OF LI&ffT. 387 



learning what has been said concerning conjugate, real and 

 virtual foci, the formation of these images will be easily 

 understood. The image of an object is determined by 

 finding the images of a number of points in the object. 



OO5. Construction for Real Images Formed by 

 Concave Mirrors.;!.) The method may be illustrated as 

 follows : Let AS represent an object in front of a concave mirror, 

 at a distance greater than the radius of curvature. Draw Ax, the 

 secondary axis for the point A. The conjugate focus of A will lie 

 in this line ( 603 [I]). From the infinite number of rays sent 

 from A to the mirror, select, as the second, the one that is 

 parallel to the principal axis. This ray, after reflection at i, will 

 pass through the principal focus ( 601). The reflected rays, iF and 

 xA (secondary axis for A), will intersect at , which is the ~con- 



jugate focus for A In similar manner, b, the conjugate focus for 

 B, may be found. Points between A and B will have their con- 

 jugate foci between a and b. 



(2.) If the eye of the observer be placed far enough back of the 

 image thus formed for all of the image to lie between the eye and 

 the mirror, it will receive the same impression from the reflected 

 rays as if , the image were a real object. All of the rays from any 

 point in the object, as A, that fall upon the mirror, intersect after 

 reflection at a, the conjugate focus. These reflected rays, after 

 intersecting at a, form a divergent pencil. A cone of these rays 

 thus diverging from a enters the eye. They originally diverged 



