74 GENERAL MICROBIOLOGY 



6. Determine how many revolutions of the drum (from 

 to 0) are necessary to move the movable line one division 

 and from this determination calculate the value in microns, 

 of one division on the drum. One determination of this 

 value is sufficient. 



B. Method of Using the Filar Ocular Micrometer. 



1. Replace the object micrometer by a slide containing 

 organisms, focus, and measure an organism, counting the 

 number of divisions the drum is turned in moving the 

 movable line from end to end of the organism. 



Example. If the drum is turned two divisions the organism was 

 two times 0.1 micron in length or 0.2 micron. 



2. To measure the microorganisms with a higher power 

 objective, the value of each division of the scale has to be 

 recalibrated. 



EXERCISE 20. DETERMINATION OF THE RATE OF 

 MOVEMENT OF MOTILE ORGANISMS 



Apparatus. Microscope; Leitz "step" micrometer; stop- 

 watch; hanging-drop preparation of motile organisms. 



Method. Using a hanging-drop preparation of the 

 organism to be examined, determine the rate of movement 

 per second, using the step micrometer and a stop-watch. 



EXERCISE 21. PREPARATION OF A HANGING DROP 



The purpose of the hanging-drop preparation is to study 

 bacteria in the living condition; to demonstrate (a) their 

 form, (6) arrangement, (c) motility (this is best ob- 

 served from twenty-four-hour cultures), (d) appearance, 

 (e) division of cells, (/) formation or presence of spores; 

 (g) to determine the presence and types^of microorganisms in 

 any material and to watch the changes in the predominat- 

 ing types of the microbial flora in a medium from day to 

 day; (h) and, in pathogenic bacteriology, to demonstrate 

 agglutination. 



