114 DETERMINATION OF THE CHARACTER OF OBJECTS [Ch. IV 



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selected for observation. If a 16 mm. objective is used instead of a 

 4 mm., 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, as the larger aperture takes in more 



oblique rays and hence the black 

 margin is narrowed (§ 202). 



§ 200a. It should be remembered 

 that the image in the compound micro- 

 scope is inverted (fig. 20); hence the 

 bright spot really moves toward the 

 mirror for air, and away from it for oil. 



§ 201. Air and oil by reflected 

 light. — Use the same preparation 

 as in § 200. Cover the diaphragm 

 or mirror so that no transmitted 

 light can reach the preparation. 

 The oil and air will appear like 

 globules of silver on a dark 

 ground. The part that was dark- 

 est in each with transmitted light 

 will be lighted, and the bright central spot will be somewhat dark. 

 Use also the condenser and dark-ground illumination (§ 123). 

 Experiments in which the substage condenser is used (§ 202-209). 

 § 202. 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 refractive than the mucilage 

 and the air less (fig. 69). 



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

 70). Use it as object and employ the 16 mm. objective and 8x or 

 iox ocular. The fragment will be outlined 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 remain distinct, but the dark 

 band will be somewhat narrower. Remove the cover-glass, wipe it 



Fig. 60. Air Bubble and Oil 



Globule in Water. 



Axis The principal optic axis. 



F, F The principal foci of the air 

 and oil. As the air is less refractive 

 than water its focus is virtual. The 

 focus of the oil globule is real, as its 

 refraction is greater than water. 



