32 



INTERPRETATION OF APPEARANCES. 



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

 preparation until an air bubble and an oil globule, each about i mm. 

 in diameter, are found in the same field of view. Light first with cen- 

 tral 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. 19.* 



§ 88. Air and Oil by Reflected Light.— Cov- 

 er the diaphragm or mirror so that no transmitted 

 light (§ 41) can reach the preparation, using the ^ ig _ 

 same preparation as in (§ 87). The oil and air on (o) and an Air-Bitbbie 

 will appear like globes of silver on a dark (A > Seen b * obli i ue Li ^ ht - 



1 x " The A rrow Indicates the Dt- 



ground. The part that was darkest in each will rection of the Light Rays . 

 be lightest, and the bright central spot will be somewhat dark.f 



§ 89. Distinctness of Outline. — In refraction images (§§ 61, 66) 

 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 each differed greatly in refractive power from the 

 medium which surrounded them, the oil being more refractive than the 

 mucilage and the air less. 



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

 Ch. V, under mounting). The outline will be very distinct with the 

 unaided eye. Use it as object and employ the 18 mm. i}/^ in.) objective 

 and high ocular. Light with central light. The fragment will be out- 

 lined by a dark band. Put a drop of water at the edge of the cover- 

 glass. It will run in and immerse the fragment. The outline will re- 

 main distinct, but the dark band will be somewhat narrower. Re- 

 move the cover-glass, wipe it dry, and wipe the fragment and slide dry 

 also. Put a drop of 50% glycerin on the middle of the slide and mount 



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

 verted (Fig. 5), hence the bright spot really moves toward the mirror for air, and 

 away from it for oil. 



fit 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 observa- 

 tion. If an 18 mm. {% i n -) ^ s usef l instead of a 3 mm. (% in.) objective, the ap- 

 pearances 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. 



