24 General Bacteriology. 



the same degree of amplification than is possible with the dry system. In using an 

 immersion lens, place a small drop of oil on the preparation, then carefully lower the 

 objective until it touches the oil drop and nearly touches the cover-glass. Apply eye to 

 the ocular and focus upward very slowly with fine adjustment until the definition is clear. 

 At the close of the day's work the oil must be removed from the objective and cover- 

 glass. This is best accomplished by wiping them with apiece of Japanese paper made for 

 the purpose. In case the oil should accidentally dry on the objective, it can be removed 

 by adding a little more oil and allowing it to stand for a few minutes; it can then be 

 wiped off with paper. If this method does not succeed, the objective should be taken to 

 the instructor. Great care must be observed since solvents of the oil are also sol- 

 vents for the lens mountings. 



REFERENCES. See Gage; A. 190; H. 104; M. & R. 93; McF. 86; N. 123; P. 206. 



SPECIAL DIRECTIONS. 



a. Examine cover-glass preparations made in (XVII) first with \ in. objective, 

 and then with the oil-immersion objective. If the specimen is satisfactory wipe off the 

 oil and mount in Canada balsam. 



b. Practice making cover-glass preparations by staining specimens from each of your 

 cultures. Use Loeffler's methylen blue for the gelatin and bouillon; aqueous solution of 

 gentian violet for agar, and carbol-fuchsin for potato. Examine, mount permanently and 

 hand to instructor for inspection. 



EXERCISE XIX. HANaiNQ-DROP PREPARATIONS. 



GENERAL DIRECTIONS. These are made by adding a small portion of bacterial cul- 

 ture from solid media to a drop of water on a clean cover-glass, or in case of fluid media 

 by placing a loop of the culture medium on the cover- glass. A hollow ground glass 

 slide having the rim of the cavity previously coated with vaseline, is inverted and lowered 

 over the cover-glass enclosing the drop. With a careful, quick movement the prepara- 

 tion is now brought right side up. 



Instead of the hollow ground glass-slide an ordinary glass-slide to which a small 

 section of a glass or rubber tube has been cemented can be used, and in some cases is 

 preferable. 



In examining the preparation under a microscope focusing is a somewhat difficult 

 process and must be carried out with great care. Use a narrow diaphragm. Find the 

 edge of the drop with the low power (f in. objective) adjusting slide so that edge of 

 drop passes through the center of the field; then turn on the high power (^ in. objective) 

 and focus without moving the slide. The edge of the drop is selected because the bacteria 

 are here nearest the cover-glass and hence more easily focused upon than where they are 

 deeper in the drop. 



REFERENCES. A. 195; H. 101; L. & K. 102; M. & W. Ill; M. & R. 94; McF. 88; 

 N. 142; P. 209. 



SPECIAL DIRECTIONS. 



a. Make hanging-drop preparation of B. subtilis from agar or bouillon. (XIII) 



6. Make same preparation of B. coli. (XIII) 



c. Make same preparation of organism supplied. (Micrococcus) 



d. Make same preparation of water containing particles of india ink or carmine in 

 suspensiqn. Study character of movement in all cases. Distinguish between vital and 

 molecular movement. 



