Reflexibitity of Light. 173 



mirror, is at one-fourth of the diameter of that 

 sphere, of which the concave surface is a section, 

 which is therefore sometimes called the centre of 

 concavity. At this point the rays reflected from 

 the mirror, are converged and cross ; and if the 

 spectator's eye is beyond this point or focus, he 

 will not see the image behind the mirror, but 

 before it, a shadowy form, suspended in the air ; 

 but, from the crossing of the rays, it appears 

 inverted. 



In fig. 63, a b is a concave mirror, cd is a 

 hand held up before it. The image, therefore, 

 you see is not placed behind the mirror, as 

 happens in every other case, but the hand seems 

 to hang suspended in the air at m. The reason of 

 this very extraordinary and striking phenomenon 

 is to be found in what was already intimated. 

 Objects are rendered visible, not by single rays, 

 but by pencils of divergent rays, proceeding 

 from the different points of the object. If these 

 pencils of divergent rays should happen by any 

 cause to be united, the object will in that point 

 cease to be visible. This happens in the focus 

 of a concave mirror, where, by the law of re- 

 flection, they are all united. If the eye, there- 

 fore, is placed in that point, it will see nothing of 

 the image. It must recede to a sufficient distance 

 to permit the rays to cross and again becpme 

 divergent. In that case the image will be seen, 

 not behind the mirror at the virtual or imaginary 

 focus, as it is in plane and convex mirrors, but 



