A MANUAL OK BACTERIOLOGY 



FIG. 2. Van Tiehem cell for work with 



The cover glass is supported on the top 

 of the cell 



Exercise 4. Motility of Bacteria 



Transfer a small quantity of bacteria from an agar culture 

 twenty-four hours old, or a drop from a hay infusion, to a clean 

 cover glass. Take care to use a small drop and place it at the 



center of the cover glass. 



Place a Van Tieghem cell 

 on a slide and quickly invert 



the cover glass on the top 

 hanging drop, as seen in section Qf the cdl (pig> ^ Exam _ 



ine this with the J objective 

 (4 mm.) and with the dia- 

 phragm nearly closed. It will be very difficult at first to see the 

 bacteria, but by careful focusing they will appear as transparent 

 dots or rods. Examine carefully to see whether they are station- 

 ary or motile, distinguishing those as motile which actually move 

 back and forth across the stage, and not those which simply 

 dance back and forth without locomotion (the Brownian move- 

 ment, exhibited by inanimate particles also). What causes the 

 bacteria to have the power of motion ? Are cocci self -motile ? 



Exercise 5. Zobglcea Forms 



The cell walls of most bacteria have a gelatinous outer sheath. 

 Examine stained preparations for chains of bacteria. Do the 



FIG. 3. Optical principles of dry lens and oil-immersion lens compared 



1, path of light in a dry-lens system; 2, path of light in an oil-immersion lens 

 system; 0, object; C, cover glass ; L, lens of the objective 



