CONCAVE MIRRORS. 



445 



and appear to proceed from a point behind the mirror 

 and close to the axis, thus producing a virtual image. 



The mode of reflection of those rays which are either 

 parallel to the axis or proceed from the focus enables 

 us to construct the path of any two rays which proceed 

 from any luminous point whatever, if placed before a 

 mirror, and to find the point where they meet after 

 reflection. This point is nothing else than the position 

 of the reflected image of the luminous point ; for from 

 the point from which two rays of light issue all other 

 rays given out by that point must clearly proceed, and 

 the point where two rays meet after reflection must be 

 the point of intersection of all other reflected rays. 



We have already seen that (the image of a luminous 

 object, whose distance from the mirror is equal to the 

 radius of curvature of the latter, is inverted, equal in 

 magnitude to the object, and at the same distance from the 

 mirror as the object.} 



Let now, as in fig. 251, an object A B be farther 



FIG. 251. 



from the mirror than twice the focal length; that is, let its 

 distance be greater than the length of the radius of 

 curvature. The ray A d, parallel to the axis, is re- 



