752 



A TEXT-BOOK OF BOTANY. 



taking up the excess of liquid by temporarily placing a piece of 

 filter paper on the opposite side (Fig. 415). 



AIR-BUBBLES. The beginner in the use of the microscope is 

 often confused by the presence of air-bubbles, mistaking them 

 for portions of the material under examination, as starch grains, 

 oil-globules, or even the cells themselves. While it is not prac- 



PIG. 415. Method of applying reagent to material already mounted, g, pipette; f, 



filter paper. 



ticable to avoid their presence entirely, their identity may be 

 determined by the manner of focussing upon them. When 

 focussing above on an airrbubble it always appears dark (Fig. 

 416, C), but when the focus is lowered, it becomes lighter (Fig. 

 416, D) ; while in the case of an oil-globule or starch grain the 

 reverse is true, i.e., it is lightest when the focus is above (Fig. 



