In the use of the condenser with oil immersion objec- 

 tives the custom prevails of using the condenser dry. 

 It is convenient especially in changing specimens and 

 meets the requirements of every day work. It is well 

 to point out, however, that both the condenser and 

 the objective lose in their efficiency when the former 

 is used dry, and for critical work the condenser should 

 be in immersion contact with the slide. 



To make immersion contact between condenser and 

 the slide place a drop of oil on the top of condenser, 

 drop the slide upon the stage, first throwing the clips to 

 one side. 



With immersion objectives the proper focusing of 

 the condenser becomes a matter of nice distinction to 

 obtain best results and can only be reliably accom- 

 plished by considerable practice and experience. 

 To obtain best position : 



Use a 2-3 objective ; focus upon the object ; adjust 

 condenser until image of window-sash or flame is in 

 the same plane with the object. 



Relation of Aperture of Condenser to 

 Objective. In the study of Bacteria and other micro- 

 organisms the objectives used being of wide aperture, 

 it is sought to have them stand out boldly in a bright 

 field. This is accomplished by bringing the diaphragm 

 to its full aperture. On all other objects, however, 



162 



