Ch. IV] DETERMINATION OF THE CHARACTER OF OBJECTS 115 



Fig. 70. Fine 

 Forceps for Plac- 

 ing Cover-glasses 

 on Specimens. 



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 narrower 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. Employ 

 the same optical arrangement as before. Ex- 

 amine the one in air first. There will be seen a 

 narrow, bright band, with a wide, dark band on 

 each side (fig. 71a). 



The one in glycerin will show a much wider 

 bright central band, with the dark borders cor- 

 respondingly narrow (fig. 71b). The dark con- 

 tour depends also on the numerical aperture of 

 the objective — being wider with low apertures. 

 This can be readily understood when it is remem- 

 bered that the greater the aperture the more ob- 

 lique 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 (fig. 69) that they 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. 



If the glass rod or any other 

 object were mounted in a me- 

 dium of the same color and re- 

 fractive power, it could not be 

 distinguished from the medium. 

 The effect of the immersing 

 liquid on the contour bands 

 around any transparent object 

 is made of practical use in the determination of the refractive index 

 of crystals. When the crystal and liquid are of the same index there 

 will be no band, and the more they differ, the wider will be the band. 

 As shown in § 194-201, lighting with oblique light, also focusing up 

 and down, will indicate whether the crystal is of greater or less index 



Fig. 71. 



Glass Rods in Air and 

 in Glycerin. 



a Glass rod in air and viewed by 

 central transmitted light. 



b Glass rod mounted in 50% gly- 

 cerin; the dark border is narrower than 

 when mounted in air. 



