26 ELEMENTS OF APPLIED MICROSCOPY. 
may be used when the strongest illumination is desirable. 
The observer should so adjust its angle in relation to the 
window or lamp that a clear and well-defined circle of 
light may be seen. 
Tf the light be uneven, or if an image of the window-bars 
or the flame be visible, either the angle of the mirror or 
its distance from the stage may be so changed as to re- 
. Fic. 18.—MeEtHop oF INSERTING OpjectIves. (After Bausch.) 
move the difficulty. If the circle is too bright so that its 
glare hurts the eyes, the angle of the mirror must be 
changed to moderate it and the diaphragm opening 
should be decreased. 
For most purposes it is desirable that the light should 
pass directly upward through the axis of the microscope— 
axial illumination. This end may be attained by focus- 
ing just below an air-bubble, and so arranging the mirror 
that the bright point inside shall be exactly at the center 
of the bubble. In examining diatoms and some other 
