149 



the slide should invariably be cleaned. The fluid 

 may be removed by a moist piece of soft linen and 

 the front then cleaned with a dry piece or with 

 lens paper. Chamois skin is not suitable, as it does 

 not absorb the fluid. 



Immersion Objectives on Test Plate. From 

 the fact that in oil immersion objectives the fluid 

 has the same refractive index as the cover and 

 front of the lens, it is not sensitive to variations in 

 thickness of cover, although many of the most 

 expert manipulators prefer adjustable mountings 

 in order to obtain the highest results, if any dis- 

 turbing element should be present. 



To determine the highest capacity on test objects, 

 ordinary daylight is not sufficient ; a flat wick oil 

 lamp is best suited. If the right hand is used on 

 micrometer screw, place the lamp at the right side 

 of the instrument, about ten inches from it, with 

 the edge of the flame turned toward the mirror. 



Place the test plate upon the stage and as the 

 diatoms in balsam are very transparent and there- 

 fore very difficult to find, a lower power objective 

 may be used as a finder ; bring No. 1, or Trice- 

 ratium favus into the center of the field and after 

 the objective has been removed, attach the immer- 

 sion objective, which we assume to be a y^, in the 

 manner prescribed. Get the best possible illumi- 

 nation with the mirror at the central point and 

 move the test plate from diatom to diatom until it 



