MICROSCOPIC TECHNIQUE AND REAGENTS. 753 



416, E) and darker when the focus is lowered (Fig. 416, F). 

 To obviate as much as possible the formation of air-bubbles, the 

 edge of the cover-glass should first be applied to the liquid on one 

 side and then allowed to drop upon it. When particular care is 

 required, a pair of forceps may be used for holding the cover and 

 lowering it gradually. 



FIG. 416. Diagrams showing the difference between an air-bubble and an oil-globule 

 in different foci: When the focus is above, as at A, the air-bubble (C) is dark gray and 

 the oil-globule (E) light gray. When the focus is at the lower portion, as at B, the air- 

 bubble (D) is light in the center and the oil-globule (F) dark gray. The same optical effects 

 as are obtained with oil-globules are observed with cell walls, starch grains and crystals. 



Frequently also simple pores in the cell-walls are mistaken 

 for cell-contents, and sometimes even the lumen of the cell has 

 been mistaken for a prism of calcium oxalate. The beginner 

 will therefore find it an advantage to study the simple pores in 

 the pith cells of elder or sassafras (Fig. 132). In sections show- 

 ing either the upper or lower wall of the cells, the pores appear 

 as circular or elliptical markings, which may be mistaken for cell- 

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