CH.III~\ INTERPRETATION OF APPEARANCES 95 



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

 side away from the mirror (Fig. yi). 



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

 clove oil with mucilage on a slide and covering as directed for air bub- 

 bles (§ 137), or use a drop of milk. 



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

 diaphragm and light as above (§ 137). Find an oil globule appearing 

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

 pear in the center as with air. Focus up and down as with air, and 

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

 ing up. 



A 



» > 



3 



o 



Fig. 91. Very small Globule of Oil (O) and an Air Bubble 

 (A) seen by Oblique Light. The arrow indicates the direction of 

 the light rays. 



§ 141. Oil Globules with Oblique Illumination. — 

 Remove the sub-stage, etc., as above, and swing the mir- 

 ror 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. 91). 



§ 142. Oil and Air Together. — Make a prepara- 

 tion exactly as described for air bubbles (§ 136), and add 

 at one edge a little of the mixture of oil and mucilage 

 (§ T 39) ! cover and examine. 



The sub-stage need not be used in this experiment. Search the 

 preparation until an air bubble and an oil globule, each appearing 

 about 1 mm. in diameter, are found in the same field of view. Light 

 first with central light, and note that, in focusing up, the air bubble 

 comes into focus first and that the central spot is smaller than that of 

 the oil globule. Then, of course, the black ring will be wider in the 

 air bubble than in the oil globule. Make the light oblique. The 

 bright spot in the air bubble will move away from the mirror while 

 that in the oil globule will move toward it. See Fig. 91.* 



§ 143. Air and Oil by Reflected Light. — Cover the diaphragm 

 or mirror so that no transmitted light (§ 64) can reach the preparation, 

 using the same preparation as in § 142. The oil and air will appear 

 like globules of silver on a dark ground. The part that was darkest in 



*It should be remembered that the image in the compound microscope is 

 inverted (Fig. 21), hence the bright spot really moves toward the mirror for air, 

 and away from it for oil. 



