HO THE POLYTECHNIC INSTITUTION. 



audience, and so the audience saw nothing in the glass, 

 but only saw through it, and, of course, only saw what was 

 actually upon the stage before them. 



Things being thus arranged, all that was necessary was 

 to allow the real person on the stage to talk and act as 

 usual until the time came for the ghosts or hobgoblins to 

 appear, when all at once a very bright light was thrown 

 upon the objects representing these things under the stage, 

 when all the spectators in the seats would see them re- 

 flected in the glass ; and, as the images of them would 

 appear as far behind the glass as the objects themselves 

 were before it, they would seem to be back upon the 

 stage, among the real actors. 



There is one curious difficulty, however, in the manage- 

 ment of such an exhibition, and that is, that without some 

 special contrivance to prevent the effect, the position of the 

 mirror, inclined at an angle of forty-five degrees more or 

 less, would have the effect of making the floor on which 

 the personations of ghosts and goblins stood under the 

 front of the stage appear in the glass in a perpendicular 

 position that is, up and down so that the images in it 

 would appear standing out upon the wall, in an impossible 

 attitude. John had observed in the advertisement of the 

 exhibition in the papers that the ghosts would " dance on 

 walls and ceilings," and he had at first imagined that the 

 being able to make them do so would be the special won- 

 der of the performance, and would require very particular 

 and extraordinary, and even, perhaps, quite complicated 

 optical arrangements, instead of being, as it really is, a 

 very difficult thing to avoid. 



Any one can see this for himself by means of any look- 

 ing-glass a small one will answer the purpose perfectly 

 well. You place this glass on a table before you, first hold- 

 ing it in an upright position. You place any object before 



