NATURAL PHILOSOPHY. 109 



CURIOUS OPTICAL ILLUSION ARTIFICIAL GHOSTS. 



An interesting optical phenomenon, namely, the production of 

 spectral apparitions, or ghosts, has been recently devised by Professor 

 Pepper, of the Polytechnic Institution, London, and most successfully 

 and practically applied in theatrical exhibitions. 



The manner in which the illusion is used for effect, in a theatrical 

 play, may be stated as follows : - 



An actor in the character of a murderer is seen asleep on a lounge 

 in the rear of the stage, which is dimly lighted. Presently he rises in 

 his sleep and begins to rave under the tortures of remorse for his crime. 

 Instantly there appears at his side a bright image of a skeleton, so 

 luminous that it sheds some light upon the obscurity around. Though 

 startlingly distinct, it is seen to be only the image of a skeleton, as ob- 

 jects on the stage are visible directly through the bones. The mur- 

 derer strikes his sword through the grizzly horror, but it is as impalpa- 

 ble as air. After a brief space the apparition vanishes as suddenly as 

 it came. It makes no movement up or down or to either hand, but 

 simply disappears, the whole effect being so perfect, that the spectator 

 can hardly bring himself to realize that he is really the subject of a 

 deception. 



The scientific explanation of the phenomena may be thus stated : 



When any one looks into a glass or metallic mirror, he sees an image 

 or ghost of himself. If his neighbor looks over his shoulder he, too, sees 

 the same ghost. What gives this business of ghost-making its chief 

 mystery is the plan of concealing adroitly the person or object who or 

 which is to be ghostified, and showing only the image or ghost, and 

 thus snapping the otherwise palpable connection between the true ob- 

 ject, animate or inanimate, and its picture or representation. If a plain 

 metallic or silvered glass mirror only were employed in producing the 

 ghost, a child would not be startled at the eifects produced, however 

 completely the original object might be concealed, from view ; but it is 

 otherwise, and all are surprised, when an image or ghost is called up 

 before the observer without any apparent connection with mirror action 

 at all. . 



Now, without going into the questions in optical science respecting 

 the reflection of light from plane -surfaces, the formation of images by 

 plane mirrors, or of multiple images formed by glass mirrors, we may 

 remark that metallic mirrors have but one reflecting surface, giving 

 only one image of an. object. With glass mirrors this is different. 

 They give rise to several images which are readily observed when the 

 image of a candle is looked at obliquely in a common looking-glass. 

 A very feeble image of a candle is seen, and then a very distinct one. 

 Behind this there are several others, whose intensity or clearness gradu- 

 ally decreases until it disappears. This phenomenon or appearance 

 arises from the looking-glass having two reflecting surfaces, that of 

 the face of the plate, and that of the layer of metal which covers the 

 hinder surface of the glass. The greater feebleness of the image re- 

 flected from the glass than from the metal surface arises from the cir- 

 cumstance that metal reflects better than glass. It completely intercepts 

 the light reflected from the original object, and throws it back to the 

 eye. 



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