16 PROPERTIES OF LENSES. 



It has been found that the thin glass used for 

 covering microscopic specimens, produces by refrac- 

 tion of the rays passing through it a sufficient nega- 

 tive aberration (Fig. 11) to destroy the adjustment 

 of an objective perfect in its performance on uncovered 

 objects. This may be remedied, as has been shown 

 by Mr. A. Ross, by under-correcting the front andover- 



Fro. 11. 



correcting the two back combinations, at the same 

 time making the distance between them susceptible 

 of alteration by means of a screw collar, this arrange- 

 ment by bringing the front and back combinations 

 nearer together, enables us to give the objective an 

 excess of positive aberration sufficient to counteract 

 the negative produced by the thin glass. 



The screw collar adjustment at the same time cor- 

 rects the errors caused by the difference in the 

 refractive indices of the various media employed for 

 the preservation and mounting of specimens. We 

 therefore consider it an indispensible adjunct to all 

 objectives of higher power than one half inch. It 

 must not be forgotten that these adjustments affect 

 the magnifying power considerably, and allowance 



