METHODS OF EXAMINATION 27 



for shutting off part of the light a blender or diaphragm (Fig. 

 8) . When the bacteria have been found, and their relation to 

 the structure is to be studied, the "Abbe'' is generally shut 

 out by the iris blender, and the structure comes more plainly 

 into view. A white light (daylight or a Welsbach burner) is 

 best for bacterial study: use the plane mirror for daylight and 

 the concave mirror for artificial light. 



For all stained bacteria the oil-immersion lens and Abbe con- 

 denser, without the use of blender. For unstained specimens, 

 oil-immersion and the narrowed blender. 



When examining with low-power objective, use a strong 

 ocular. When using high-power objective use weak ocular. A 



Fig. 8. Iris blender 



revolving nose-piece will be found very useful, since it is some- 

 times necessary to change the objective on the same field, and 

 this insures a great steadiness of the object. 



Great cleanliness is needed in all bacteriologic methods, but 

 nowhere more so than in the microscopic examination. 



The cover-glass should be very carefully washed in alcohol, 

 and dried with a soft linen rag. To remove the stains on the 

 cover-glasses that have been used, they should be soaked in 

 hydrochloric acid or placed in a 6 per cent, aqueous solution of 

 potassium dichromate with 6 per cent, of strong sulphuric acid, 

 washed in water, and kept in absolute alcohol. 



Examination of Unstained Bacteria. As the coloring of 

 bacteria kills them and changes their shape to some extent, it 



