CH. XIV] 



LENSES AND THEIR ACTION 



the axis. It results from this that the border rays cross the axis 

 considerably nearer the lens than the central rays, hence, with 

 parallel rays, instead of one focus, there are many foci drawn out in 

 a line. This is shown by the bright core in the photograph of the 

 cone of rays in fig. 322. 



Except with a symmetri- 

 cal, double convex lens the 

 amount of spherical aberra- 

 tion depends upon which face 

 of the lens receives the inci- 

 dent light, and whether the 

 incident light is parallel, 

 diverging or converging. 



With plano-convex lenses, 

 as shown in fig. 320-323, the 

 spherical aberration with 

 parallel incident light is less 

 when the parallel light is 

 incident on the convex face 

 than when the lens is turned 



Fic.~3i9. THE PRINCIPAL Focus OF A 



CONVEX AND OF A CONCAVE LENS. 



(From The Microscope) 



Axis, Axis. The principal optic axis of 

 the lenses. 



F The focus. In the convex lens it is 

 where the light rays actually cross the axis. 

 In the concave lens it is where they would 

 cross if produced backward as indicated 

 SO that the light IS incident by the broken lines. 



upon the plane face. 



For diverging rays the plane face should receive the incident 

 light, and for converging rays the convex surface should receive the 

 light to insure minimum spherical aberration. With all lenses, 

 the general rule to follow is that for minimum spherical aberration, 

 the light rays should be equally bent on entering and on leaving the 

 lens i. e., at both refracting surfaces. Furthermore, with the same 

 light beam, the aberration is greater for lenses of large curva- 

 ture than for lenses of small curvature. 



To overcome this aberration, a concave lens is combined with a 

 convex lens, and so proportioned that the too great converging 

 effect of the outer zone of the convex lens is just counterbalanced 

 by the diverging effect of the concave lens in its various zones (fig. 

 324). A perfectly corrected, or aplanatic combination brings all 

 the parallel rays to one focus. 



