Ch. IV] DETERMINATION OF THE CHARACTER OF OBJECTS 113 



§ 197. Oil globules. — Prepare these by beating a small drop of 

 clove or other oil with gum arabic mucilage on a slide and covering as 

 directed for air bubbles (§ 194), or use a drop of milk, in a drop of water. 

 § 198. Oil globules with central illumination. - - Use the same 

 diaphragm and light as above (§ 195). Find an oil globule appearing 

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

 appear in the center. Focus up and down and note that the dark 

 ring is narrower than with air and that the bright 

 center of the oil globule is clearest last in focus- 

 ing up. 



§ 199. Oil globules with oblique illumination. 

 — Remove the substage, etc., as above, 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. 68). 



§ 200. Oil and air together. — Make a pre- 

 paration exactly as described for air bubbles 

 (§ 194), and add at one edge a little of the mixture 

 of oil and mucilage (§ 197); cover and examine. 



The substage need not be used in this experi- 

 ment. 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 (fig. 68). 



As the air bubble is of less refractive index than the mucilage it 

 will act like a concave lens (fig. 69), while the oil globule, having a 

 greater refractive index than the mucilage, will act as a convex lens 

 (fig. 69, § 200a). 



It is possible to distinguish oil and air optically, as described above, 

 only when quite high powers are used and very small bubbles are 



Fig. 68. Small 

 A ik Bubble [A) 

 and On. Globule 

 (0) with Oblique 

 Light. 



The arrow indi- 

 cates the direction 

 of the light. 



