254 THE USE OF THE MICROSCOPE 



water or balsam. The dry objective of 0.85 aperture 

 (with correction collar), or the oil immersion of the same 

 aperture, may also be tried. (Coles, Metz.) 



17. Dark Field with Cardioid or Bicentric Condenser. — 

 The cover-glass and slide should be especially cleaned. The 

 sHde should be 1 miUimeter thick, and the cover 0.17 

 millimeter thick. Put an appropriate centered bright-field 

 condenser in the condenser sleeve. Focus the source of 

 light on the slide, and center this source with a 10-times 

 objective which is concentric with the oil-immersion 

 objective. Exchange the bright-field condenser for the 

 cardioid, without shifting the lamp or the mirror. Put 

 water (or immersion oil, or xylol) between the condenser 

 and the slide, which should show a sufficient number of 

 refractive particles. A special slide may be used for 

 centering, having a piece of wet lens paper under the cover- 

 glass. Center the cardioid condenser by moving the lever 

 and the rotating ring in the proper directions, one at a time. 

 Replace the slide with lens paper by the slide with bacteria, 

 etc., for dark-field observation with the special oil-immer- 

 sion objective. The intense light of the direct image of the 

 incandescent filament or ribbon must be shielded by a 

 dense screen during focusing and centering by the bright- 

 field condenser. Then this screen is removed. Slide and 

 cover-glass must be parallel, and the lack of this causes 

 failure. The layer of liquid should be as thin as practicable. 

 A special dark-field chamber with an island within a groove 

 is useful. The preparation may be temporarily sealed with 

 soft paraffin. (Jentzsch, Zeiss, Leitz.) 



18. Dark Field with Cassegrain or Leuchtbild Condenser. 

 Take a thin enough slide, with specimens of bacteria 

 or spirochsetes slightly stained with fuchsin, and mounted 

 in immersion oil or balsam, with cover 0.17 millimeter 

 thick, or without a cover-glass. Use a powerful light, such 

 as a 6-volt, 100-watt tungsten lamp, coiled filament or 

 ribbon. Center the light, and focus the preparation with 

 the usual bright-field condenser, using a dense enough 

 screen. Remove this condenser without shifting the mirror. 



