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- 



Fig. 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 air-bubble 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. 



