CORRECTION FOR COVERING-GLASS. 17 



passage through it, in the directions t r, t' r' ; and on emerging 

 from it again, they will pass on towards E and e'. Now if the 

 course of these emergent rays be traced backwards, as by the 

 dotted lines, the ray e r will seem to have issued from x, and the 

 ray e' r' from y ; and the distance x y is an abberration quite suffi- 

 cient to disturb the previous balance of the aberrations of the lens 



Fig 



composing the object-glass. The requisite correction may be 

 effected, as Mr. Ross pointed out, by giving to the front pair (Fig. 

 10, i)^of the three of which the Objective is composed, an excess 

 of positive aberration (*. e., by under-correcting it), and by giving 

 to the other two pairs (2, 3) an excess of negative aberration (i. e. 

 by over-correcting them), and by making the distance between the 

 former and the latter susceptible of alteration. For when the 

 front pair is approximated most nearly to the other two, and its 

 distance from the object is increased, its positive aberration is 

 more strongly exerted upon the other pairs, than it is when the 

 distance between the lenses is increased, and the distance between 

 the front pair and the object is diminished. Consequently, if the 

 lenses be so adjusted that their correction is perfect for an uncovered 

 object, the front pair being removed to a certain distance from the 

 others, its approximation to them will give to the whole combi- 

 nation an excess of positive aberration, which will neutralize the 

 negative aberration occasioned by covering the object with a thin 

 plate of glass.* This correction will obviously be more important 



* The mode in which this Adjustment is effected will be more fitly 

 described hereafter (§§ 113, 114). 



C 



