CH. Ill] INTERPRETATION OF APPEARANCES 105 



light with oblique light. The bright spot will be eccentric, and 

 will appear to be on the same side as the mirror (Fig. 98,0). 



§ 155. Oil and Air Together. — Make a preparation exactly 

 as described for air bubbles (§ 149), and add at one edge a little of 

 the mixture of oil and mucilage (§ 152); 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 ap- 

 pearing 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.* 



§ 156. Air and Oil by Reflected Light. — Cover the dia- 

 phragm or mirror so that no transmitted light (§ 73) can reach the 

 preparation, using the same preparation as in § 155. The oil and 

 air will appear like globules of silver on a dark ground. The part 

 that was darkest in each with transmitted light will be lighted, and 

 the bright central spot will be somewhat dark.f 



§ 157. Distinctness of Outline. — In refraction images this 

 depends on the difference between the refractive power of a body 

 and that of the medium which surrounds it. The oil and air were 

 very distinct in outline as both differ greatly in refractive power 

 from the medium which surrounds them, the oil being more refrac- 

 tive than the mucilage and the air less. (Figs. 61-63.) 



Place a fragment of a cover- glass on a clean slide, and cover it 



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

 inverted (Fig. 26) , hence the bright spot really moves toward the mirror for 

 air, and away from it for oil. 



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

 when quite high powers are used and very small bubbles are selected for ob- 

 servation. If a 16 mm. ( % in. } is used instead of a 3 mm. ( yb in. ) objective, 

 the appearances will vary considerably from that given above for the higher 

 power. It is well to use a low as well as a high power. Marked differences 

 will also be seen in the appearances with objectives of small and of large 

 aperture. 



