MYSTERIOUS DAGGER. 143 



rays of light, in place of proceeding without obstruction 

 to an eye at 0, are reflected as it were from those minute 

 particles of which the smoke is composed, in the same 

 manner as a beam of light is rendered more visible by 

 passing through an apartment filled with dust or smoke. 



It has long been a favourite experiment to place at A, 

 a white and strongly-illuminated human skull, and, to 

 exhibit an image of it amid the smoke of a chafing dish 

 at B ; but a more terrific effect would be produced if a 

 small skeleton, suspended by invisible wires, were placed 

 as an object at A. Its image suspended in the air at B, 

 or painted upon smoke, could not fail to astonish the 

 spectator. 



The difficulty of placing a living person in an inverted 

 position, as an object at A, has no doubt prevented the 

 optical conjurer from availing himself of so admirable a 

 resource ; but this difficulty may be removed by employing 

 a second concave mirror. This second mirror must be so 

 placed as to reflect towards M N, the rays proceeding from 

 an erect living object, and to form an inverted image of 

 this object at A. An erect image of this inverted image 

 will then be formed at B, either suspended in the air or 

 depicted upon a wreath of smoke. This aerial image will 

 exhibit the precise form and colours and movements of 

 the living object, and it will maintain its character as an 

 apparition if any attempt is made by the spectator to 

 grasp its unsubstantial fabric. 



A deception of an alarming kind, called the Mysterious 

 dagger, has been long a favourite exhibition. If a person 

 with a drawn and highly-polished dagger, illuminated by 

 a strong light, stands a little farther from a concave 

 mirror than its principal focus, he will perceive in the air, 

 between himself and the mirror, an inverted and di- 

 minished image of his own person with the dagger 

 similarly brandished : if he aims the dagger at the centre 



