84 INTERPRETATION OF APPEARANCES. [CH. III. 



cess of these experiments will depend entirely upon the care and skill 

 used in preparing the objects, in lighting, and in focusing. 



Employ a 3 mm. (}i in.) or higher objective and a high ocular for all 

 the experiments. It may be necessary to shade the object (§ 102) to 

 get satisfactory results. When a diaphragm is used the opening should 

 be small and it should be close to the object. 



§ 127. Air Bubbles. — Prepare these by placing a drop of thin muci- 

 lage on the center of a slide and beating it with a scalpel blade until the 

 mucilage looks milky from the inclusion of air bubbles. Put on a cover- 

 glass, but do not press it down. 



Fig. 87. Diagram showing 

 how to place a cover-glass upon 

 an object with fine forceps. 



§ 128. Air Bubbles with Central Illumination. — Shade the ob- 

 ject ; and with the plane mirror, light the field with central light (Fig. 



23)- 



Search the preparation until an air bubble is found appearing about 

 1 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, circular margin 

 and a small bright center. If the bright spot is not in the center, ad- 

 just 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 (§ 61). 



Focus both up and down, noting that, in focusing up, the central 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. 



§ 129. 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. 23, C). 

 The bright spot will appear no longer in the center but on the side away 

 from the mirror (Fig. 88). 



§ 130. Oil Globules. — Prepare these by beating a small drop of clove 

 oil with mucilage on a slide and covering as directed for air bubbles 

 (§ 128), or use a drop of milk. 



§ 131. Oil Globules with Central Illumination. — Use the same 

 diaphragm and light as above (§ 128). Find an oil globule appearing 

 about 1 mm. in diameter. . If the light is central the bright spot will ap- 



