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. 



If 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- 



■Method of Inserting Objectives. (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 Ught 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 



