104 INTERPRETATION OF APPEARANCES [CI/. Ill 



Search the preparation until an air bubble is found appearing 

 about i mm. in diameter, get it into the center of the field, and if 

 the light is central the air bubble will appear with a wide, dark, cir- 

 cular margin and a small bright center. If the bright spot is not in 

 the center, adjust the mirror until it is. 



This is one of the simplest and surest methods of telling when 

 the light is central or axial when no condenser is used (§ 74). 



Focus both up and down, noting that, in focusing up, the cen- 

 tral spot becomes very clear and the black ring very sharp. On 

 elevating the tube of the microscope still more the center becomes 

 dim, and the whole bubble loses its sharpness of outline. 



§ 151. Air Bubbles with Oblique Illumination. — Remove 

 the sub-stage of the microscope and all the diaphragms. Swing the 

 mirror so that the rays may be sent very obliquely upon the object 

 (Fig. 28, C). The bright spot will appear no longer in the center 

 but on the side away from the mirror (Fig. 98, A). 



§ 152. Oil Globules. — Prepare these by beating a small drop 

 of clove oil with mucilage on a slide and covering as directed for 

 air bubbles (§ 150), or use a drop of milk. 



§ 153. Oil Globules with Central Illumination. — Use the 

 same diaphragm and light as above (§ 150). Find an oil globule 

 appearing about 1 mm. in diameter. If the light is central a bright 

 spot will appear in the center as with air. Focus up and down as 

 with air, and note that the bright center of the oil globules is clear- 

 est last in focusing up. 



A 



w 



Fig. 9S. Very small Globules of Oil (O) and an Air Bnb- 



» > bles (A) seen by Oblique Light Surface view. The arrow 



d$\/ K^fek. indicates the direction of the light rays. 



O 



§ 154. Oil Globules with Oblique Illumination. — Remove 

 the sub-stage, etc., as above, and swing the mirror to one side and 



