The tendency of a concave mirror is to increase the convergence 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 parallel 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. The reflected rays will still diverge, but not so rapidly as the incident rays. If the divergence be less than that first mentioned, viz., that of rays issuing from a point further from the mirror than the principal focus, the divergence will be changed to convergence and a real focus will be formed.