REFLECTION OF LIGHT. 



385 



tween principal and secondary axes is almost wholly one 

 of convenience. Rays diverging from B will form a focus 

 at b. The angle of incidence being necessarily equal to the 



FIG. 285. 



angle of reflection, it is evident that rays diverging from b 

 would form a focus at B. On account of this relation 

 between two such points, they are called conjugate foci. 

 Therefore, conjugate foci are two points so related 

 that each forms the image of the other. 



603. Construction for Conjugate Foci. In the case 

 of concave mirrors, to locate the conjugate focus of a luminous 

 point, it is necessary to find the point at which at least two reflected 

 rays really or apparently intersect. The method may be illustrated 

 as follows : 



FIG. 286. 



(1.) Let 8 (Fig. 286) represent the luminous point whose con- 

 jugate focus is to be located. It may or may not lie in the principal 

 axis. Draw the axis for the point 8, i.e., a line from S through (7, 

 17 



