REFRACTION OF LIGHT. 403 



625. Conjugate Foci. Eays diverging from a 

 luminous point in the principal axis at a small distance 

 beyond the principal focus on either side of the lens will 

 form a focus on the principal axis beyond the other prin- 

 cipal focus. Thus, rays from L will converge at /; con- 

 versely, rays from I will converge at L ( 602). If the 

 luminous point be in a secondary axis, the rays will con- 

 verge to a point in the same secondary axis. Two 



FIG. 306. 



points thus related to each other are called con- 

 jugate foci; the line joining them always passes 

 through the optical centre. 



(a.) If the luminous point be more than twice the focal distance 

 from the lens, the conjugate focus will lie on the other side of the 

 lens at a distance greater than the focal distance, but less than twice 

 the focal distance. If the luminous point be moved toward the 

 lens, the focus will recede from the lens. When the luminous 

 point is at one secondary focus, the rays will converge at the other 

 secondary focus. When the luminous point is between the second- 

 ary and principal foci, the rays will converge beyond the secondary 

 focus on the other side of.the lens. W r hen the luminous point is at 

 the focal distance, the emergent rays will be parallel and no focus 

 will be formed. When the luminous point is at less than the focal 

 distance, the emergent rays will still diverge as if from a point on 

 the same side of the lens, more distant than the principal focus. 



