REFLECTION OF LIGHT. 383 



ror, and C the centre of the corresponding sphere. O is called 

 the centre of curvature; A is the centre of the mirror. A 

 straight line of indefinite length drawn from A through 

 C, as ACX, is called the principal axis of the mirror. A 

 straight line drawn from any other point of the mirror 

 through (7, as 

 JCd, is called a 

 secondary axis. 

 The point F, 

 midway between 



A 



' FIG. 



called the prin- 



cipal focus. The distance AF is the focal distance of the 

 mirror ; the focal distance is, therefore, one-half the radius 

 of curvature. The angle MCN is called the aperture of 

 the mirror. 



(a.) A curved surface may be considered as made up of an infinite 

 number of small plane surfaces. Thus, a ray of light reflected from 

 any point on a curved mirror may be considered as reflected from a 

 plane tangent to the curved surface at the point of reflection. This 

 reflection then takes place in accordance with the principles laid 

 down in 591. It should be borne in mind that the radii drawn 

 from C to points in the mirror as / and J are perpendicular to the 

 mirror at thesfc points. Thus, the angles of incidence and reflection 

 for any ray may be easily determined. 



GOO. Effect of Concave Mirrors. The ten- 

 dency of a concave mirror is to increase the con- 

 vergence or to decrease the divergence of incident 

 rays. 



(a.) If the divergence be that of rays issuing from the principal 

 focus, the mirror will exactly overcome it and reflect them as par- 

 allel rays. If the divergence be greater than this, viz., that of rays 

 issuing from a point nearer the mirror than the principal focus, the 

 mirror cannot wholly overcome the divergence, but will diminish it. 



