22 General Bacteriology. 
The preparation can be made permanent either by allowing the water under the 
cover-glass to dry before it is removed, or by floating it off with water and afterwards 
drying. When dry a drop of Canada balsam, dissolved in xylene, is placed on the cover- 
glass and this is then lowered on to the slide again. 
Resume. 
a. Spread film, 
. Air dry, 
Fix, 
. Stain, 
Mount, 
Examine, 
Mount in balsam, or, 
Mount in balsam, 
. Examine. : 
‘‘The great mistake made by beginners is to take too much growth.’’ (M. & R.) 
REFERENCES. A. 151; H. 71; L. & K. 104; M. & W. 89; M. & R. 95; MecF. 91; 
N. 147; P. 198; P. B. C. 11; S. 25. 
SPECIAL DirEcTIONS. Make cover-glass preparation from agar streak of B. subtilis 
(XIII) staining with an aqueous solution of gentian violet for 5 minutes. , 
euehws ass 
EXERCISE XVIII. USE OF MICROSCOPE. 
GENERAL DirEcTIons. For bacteriological purposes a microscope with a magnifying 
power of at least 500 diameters is needed. There should be a coarse adjustment (rack 
and pinion) as well as a fine micrometer screw; and the following accessories: two eye 
pieces, one 1 in. (25 mm.) and one 2 in.(50mm.); three objectives, one $ in. (16 mm.), 
one+in. (4mm.), or¢ in. (3.5 mm.) and one oil immersion 7y in. or 7s in. (2 mm.); a 
triple nose-piece, and an Abbe substage condenser with iris diaphragm mounting. 
In the use of the microscope the following points should be noted: 
a. Liant. The proper angle at which the mirror should be placed is best determined 
by removing the eye-piece and so arranging the mirror that the unobstructed light from 
the window covers the whole field. The ideal light is that from a white cloud. Direct 
sunlight should never be used. 
b, ABBE CONDENSER. The purpose of the condenser is to furnish a large cone of light, 
and as it is corrected for parallel rays the plane side of the mirror should always be used, 
except when artificial light is employed. When highly stained objects are to be exam- 
ined, the open diaphragm should be used, but when the structural rather than the color 
picture is desired, it will be necessary to diminish the light by closing the diaphragm. 
When the high powers are employed, raise the condenser as high as possible; for low 
powers a lower position will give better definition. 
c. Focusing. Turn the proper objective in place and rack down until the objective 
nearly touches the cover-glass. This should be done while the eye is held at one side and 
directs the movement. Then with the eye at the tube slowly move up with the micrometer 
screw. Never rack down with the eye at the tube. 
d, UsE or O1L-Iumersion. The oil-immersion objective is indispensable to the proper 
study of bacteria. It is constructed upon the principle that a drop of fluid having the 
same refractive index as the objective, prevents the dispersion of light, thus permitting 
the use of lenses having a greater numerical aperture and longer working distance for 
