3 1 INTERPRE TA TION OF APPEARANCES. 



the experiments. It may be necessary to shade the object (§ 66) to get 

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

 small (§ 44). 



§ 81. Air Bubbles. — Prepare these by placing a drop of thin mucilage 

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

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

 glass (Ch. V), but do not press it down. 



§ 82. Air Bubbles with Central Illumination. — Shade the object ; 

 and with the plane mirror, light the field with central light (PI. II, Fig. 



13, §42). 



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 (§ 52). 



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 

 body still more the center becomes dim, and the whole bubble loses its 

 sharpness of outline. 



§ 83. Air Bubbles with Oblique Illumination. — Remove the sub- 

 stage of the microscope (Fig. 10), and all the diaphragms. Swing the 

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

 13, C). The bright spot will appear no longer in the center but on the 

 side away from the mirror (Fig. 19). 



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

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

 81). 



§ 85. Oil Globules with Central Illumination. — Use the same dia- 

 phragm and light as above (§ 82). Find an oil globule appearing about 

 1 mm. in diameter. If the light is central the bright spot will appear 

 in the center as with air (§ 82). Focus up and down as with air ; and 

 note that the bright center of the oil globule is clearest last in focusing 

 up. 



§ 86. Oil Globules with Oblique Illumination. — Remove the sub- 

 stage, etc., as above, and swing the mirror to one side and light with 

 oblique light. The bright spot will be eccentric, and will appear to be 

 on the same side as the mirror (Fig. 19). 



§ 87. Oil and Air Together. — Make a preparation exactly as de- 

 scribed for air bubbles (§ 81), and add at one edge a little of the mix- 

 ture of oil and mucilage (§ 84) ; cover and examine. 



