CONDENSERS 113 



that the dry achromatic condensers of N.A. 1-0 and 

 aplanatic cone 0-9 are entirely suitable, and most 

 workers will find this type the best for all-round use. 



Objectives as Condensers. It is possible to use objec- 

 tives in place of condensers, and centring mounts with 

 the R.M.S. screw are supplied for that purpose. Nothing, 

 however, is gained by so using them, as, even if they are 

 better corrected than condensers, their corrections are 

 upset by making them work through a slip which is much 

 thicker than the cover-glass for which they are adjusted. 



