

 tance 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 exhibition. 

 This defect may be removed by simultaneously 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 inter- 

 mediate magnitudes, till it vanishes in a cloud of light. 



Those who have studied the effects of concave mirrors 

 of a small size, and without the precautions necessary to 

 insure deception, cannot form any idea of the magical 

 effect produced by this class of optical apparitions. 

 When the instruments of illusion are themselves con- 

 cealed when all extraneous lights but those which 

 illuminate the real object are excluded when the mirrors 

 are large and well polished and truly formed the effect 

 of the representation on ignorant minds is altogether 

 overpowering, while even those who know the deception, 

 and perfectly understand its principles, are not a little 

 surprised at its effects. The inferiority in the effects of 

 a common concave mirror to that of a well-arranged 

 exhibition, is greater even than that of a perspective 

 picture hanging in an apartment to the same picture 

 exhibited under all the imposing accompaniments of a 

 dioramic representation. 



