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. 938; McF. 86; N. 123; P. 206. 
SPECIAL DIRECTIONS. 
a. Examine cover-glass preparations made in (XVID) first with 4 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. HANGING-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 eases 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 (4 in. objective) adjusting slide so that edge of 
drop passes through the center of the field; then turn on the high power (4 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. 111; M. & R. 94; MeF. 88, 
N. 142; P. 209. lead 
SPECIAL DIRECTIONS. 
a. Make hanging-drop preparation of B. subtilis from agar or bouillon. (XIII) 
b. 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 
suspension. Study character of movement in all cases. Distinguish between vital and 
molecular movement. 
