1 96 Prominent Image from concave Mtrrtr. [Book III. 



ject. If the eye, therefore, is withdrawn to a fufHcient 

 diftance (to o for example) for the rays to crofs each 

 other, it will perceive the image at E between the 

 mirror and itfelf. The reafon of this depends upon 

 what has been already ftated. Every object is ren- 

 dered vifible to us by pencils of divergent rays from 

 every point of that object ; it therefore ceafes to be vi- 

 fible if thefe rays become parallel or convergent ; and 

 this happens when the object is not nearer to the 

 mirror than the principal focus. To render, there- 

 fore, an object thus fmmted vifible, it is neceflary that 

 the eye fhould recede fo far beyond the place of the 

 image E, as to allow the rays to crofs each other, and 

 meet the eye in a ftate of divergence. 



The image is in this cafe always inverted. Such is 

 the image ba of the object A B (fig. 2.) From this 

 property of the concave reflector to form the image of 

 an object, in thefe cafes, before the reflector, many de- 

 ceptions have been produced, to the great furprize of 

 the ignorant fpectator. He is made to fee a bottle, 

 half full of water inverted in the air without lofing a 

 drop of its contents ; as he advances into a room, he 

 is tempted to exclaim with Macbeth, " Is this a dag- 

 ger that I fee before me !" and when he attempts to 

 grafp it, it vanifhes into the air. 



A variety of fimilar appearances may be reprefent- 

 td, which are all produced by means of a concave 

 mirror, having an object before it ftrongly illuminat- 

 ed, care being taken that only the rays of light re- 

 flected from the object fhall fall upon the concave 

 reflector, placed in fuch a manner that the image (hall 

 be in the middle of the adjoining room ; or, if in the 

 fame room with the object and reflector, a fcreen muft 

 be placed fo as to prevent the fpectator from difco- 

 vering them. A hole is then made in the partition 

 4 between 



