100 



METHODS OF OBSERVING BACTERIA 



an electric-arc lamp is used the condensing lens is combined with it; 

 when a Welsbach burner is used the lens is separate. 



6. Manipulate the reflector so that the microscopic field appears 

 uniformly dark, while cells, bacteria, granules, etc., appear as exceed- 

 ingly light, highly refractive objects. 



The dark-field illuminator is not at all difficult to use, and with it 

 fine objects, such as spirochetse, etc., are found much more easily 

 than with the ordinary hanging-drop or moist-chamber method. 

 The high-power dry lenses ^ and ^-inch focal distance are gen- 

 erally best for use with the dark-field illuminator. There is^no great 

 advantage in the use of oil-immersion lenses. If they are to be used 

 at all it is necessary to screw into the interior of the lens a small 

 metal funnel in order to limit the field of vision; this is necessary for 

 optical reasons. It is also necessary to place a drop of immersion oil 

 on the upper surface of the cover-glass in addition to the drop which 

 was placed on the upper surface of the dark-field condenser. The 

 latter is generally supplied with a diaphragm to regulate the amount 

 of light admitted. 



FIG. 43 





Glass slide. 



FIG. 44 



FIG. 45 



Square and round cover-glass. 



Small Utensils. A number of small appliances and utensils are 

 necessary for the first elementary studies in bacteriology. 



Slides and Cover-glasses. Slides and cover-glasses are used 

 similarly as in normal and pathologic histology. It has previously 

 been pointed out that the cover-glasses or cover-slips for use in work 

 with bacteria should be the thinnest kind, that is, No. 1 (they are 

 from 0.15 to 0.17 mm. in thickness). Both slides and covers should 

 be very clean and free from dirt and grease. As they are generally 



