88 MICROSCOPICAL EXAMINATION OF BACTERIA 



fluid makes contact with the under surface of the slide ; 

 avoid the formation of air bubbles. 



10. Adjust the substage mirror so that the light is 

 reflected upward. A bright spot will be seen on the 

 fresh preparation near the centre of the field. 



11. Replace the -inch objective by the T V-mch 

 oil immersion lens which has been fitted with the special 

 stop to reduce its N. A. ; place a drop of immersion oil 

 upon the centre of the cover-glasses of the fresh prepa- 

 ration and lower the microscope tube until the front 

 lens of the objective has entered the oil drop. 



12. Focus the bright spot referred to 

 in step 10. If it no longer occupies the 

 centre of the field, alter the angle of the 

 sub-stage mirror until it does. 



13. Now focus the lens accurately on 

 the film, cautiously vary the height of 

 the dark ground condenser until the best 

 position is found. The intensely illumi- 

 nated bacteria will stand out in vivid 

 contrast to the dark background. 



Microscopical Examination of the 

 Stained Specimen. (The body tube of 

 the microscope may be vertical or inclined to an 

 angle.) 



1. Secure the slide on the stage of the microscope by 

 means of the spring clips. 



2 . Place a drop of cedarwood oil on the centre of the 

 cover-slip. 



The immersion oil is pure cedarwood oil, and is kept in a small 

 bottle of stout glass (Fig. 70), the cavity of which is shaped like an 

 inverted cone, and is provided with a safety funnel (so that the oil 

 does not escape if the bottle is accidently overturned) and a dust 

 cap of boxwood fitted with a wooden rod with which the drop of 

 oil is applied to the coverglass or lens. 



3 . Use the T V-inch oil immersion lens of the micro- 

 scope. Rack down the body tube till the front lens 



