properties of lenses. 

 Fig. 11. 



17 



correcting the two back combinations, at the same 

 time making the distance between them susceptible 

 of alteration by means of a screw collar, this by 

 bringing the front and back combinations nearer 

 together, enables us to give the objective an excess of 

 positive aberration sufficient to counteract the nega- 

 tive produced by the thin glass. 



The screw collar adjustment at the same time 

 corrects 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 indispensable 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 allow- 

 ance must be made for this when accurate measure- 

 ments of a covered object are required. 



Many methods of practically making these cor- 

 rections are mentioned in the various manuals, but I 

 think the following will be found the easiest, and 

 sufficiently correct for all purposes. 



Turn the index (A Fig. 12) to Zero, in other words 



c 



