CH. IX] CORRECTION OF THE ABERRATIONS 285 



ray near the axis. If then the full aperture is filled, as shown in the 

 figure, with rays parallel with the axis, there will be a series of foci, 

 those of the border rays being nearer the lens than those near the 

 middle of the lens (fig. 168, fi, 2, 13). 



459. Correction of spherical aberration. It is possible by 

 selecting convex and concave lenses of different material and hence 

 of different refractive power, to overcome the spherical aberration 

 of the convex lens with an equal and opposite aberration in a concave 

 lens without overcoming the converging 

 action of the convex lens. Consequently 

 rays will all come to one focus. Such a 

 lens combination is said to be aplanatic 

 or spherically corrected. 



If the correction were not quite suffi- FIG. 168. SPHERICAL ABER- 

 cient so that the border rays still came RATION - IN LENS ES. 



to a focus slightly nearer the lens than Axis The principal optic 

 the middle rays, the combination would 7 ' 2 3 Ray x at the ^ e 



be under-corrected. If the concave lens comes to a focus at /i; ray 



Al i , - Al 2 at /2, and ray 3 at fa, that 



were too strong, the border rays of the is> the nearer the optic axis 



convex lens would come to a focus farther tne longer the focus, and the 

 r ,, , ,, ., . j,, , nearer the edge of the lens 



trom the lens than the middle rays, and t h e shorter the focus. 



the combination would be said to be over- 

 corrected. Sometimes under-correction or over-correction is designed 

 to compensate for parts of the optical apparatus which the rays will 

 meet later, or for aberrations produced before the light reaches the 

 objective. The common and almost universal example is the spher- 

 ical aberration introduced by the cover-glass over the specimen 

 (fig. 169). 



460. Cover-glass correction. By referring to fig. 169 it will 

 be seen that the effect of the cover-glass is precisely like the 

 spherical aberration due to the unequal refraction of the different 

 zones of a convex lens; that is, the border rays are more bent than 

 those nearer the axis, as the obliquity of the rays is greater 

 (446). 



Now to overcome this there must be introduced into the objective 

 an under-correction just sufficient to balance the effect of the cover- 



