94 METHODS OF OBSERVING BACTERIA 



consists in lowering it so much that it is brought beyond the proper 

 focal distance. He is then likely to lower it still farther until the 

 front lens touches the cover-glass and the pressure may be great 

 enough to crack or dislocate the former. The expensive objective is 

 then ruined and can only be repaired at a considerable outlay of 

 money. 



FIG. 36 



Hollow slide with cover-glass. 



Steps in Using the Microscope for Unstained Bacteria in the Hanging 

 Drop or Moist Chamber. 1 1. The same as step No. 1 for stained 

 preparations. 



2. The same as step No. 2 for stained preparations. 



3. Illuminate the object that is, the live bacteria in the hanging 

 drop of water or bouillon by manipulating the plane reflector (do 

 not use the concave mirror). Have the iris diaphragm closed sb that 

 the field is only very dimly lighted. Slowly lower the tube on the 

 coarse adjustment until the margin of the drop can be recognized. 

 Move the margin of the drop near the centre of the field so that the 

 centre of the lens is just over the thinnest portion of the drop. Clamp 

 the slide and see once more whether the desired spot is in the centre 

 of the field. (All this has to be done under guidance of the eye, 

 looking through the low-power dry lens.) 



4. Raise the substage condenser to its highest level; raise the tube; 

 place a drop of immersion oil on the cover-glass, which is situ- 

 ated over the concavity of the slide. Now bring the oil-immersion 

 lens into place and lower it until it touches the cedar oil. Keep on 

 lowering the lens on the coarse adjustment, very carefully and slowly, 

 until it firmly touches the cover-glass. This can be ascertained by 

 watching the vaselin which is between the cover-glass and the slide. 

 As soon as the oil-immersion objective presses firmly on the coyer- 

 glass resting over the concavity the vaselin will be squeezed out 

 from under it. In so manipulating the immersion lens there is prac- 

 tically no danger of injuring it, because a little excess pressure will 

 crack the cover-glass, due to the poor backing given it by the con- 

 cavity of the slide. If this occurs the hanging drop will have to be 

 made over again. 



5. After the immersion lens has been lowered as described it is 

 below the proper focal distance. Open the iris diaphragm and see 

 that the field is well and uniformly lighted (if this is the case there 

 is no danger of having an oblique illumination). Now, close the iris 

 diaphragm so that only a small opening is left and the field is very 



1 For steps in preparing moist chamber, see p. 103. 



