314 PHI 'SIOL OG1 CA L PH YSICS. [Chap. XXV. 



it. Thus, in the case of ray 1, the refraction on 

 entering the lens is towards the normal n, on leaving, 



because the ray passes from 

 a rarer to a denser medium, 

 the bending is away from the 

 normal n', the result of both 

 refractions being to direct the 

 ray towards F, the principal 



Tig. 140.-Principal Focus of J CUS - J he a ^ le . LFL/ f rUied 



a Convex Lens. by rays irom the circumference 



of the lens and the principal 

 focus is the aperture of the lens. 



On the other hand, if the luminous body be at F, 

 the rays after emergence from the lens will be parallel 

 to one another. 



2. Let the rays diverge from a luminous point, 

 and fall on a convex lens, they are no longer focussed 

 at the principal focus. Suppose, as in Fig. 141, the 

 rays proceed from a 

 pointy* which is farther 

 from the lens than the 

 focal distance. After 

 refraction they w.ll 

 meet in a point f 

 outside of the principal Fig . 141 ._ Coujugate F oci of a Convex 

 focus F. Or if the Lens, 



luminous point be at/'', 



the refracted rays will meet &tf. Because of the re- 

 lation thus existing between f and f, they are called 

 CONJUGATE FOCI. That is, f is the conjugate focus of 

 /"', and/' of/'. Just as in the case of mirrors, as/" 

 comes more and more nearly to be at the focal dis- 

 tance from the lens its conjugate focus f moves 

 farther and farther off, till if f coincide with the focal 

 distance, the emergent rays will be parallel, and there 

 will be no conjugate focus. Again, as y moves farther 

 and farther from the focal distance its conjugate focus 



