on ZOOLOGICAL EXERCISES. 
EXERCISE IY. 
a. 'Take two slides and two cover-glasses. Clean as in 
Exercise I. Shake a little gum and water up in a 
bottle until it is in a froth, and place a drop on the 
centre of one of the slides. Put on a cover-glass. 
On the other slide, put a drop of milk and water, 
and cover. 
6. Put on the quarter objective, and arrange the mirror 
| for direct illumination. Place the slide of gum-water 
on the stage and examine. A number of small 
circles with dark edges will be seen. These are air- 
bubbles. Select one and focus accurately. Then, 
placing the finger on the fine adjustment, raise and 
depress the objective. Notice carefully the effect. 
When the objective is depressed, the air-bubble will 
become more luminous; when the objective is raised, 
the air-bubble will become darker. This is because 
air has a lower refractive power than water, and the 
rays of light from it diverge as they ascend. 
c. Draw back the body, remove the slide*, and place the 
slide with milk on the stage. On examination, this 
will be found to contain a large number of small oil 
globules. Go through the same process as described 
in 6. The effect will be just the opposite. When 
the objective is depressed, the oil-globules will become 
darker ; when it is raised, they will become more 
luminous. This is because the oil is more highly 
refractive than the surrounding water, and conse- 
quently the rays of light converge as they ascend. 
* When a high power is in use, always draw back the body before 
attempting to remove the slide, for fear of both the slide and the objective 
being damaged, 
