PEPPER'S GHOST. 303 



Thus, when the sun apparently sinks below the horizon (say at 

 a seaside spot with westerly sea view), the disappearance is 

 delayed to a notable extent by refraction; after the sun has sunk 

 to such an extent that no rays passing in a perfectly straight line 

 could reach the observer's eye, the disc will still be visible above 

 the horizon, the rays traversing a curved path, somewhat like that 

 in the case of sea mirage (fig. 144), whilst passing through the 

 lighter exterior layer of the atmosphere towards the denser layers 

 at the sea level, the action here being similar in general effect to 

 that illustrated in Expt. 333 with the coin and cup of water, 

 whereby the observer is enabled to see round the corner, as it were, 

 in virtue of refraction. 



Expt. 340. Pepper's Ghost. This beautiful illusion substan- 

 tially depends on the same principle as that involved in Amici's 

 camera lucida (Expt.. 338), viz., that a plate of glass can act 

 simultaneously as reflector and as a transparent medium, through 

 which objects can be rendered visible by suitably illuminating 

 them. As worked in public exhibitions a large sheet of glass is 

 arranged at the front part of the stage, sloping forwards at an 

 angle of about 45 towards the spectators. In front of this the stage 

 is cut away (fig. 145), so as to form a kind of well, which is 

 invisible to the audience ; this well is lined with black cloth and 

 furnished with 

 powerful illu- 

 minating lamps, 

 which can be 

 turned up and 

 down as re- 

 quired ; simi- 

 larly the scenes 



th lT k^ *t Fig * 145 ' Pe PP er ' s Ghost - 



of the stage can be brightly illuminated or thrown practically 

 into darkness by working another set of lights. When the 

 lamps lighting the well are turned down, nothing in the well 

 is visible to the audience, either by direct vision or by reflection ; 

 if the other lights are more or less turned up, the actors and scenes 

 behind the enclosed glass plate are seen through the glass, precisely 

 as in the case of an ordinary theatre, saving that with this latter 

 there is no intermediate glass plate. When, on the other hand, 

 the ordinary stage lights are turned down and those illuminating 

 the well are turned up, the scenes behind the glass become 

 invisible for want of light, whilst any object or actor, AB, placed 

 horizontally in the well is seen by a spectator in front at P by reflec- 



