LABORATORY WORE 339 
several drops of one of the staining solutions and allow to (c) stain for several min- 
utes. The length of time necessary for this varies with conditions, one to five 
minutes being usually sufficient. Wash off the stain in running water and then dry 
the slide by gentle heat. Place a drop of immersion oil upon the stained bacteria 
and place the slide under the microscope. Use a }(2 inch immersion, lowering the 
objective into the immersion oil and focusing very carefully. If the microscope 
has an Abbe condenser or a diaphragm it is best to have this widely open. The 
bacteria are so minute that it is hardly possible to study them with a lower magnify- 
ing power than a }9, although they can be seen with a 1é inch. Examine the bac- 
teria and sketch. In this way make a microscopic examination of all of the cultures 
isolated and purified, and compare with Figs. 7 and o. 
Motility —To determine the motility of bacteria transfer a small quantity from 
an agar slant to a bouillon tube, and allow to grow for 24 hours. Place a drop of the 
24-hour-old bouillon culture on a slide and put upon it a cover-glass. Examine 
this with a 4 inch objective and with the diaphragm nearly closed. The best light 
for the purpose is artificial light (electric) placed near the microscope and reflected 
through by the plain surface of the mirror. It will be very difficult at first to see the 
bacteria, but with careful focusing they will appear as transparent dots or rods. 
Examine carefully to determine whether they are stationary or motile, calling 
only those motile that move back and forth across the stage and not those that 
simply dance back and forth without locomotion (the Brownian motion). It is 
sometimes desirable to keep the specimen under observation for some time in which 
case a hanging-drop method may be used. A concave slide is to be used and a ring 
of vaseline painted around the depression. The drop containing the living bacteria 
is placed in the middle of a large cover-glass and inverted over the concavity of the 
slide. By pressing it firmly into the vaseline ring it will be sealed so as to prevent 
evaporation and may be kept under observation for hours. 
No. 9. Bacteria in the Mouth.—With a clean knife scrape a little of the material 
attached to the teeth and spread it in a very thin layer over a slide. Dry, fx and 
stain, and with a microscope note the large numbers of bacteria present. Sketch the 
varieties seen. 
No. ro. Bacteria in Milk.—Place a small drop of milk on a clean microscopic 
slide. ‘With a needle spread it out over the slide into a thin smear. Dry with mild 
heat, on a level surface over a steam radiator, for example. When dry place in 
xylol to dissolve out the fat. Then immerse in alcohol for a few minutes to fix the 
milk to the slide. Next immerse in a methylene blue solution (see No. 8) for about 
three minutes. Wash in water. Place in alcohol again until the smear of milk is 
almost decolorized. Then return to the stain again for a few seconds. Wash and 
dry. Then examine with oil immersion lens of a microscope. The milk should 
be stained a faint blue, the bacteria a dark blue. Notice whether the bacteria are 
all alike or whether there seem to be several kinds. 
This method of staining should first be tried on rather old milk (but not yet 
sour) in which the bacteria are numerous. Later it may be tried in milk containing 
fewer bacteria. If it is desired to use this method for counting the bacteria in milk, 
