15 
twice with xylol, this being allowed to act a few moments each time before it 
is drained off, after which treatment, the slide is allowed to dry. The Canada 
balsam, dissolved in xylol, should be of such a consistency that a drop one- 
eighth of an inch in diameter will spread under the weight of the cover glass 
so as to fill the space beneath the cover glass completely. The cover glass 
should not be dropped on the Canada balsam, but should be held in an inclined 
position and very slowly lowered on the drop. The preparation should be 
placed in a horizontal position for several days until the balsam has hardened. 
8. Remove some of the tartar from the teeth with a tooth pick and spread 
m a thin, even film without water. Fix and stain three minutes with methy¬ 
lene blue. 
9. Prepare a spread of some old milk which is not curdled. Be sure the 
spread is thin. Do not dilute with water. Dry and immerse in xylol three to 
five minutes to remove the fat. Wash in alcohol to remove the xylol. Wash 
in w r ater. Stain three to five minutes with methylene blue. 
10. From each of the pure cultures of bacteria furnished make a preparation 
in the following manner: 
\\ ith the wire loop place a small drop of tap water on a clean slide. Trans¬ 
fer a minute portion of the bacterial growth on the solid nutrient agar to the 
drop of water by touching the growth with the tip of the wire needle. Mix the 
bacteria with the w r ater, w r hich should then be spread in an even film over an 
area as large as a dime. The turbidity produced in the drop of water by mix¬ 
ing the bacteria in it should be evident to the eye. Allow the preparation to 
dry. Fix it and stain with the dye indicated by the instructor. 
11. Examine the preparations in order, as was done in Exercise 3, using the 
oil immersion (1.8 m. m.) objective only. Draw and describe one good field 
from each preparation. 
(a) What can you see in the infusion? 
(b) How r can you tell bacteria from debris? 
(c) Can you tell the protozoa from the bacteria? 
( d ) How' many different forms of bacteria are visible? Compare 
them with Fig. 5. 
C e ) What can be seen in the tartar? 
(0 Do spheres, rods or spirals predominate? 
(g) Are there many different forms of bacteria in milk? 
(h) What is the amorphous blue material in the stained milk 
spread? 
( i ) What morphological types of bacteria were found in the pure 
cultures? 
(j) What is the value of anilin dyes in the study of bacteria? 
