96 INTERPRETATION OF APPEARANCES [CH. Ill 



each will be lightest, and the bright central spot will be somewhat 

 dark.* 



§ 144. 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. (Figs. 54-56.) 



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

 (see under mounting). The outline will be distinct with the unaided 

 eye. Use it as object and employ the 16 mm. (Jfi in.) objective and 

 high ocular. Light with central light. 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 dis- 

 tinct, but the dark band will be somewhat narrower. Remove 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 the 

 fragment of cover-glass in that. The dark contour will be much nar- 

 rower than before. 



Draw a solid glass rod out to a fine thread. Mount one piece in 

 air, and the other in 50% glycerin. Put a cover-glass on each. Em- 

 ploy the same optical arrangement as before. Examine the one in air 

 first. There will be seen a narrow, bright band, with a wide, dark 

 band on each side. 



The one in glycerin will show a much wider bright central band, 

 with the dark borders correspondingly narrow (Fig. 92, b). The dark 

 contour depends also on the numerical aperture of the objective — being 

 wider with low apertures. This can be readily understood when it is 

 remembered that the greater the aperture the more oblique the rays of 

 light that can be received, and the dark band simply represents an 

 area in which the rays are so greatly bent or refracted (Figs. 54-56) 

 that the}' cannot enter the objective and contribute to the formation of 

 the image ; the edges are dark simply because no light from them 

 reaches the observer. 



*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 observa- 

 tion. If a 16 mm. {% in.) is used instead of a 3 mm. {y& 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. 



